1.4- Addition reactions

Addition of an alkali-metal fluoride, frequently KF, CsF, (Me2N)3S+Me3SiF2 (soluble in organic solvents) or a tetraalkylammonium salt, to a fluoroaUcene in an aprotic dipolar solvent is usually the method used to generate perfluorocarbanions. Some of these intermediates have been directly observed by NMR [58, 121], The intermediate carbanions may be trapped by a variety of electrophiles, and some examples are given in Table 7.7. 

Reactions of additions of 3-(2-chloro-l-azaazulenyl)methylene with cis- and trans-stilbenes and p-substituted styrenes to form 3-cydopropyl-l-azaazulenes have been accomplished [65]. The reaction proceeded in a stereospecific manner to afford the 

2-addition of nitroform across the double bonds of vinyl ethers has been used to synthesize a large number of a-trinitromethyl ethers (113-115) have been synthesized from 

Shackelford and co-workers studied the 1,2-addition of 2,2-dinitropropanol, 2,2,2-trinitroethanol, and 2-fluoro-2,2-dinitroethanol across the double bonds of vinyl ethers. These reactions are Lewis acid catalyzed because of the weak nucleophilic character of alcohols which contain two or three electron-withdrawing groups on the carbon p to the hydroxy functionality. Base catalysis is precluded since alkaline conditions lead to deformylation with the formation of formaldehyde and the nitronate salt. 

The reaction of 2-fluoro-2,2-dinitroethanol (119) with divinylether (118) under different conditions gives three products, namely, the expected vinyl acetal (120) and the bis-acetal (121) from addition of one and two equivalents of 2-fluoro-2,2-dinitroethanol, respectively, and the vinyl ether (122), which results from franx-etherification of (118) with loss of acetaldehyde. Shackelford and co-workers found that by altering the nature of the Lewis acid catalyst and the reaction stoichiometry they were able to alter the distribution ratio of these products. 

The franx-etherification of vinyl ethers is not uncommon under Lewis acid catalysis. 1,1-Bis(2-fluoro-2,2-dinitroethyl)ethyl acetal (124) is obtained on reaction of two equivalents of 2-fluoro-2,2-dinitroethanol (119) with vinyl acetate (123) in the presence of mercuric sulfate.  

The acetylenic bond of propargyl ethers can react with polynitroaliphatic alcohols, as in the case of ethoxyacetylene (125), which reacts with two equivalents of 2-fluoro-2,2-dinitroethanol (119) to give the orthoester (126).  

In addition to their application as solvents or intermediates in chemical syntheses, chlorophenols are industrially important due to their broad spectrum of antimicrobial properties and their use as fungicides, herbicides, insecticides, ovicides, and algicides. 

Up to the early 1980s VCM was produced by addition of hydrogen chloride to acetylene. In this process the gaseous reactants are brought into contact with the catalyst at slightly increased pressure and 100-250 °C [1]. Mercury(II) chloride on activated carbon is used as a catalyst in this heterogeneous process. Today, however, this reaction has no economical importance. Nowadays, VCM is exclusively produced by thermal decomposition of DCE. 

In the LTC process, ethylene and chlorine react in DCE as solvent at temperatures below the boiling point of DCE (usually between 20 and 75 °C). The re- 

The activity and selectivity of the FeCla catalyst is easily increased by addition of various additives. In the early 1980s it was found that addition of anunonia 

The use of well-designed chemical processes, aided hy chiral molecular catalysts, can provide truly practical and efficient synthetic methods for the production of hioactive substances and functional materials. In particular, the growing awareness of the importance of chirality to the function of chemical substances has led to the development of many new chiral molecular catalysts for use in large-scale production. Chiral molecular catalysts consisting of a metal atom or ion and a chiral organic ligand(s), under the appropriate conditions, can not only repeatedly accelerate organic reactions, but also simultaneously control the absolute stereochemical outcome [1]. 

The addition of electrophiles like Me [30] to the complexes 4 results in the formation of lla,b corresponding to the reaction in Eq. (11). Whereas complex 11a is not stable at room temperature and must be prepared and 

In some addition compounds, if there is close proximity of an H atom and an OH group they can eliminate a molecule of water. The elimination of a small molecule, such as water, from an organic compound after an addition reaction is called a condensation reaction. 

Nucleophiles seek areas of low electron density, and electrophiles, those of high electron density 

These reactions are useful in chemical synthesis to introduce an extra carbon atom into a molecule 

Addition reactions with sodium hydrogensulfite, NaHSOs 

It can be seen that, after addition, one of the H atoms is in close proximity to the OH in the molecule this results in the elimination of water. The resulting compound is called a hydrazone. 

Michael addition of secondary phosphines on conjugated olefins is a well known reaction in organic synthesis. Accordingly, addition of diphenylphosphine on hydrophilic activated alkenes in CH3CN or in CH3CN/H20 solution leads to various tertiary phosphines [33] examples include 1, 25, 27. In order to avoid the formation of phosphine oxides and/or the hydrolysis of some alkene derivatives (e.g. acryl esters) a small amount of Et N+OH was used as base, and a small quantity ofditertbutylphenol was 

In ethanol/water mixtures addition of sodium mercaptoalkane sulfonates on vinyldiphenylphosphine proceeds smoothly at room temperature and yields a variety of tertiary phosphines such as 24. Interestingly, at the beginning of the reaction the ethanolic solution of the vinylphosphine and the aqueous solution of the educt comprise two separate phases and this is favourable for the high yields obtained (59-97%) [30], 

Addition reactions — The fullerenes Ceo and C70 react as electron-poor olefins with fairly localized double bonds. Addition occurs preferentially at a double bond common to two annelated 6-membered rings (a 6 6 bond) and a second addition, when it occurs is generally in the opposite hemisphere. The first characteriz-able mono adduct was [C6oOs04(NC5H4Bu )2]. formed by reacting Cgo with an excess of OSO4 in 4-butylpyridine. The structure is shown in 

Other addition reactions are shown in the scheme. Thus, Ceo reacts as an olefin towards [Pt (PPh3)2] to give the t adduct [Pt( j -C6o)(PPh3)2]. Indeed six centres can simultaneously be coordinated by a single fullerene cluster to give [C6o M(PEt3)2 6], (M = Ni, Pd, Pt), with the 6M arranged octahedrally about the core. Likewise, reaction 

Hirsch, Angew. Chem. Int. Edn. Engl. 32, 1138-41 (1993) and references cited therein. 

For clarity only the front sides of the fullerenes are shown.  

Returning now to the reaetions in the seheme it ean be seen that earbenes and silenes 

Addition reactions are characterized by two groups adding across a double bond  

Addition reactions occur in compounds having n electrons in carbon-carbon double (alkenes) or triple bonds (alkynes) or carbon-oxygen double bonds (aldehydes and ketones). Addition reactions are of two types electrophilic addition to alkenes and alkynes, and nucleophilic addition to aldehydes and ketones. In an addition reaction, the product contains all of the elements of the two reacting species. 

Alkenes and alkynes readily undergo electrophilic addition reactions. They are nucleophilic and commonly react with electrophiles. The n bonds of alkenes and alkynes are involved in the reaction, and reagents are added to the double or triple bonds. In the case of alkynes, two molecules of reagent are needed for each triple bond for the total addition. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

The n electrons attack the electrophile, the positive part of the reagent, usually the H , and form a carbocation intermediate. 

The nucleophile (Nu ), the negative part of the reagent, usually X HO and so on, attacks the carbocation to form the product. 

Addition reactions are characteristic of unsaturated compounds. In addition reactions, an unsaturated bond (C = C, C C, etc.) is completely or partially saturated by addition of a molecule across the multiple bond. 

The most important addition reactions are the addition of hydrogen (H2), the halogens (X2), the hydrogen halides (HX) and water (H—OH). 

In an addition reaction, one K bond and one o bond are converted into two o bonds. The heat released in the formation of two o bonds is greater than that needed to break one o bond and one n bond, therefore addition reactions are exothermic. The reverse of the addition reaction is known as elimination reaction. 

Addition Reactions.—As the years go by, the importance of electron transfer processes is becoming increasingly apparent, and hardly a month passes without the reinterpretation of a reaction as involving such a process. This has stimulated the publication of review articles such as that by Mattes and Farid on the electron transfer reactions of alkenes, and the more specific reviews by Mariano on the application of electron transfer photochemistry to iminium salts. In this area Mariano and his co-workers have reported further on the electron-transfer-initiated photochemistry of iminium salts (1), and in detail on the spiro-cyclization methodology of iminium salt cyclization.  

In acetone, the three products (3), (4), and (5) are formed, whereas in cyclohexane and benzene, the last of these, (5), is accompanied by a new reduction product (6) and an unknown rearranged product. In contrast, the alkene (7) is photoinert in cyclohexane and benzene. 

Catalytic effects in the acid-catalysed hydration of the aryl alkenes (8) and (9) have been studied, and photohydration of these alkenes occurs via the Si state. The hydroxyisoflavenes (10) are converted photochemically into the benzofurobenzofurans (11) on irradiation in methanol. The actual mechanism of the process is not known and could involve either of the intermediates (12) or (13). 

Battersby and his co-workers have reported the synthesis of the chlorin (14) via photocyclization of the IS-ir-systein (15). Other examples have also been reported.  

irradiation of the gibberellin derivative (16a) in CeHe-T O gave the tritiated product (16b). 

Addition Reactions.- A pulsed-laser induced SET from stilbene to 

Addition reactions occur when two starting materials add together to form only one product. 

The mechanism of these reactions can involve an initial electrophilic or nucleophilic attack on the key functional group. 

Returning now to the reactions in the scheme it can be seen that carbenes and silenes 

Heteroatom fullerene-type clusters — The possibility of incorporation of hetero atoms into C clusters has excited the attention of both theoreticians and experimentalists since the earliest days of fullerene chemistry, particularly in view of the known stability and ubiquity of organic heterocycles. The structural relationship between Cgo and /1-rhombohedral boron has already been alluded to (p. 142). 

Laser vaporization of a composite pressed disc of graphite and BN using He as carrier gas, followed by mass spectrometric analysis, gave a range of clusters with even numbers of atoms from less than 50 to well above the peak 

Addition Reactions.- The irradiation through Pyrex of the electron-acceptor alkenes (20) in acetonitrile solution with phenanthrene and hexamethyl disilane brings about a regio- 

The reaction proceeds via an electron transfer from phenanthrene 

Boiety is electron donating are poor yields obtained. The 

RearrancBBent Reactions.— Kira et ml. have described the photochemical isoBerization of allyl silyl derivatives. The irradiation at 254 nn in hexane brines about the conversion of (25, R = Me, Ar = Ph) into (26, R = Me, Ar s Ph) affordins a product mixture of the two in the ratio of 29 54. Similar behaviour was shown for the derivatives (25, R = Me, Ar = naphthyl) and (25, R = H, Ar = phenyl). The reaction was shown to be intramolecular. When the irradiation of a derivative with an optically active silyl centre was studied the reaction proceeded with inversion of oonfisuration of the micratins 

Hoffmann postulates. A quantitative yield of the trans-alkene 

Addition Reactions.—The photochemical conversion of the optically active aniline (1) into the indoline (2), and the methanol adducts (3, 4) of (1), have been described. Irradiation of the substituted naphthoic acid (5) yields the cyclized lactone (6). This is a key step in the synthesis of the quinone (7).  

Photoaddition of acetaldehyde to the stereoisomers of (8) gave only two (9) of the eight possible adducts. The acetyl group apparently enters specifically at C-4 trans- to the hydroxy-group. 

Addition products are formed when iminium salts (10) are irradiated in ethers or alcohols. The mechanism of the addition involves an electron transfer as shown 

Addition Reactions.—Methanol has been shown to quench the fluorescence of 

2-diphenylcyclobutene. This is good evidence that the photo-reaction of methanol with the cyclobutene to afford (1) and (2) arises from the singlet state.1 The mechanism of the addition is thought to involve the cation (3) which either is 

Addition reactions with alkenes to form cyclopropanes are the best-studied reactions of carbene intermediates, in terms of understanding carbene mechanisms and synthetic applications. Doering, in 1954, first reported the formation of cyclopropanes by the 1,2-addition of carbenes to alkenes. Singlet and triplet carbenes exhibit some important differences. Because it has an empty p orbital (like a carbocation) and a nonbonded pair of electrons (Hke a carbanion), the singlet carbene exhibits both carbocation and carbanion character. However, the triplet carbene behaves more as a diradical. These characteristics influence the types and stereochemistries of carbene reactions. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. 

In general, the following reactivity sequence applies to addition of carbene to olefins  

Me2C=CMe2 Me2C=CHMe Me2C=CH2 H2C=CH-CH=CH2 [ PhCH=CH2 

However, reaction of the triplet carbene is not stereospecific and the addition follows a radical pathway, and is stepwise, producing a mixture of diastereomeric 

The stereochemistry of these cycloadditions is so specific that it may be used as a diagnostic test for the identification of singlet and triplet carbenes. If the reaction is conducted in the presence of triplet quenchers, substances such as butadiene, which selectively remove any triplet carbenes, the addition is again stereospecific. Reactions involving free carbenes are very exothermic because two new a bonds 

Addition Reactions. — A detailed study of the free-radical chlorination of penta-2,3-diene with t-butyl hypochlorite shows that substitution products are obtained by both allylic and allenic hydrogen-abstraction addition 

Oxymercuration studies on optically active penta-2,3-diene using mercuric acetate in methanol suggest that formation of the unsymmetrical mercurinium 

Reduction of organomercurials formed from cyclic allenes (10—14-membered rings) with sodium borohydride yields an increasing ratio of trans. cis monosubstituted olefins as the ring size is increased. The stereochemistry and mechanism of the reduction of cyclic allenes using di-imide and sodium in liquid ammonia have been investigated. A stereospecific reduction with sodium in liquid ammonia of the intermediate cyclopropylallene 

1 Addition Reactions - The allyl ketone (9) is the product formed on photohydration of the diyne (10). The photohydration step arises from the triplet excited state of (10). The polarization within the excited state is the key to the outcome of the reaction and is such that the protonation occurs at Cl. Shim has published a short review dealing with the hydration of such systems. 

Direct irradiation of the arylalkenes (11) results in their conversion into the cyclic ethers (12) and (13). These cyclizations are the result of proton transfer within the excited state and cyclization of the resultant zwitterion. Using 2,4,6-triphenylpyrylium tetrafluoroborate as an electron-accepting sensitizer the compounds (11, = 1, R = CH3) and (11, n = 1, R = CH3O) undergo oxidative cleavage even when the reactions are carried out under argon.  

Smith and Richards S report that there is doubt over the formation of the products claimed by Padwa et alP from the irradiation of compounds such as (14). In the earlier report it was claimed that products, the result of ring contraction, were formed by a two-photon path but this has now been shown to be unlikely and the product formed on irradiation of (14) is merely that of addition of methanol to the ethene bond affording the adduct (15).  

1 Addition Reactions - An examination of the photochemical behaviour of styrenes encapsulated in zeolites has shown that both oxidation and hydration take place and one of the major reactions encountered is the formation of 2-phenylethanol. Irradiation (X, 300 nm) of the allenes (43) and (44) in deuteriochloroform solution in the presence of a 1 1 molar ratio of (PhS)2 and (PhSe)2 provides a convenient method for thioselenation and affords good yields of adducts such as (45, 99%, 22 78 E Z ratio) and (46, 75%, 40 60 E Z ratio), respectively.  

The telluroglycoside (47) undergoes C-Te bond fission on irradiation in benzene solution at 100 °C to give the glycosyl radical (48). The radicals 

Medium ring cycloalkenes undergo carbonylation when they are irradiated over a Co(acac)2 catalyst in the presence of CO and methanol and the yields of methyl cycloalkanecarboxylates are high.  

In this section, additions to the multiple bonds C=C, C=C and C=0 as well as to the epoxide bond C—C are described. Sections 3.1—3.4 deal 

X can be halogen, OH, O-alkyl, alkyl and aryl. The published data on 

Past work on the addition of simple alkyl-substituted alkenes to photo-excited benzene has shown that the reaction favours the formation of adducts which appear to be derived from initial bonding between the alkene and the meta positions of the aromatic ring. The Intermediate species which Is thought to be produced shows evidence for some separation of charge, as shown in (78) for the addition of ethylene to benzene. The isolated product, e g. (79). results from coupling between C-1 and either C-3 or C-5 of the intermediate. 

Corneiisse and co-workers have now described the results of CNDO/S and MNOO calculations performed for this reaction. Their results are consistent with initial bonding between the alkene and the meta positions of the ring followed by very rapid, almost synchronous, closure of the intermediate to the product. The 

The results confirm that It is the state of the arene which reacts with ground state alkene but do not allow any conclusion to be made about the intermediacy of an exciplex or any other species along the reaction path. Also in the same laboratory, the products of the meta photo-addition of cyclopentene with alkyl and alkoxy benzenes have been determined and the effect of increasing size of the alkyl or alkoxy group examined. The major product isolated is (80). although (81) and other isomers become more important for alkyl benzenes as the substituent s size is increased. The results are consistent with a polar mechanism involving the formation of (82) where the substituent stabilises the positive charge and steric interactions are avoided by the endo orientation of the cyclopentane. Also consistent with a polar intermediate are the meta adducts 

The groups of Gilbert and of Wagner have reported examples of 

Wagner produces convincing evidence that it is the triplet 7r- zr states of (99) and (100) which are responsible for the reactlon and this may be the origin of the ortho rather than meta mode of addition for these compounds. 

Cycloaddition reactions of derivatives of chromium and tungsten pentacarbonyl compounds 

Addition reactions of metal pentacarbonyl a, P-unsaturated Fischer carbene complexes 

The unsaturated osmium hydrido carbonyl cluster ( i2-H)2Os3(CO)io reacts with a wide variety of Lewis bases, L,264 according to the following equation  

For alkenes and alkynes, addition to the double or triple C-C bond is common. A typical example is 

This kind of reaction can proceed in solution by a carbocation mechanism, but in the radiation-induced case, it proceeds almost exclusively by a radical mechanism. In most cases, the radiation initiates reactions that are of chain character. 

Addition to an unsaturated compound (U) can occur in an alternating chain as exemplified here  

The chain length is therefore adversely afFected by the irradiation dose rate being inversely proportional to its square root. Wagner (1969) lists a large class of unsaturated compounds in which addition reactions can be induced by irradiation. Typical examples involving long chain lengths are for the addends HC1, Cl2, and HBr in ethylene, benzene, toluene, and so on. where the products are telomers or hexachlorides. 

BF3 Et20 facilitates the addition of moderately basic nucleophiles like alkyl-, alkenyl-, and aryllithium, imines, Grignard reagents, and enolates to a variety of electrophiles. 

The cuprate 1,4-conjugate addition step in the synthesis of (+)-modhephene is difficult because of the neopentyl environment of C-4 in the eneone, but this can occur in the presence of BF3 Et20 (Eq. 75) [129]. 

This reagent is used as a Lewis acid catalyst for the intramolecular addition of diazo ketones to alkenes [130], The direct synthesis of bicyclo[3.2.1]octenones from the appropriate diazoketones using BF3 Et20 (Eq. 76) is superior to the copper-catalyzed thermal decomposition of the diazo ketone to a cyclopropyl ketone and subsequent acid-catalyzed cleavage [131]. 

BF3 Et20 reacts with fluorinated amines to form salts which are analogous to Vils-meier reagents, Arnold reagents, or phosgene-immonium salts (Eq. 77) [131]. These salts can be used to acylate electron-rich aromatic compounds, introducing a fluorinated carbonyl group (Eq. 78). 

The two possible modes of addition of a reagent X—Y to an alkene are shown in (a). In syn addition, both groups add to the same side or face of the molecule. In anti addition, the groups add to the opposite faces of the molecule. The consequences of syn and anti additions are shown in (b). Geometric isomers can result from the addition of a reagent X—Y to the double bond of a cycloalkene. Syn addition produces a cis product, whereas anti addition produces a trans product. 

In hydroboration—oxidation, a reaction sequence used to synthesize alcohols, a-pinene is converted to the alcohol shown below. What molecule has been added to the alkene What is the stereochemistry of the addition reaction  

The methyl carbocation has an sp hybridized carbon. The H—C—H bond angles are 120°, and the carbon atom has a vacant 2p orbital that isperpen-dicular to the plane of the four atoms. 

Conjugate Addition Reactions of a,/ -Unsaturated Carbonyl Substrates 

Chen and co-workers later reported the successful asymmetric 1,4-addition of aryl thiols to a,/ -unsaturated cyclic enones and imides using Takemoto s elegantly simple catalyst (3) [43]. This bifunctional amine-thiourea catalyst gives optimal reactivity and reproducibility when used at 10 mol% loading in the presence of freshly dried 4 A molecular sieves (MS). This combination afforded the expected addition products in high yields (90-99%) and moderate to good enantioselectiv-ities (55-85% ee) for a variety of cyclic and acyclic Michael acceptors (Table 6.2). 

Catalyst 3 is proposed to function in a manner similar to the cinchona alkaloid catalysts (1 and 2), with the tertiary amine providing activation for the nucleophilic thiol, which is held in close proximity to the thiourea-bound carbonyl substrate. 

Conjugate Addition Reactions. - Two protocols for achieving the enantioselective conjugate addition of an alkyl group to an enone using a cuprate containing a chiral ligand have been reported. In one case the lithium amide (28) was used to prepare an amidocuprate 

4- to enones in good yield. The reagents have advantages over cuprates of better solubility and thermal stability and their chief drawback, that only one of the three ligands is transferred, can be 

5- disubstituted a,a-unsaturated ketones by the Michael addition -elimination reaction of a variety of cuprates with B-alkylthioenones has been published.1 

The conjugate addition chemistry of a number of formyl- and acyl- anion equivalents has been studied these include the 

Basic alumina, in the absence of solvents, has been found to 

Conjugate Addition Reactions. - A review of the very important 

2-addition by-product than are obtained when the chlorosilane 

Several publications have appeared describing conjugate 

Reagents i. Sn(OTf)2, RCHO ii, Na2C03, MeOH iii, NaBH. iv, NalO/, 

4-addition of enethiolates to enones has been studied and found to give -alkylated products with thioketone derived enethiolates, C-alkylated products with thioester derived enethiolates, but interestingly 1,2-addition with thioamide 

Among the addition reactions, those proceeding via radical intermediates should be privileged when carried out under sonication. The mechanisms, however, are not always easily determined due to the simultaneity of the sonochemical and mechanical effects. The synthetically important cycloadditions raise specific problems and necessitate specific discussions (Qi. 3). 

Enolate anions formed by Michael addition may be trapped by methanesul-phinyl chloride. The resulting sulphoxide undergoes ready elimination to form a new unsaturated ketone, capable of further substitution by conjugate addition [equation (70)]. 

Fujisawa, A. Noda, T. Kawara, and T. Sato, Chem. Lett., 1981,1159. 

Using the latter approach Posner et have developed routes to enantiomeri-cally pure cyclopentanone intermediates for chiral steroid syntheses. 

Like substitution reactions, additions of organozinc compounds on electrophiles are very slow and need often activation of the reaction partners. 

Alkylzinc halides react only sluggishly with aldehydes or ketones. This reactivity can be improved by activating the carbonyl derivative with a Lewis acid. Excellent results are obtained with titanium alkoxides,137 Me3SiCl,138 

Although alkylzinc derivatives add only slowly to aldehydes, alkenylzinc derivatives display a higher reactivity (Equation (74)).142 Reactive benzylic or related zinc reagents smoothly add to aldehydes, providing the allylic alcohol in almost quantitative yield (Equation (75)).143 

The formation of activated iminium intermediates derived from nitrogen heterocycles has been reported by Comins and co-workers.163,163a The activation of pyridine derivative with phenyl chloroformate provides pyridinium salt, which smoothly reacts with the zinc homoenolate (Equation (94)).163 163a 164 The reaction of unsaturated amide with Ph jC 1 BI 4 produces iV-acyliminium ions, which react with PhaZn in CH2CI2 producing the desired a--substituted amine (Equation (95)).165 

Zinc enamides such as 50 are reactive organozinc species, which can undergo addition to unactivated olefins with good to excellent yields. After trapping of the organozinc intermediate 51 with an electrophile and hydrolysis, a variety of functionalized primary, secondary, and tertiary a-alkylated ketones are isolated (Equation (100)).170 

2 Miscellaneous Reactions. 1.2.1 Addition Reactions - A novel photochemical reaction of stilbene in carbon tetrachloride solution has been described. Irradiation of this system populates the first excited singlet state of stilbene which then abstracts a halogen from the solvent. The resulting radical pair composed of a trichloromethyl radical and (14) yields the products. 

Du et al. reported the chiral Zn( 11)-catalyzed enantioselective Michael addition of nitroalkanes to nitroalkenes to synthesize optically active 1,3-dinitro compounds 

As the name suggests, these reactions involve addition of a regent to an unsaturated group and as such nominally display 100% atom economy. 

When the addition is initiated by attack of the jr-electrons in an unsaturated bond on an electrophile to form a carbocation the reaction is an electrophilic addition, a very common class of reactions for alkenes. The reaction is governed by Markovnikov s rule, which states that in addition of HX to a substituted alkene, the H will form a bond to the carbon of the alkene carrying the greater number of hydrogen atoms. 

Diels-Alder reactions provide one of the few general methods of forming two carbon-carbon bonds simultaneously. The main features of these reactions are described in Box 1.3. The reaction finds widespread industrial use for example hardeners for epoxy resins are made by reaction of maleic anhydride with dienes such as 2-methyl-1,4-butadiene. 

In this chapter, we will be studying addition reactions to carbon-carbon multiple bonds this is the converse process of the eliminations that we studied in the previous chapter. Addition to carbon-heteroatom multiple bonds is coming up in Chapter 14. Nucleophiles, electrophiles, and radicals can all add across double bonds first, we will concentrate on electrophiles and radicals, as nucleophiles only add readily when the double bond bears a group (such as a carbonyl, nitro, or nitrile Chapter 17) capable of accepting electron density. Reactions with electrophiles or radicals add two moieties, atoms or groups, by a stepwise process the two atoms or groups are not added simultaneously. However, there is another class of reactions where the two new bonds are made simultaneously—these are called concerted reactions. 

We should first recall the electronic structure of alkenes, with carbon atoms sp hybridized and a sigma framework of bonds at approximately 120 ° to each other. Above and below the plane of the molecule is a jt-orbital, derived from the two remaining p orbitals (11.1). 

The addition of two atoms or groups to an alkene is the most important reaction for this type of compound. The two electrons of the it-bond provide two of those needed to make the two new a-bonds. 

The reaction of an organometallic reagent with an a,j3-unsaturated ketone can occur by 1,2-addition to the carbonyl group and by 1,4- 

At present, the accepted mechanism of 1,4-addition involves the formation of either a charge-transfer complex or an anion-radical species by partial or complete electron transfer, respectively [Eq. (92)]. Collapse of the charge-transfer complex or transfer of an organic group from the copper(II) species which results from the second process, completes the addition sequence 139). Supporting evidence for this view of the 

An alternative mechanism for conjugate addition to a,j8-unsaturated ketones is a free-radical chain process in which copper(I) would serve as the metallic center in a radical displacement reaction. However, the Li(RjCu) ------------------------ R- (93) 

Marshall and Ruden (196) have reported a conjugate addition of lithium dimethylcuprate to a cyclopropyl enone. The major products are the 1,4-adduct and one from a previously unreported 1,6-addition to the cyclopropyl ring [Eq. (96)]. 1,6-Conjugate additions are known for dienoates (206) and 2,2-di(carboxyethyl)vinylcyclopropane 94, cf. 59). 

Benzylmagnesium chloride reacts with methyl vinyl ketone to give a mixture of the 1,4-and 1,2-adducts in the ratio 4 1. Catalysis by copper(I) chloride does not dramatically increase this ratio. For 3-methoxybenzyl-magnesium chloride and the same ketone [Eq. (97)] the ratio of 1,4-adduct 1,2-adduct 1,4-ortho adduct is 2 1 8. Copper catalysis alters this ratio to 15 1 3. Thus copper compounds may be very effective (93) in circumventing the benzylic rearrangement. 

The design and use of new initiators to mediate addition reactions has attracted some attention with new peroxide and azo-derived initiators being described. The additions of 2-cyanoisopropyl radicals (derived from homolysis of the common radical initiator AIBN) to a range of alkynes have been examined. The reactions were regioselective with alkynes bearing electron-withdrawing substituents but failed with hindered or alkylacetylenes. The same radical addition to Ceo has been studied by EPR. Two different types of adduct radicals were proposed.  

The addition of silanes across alkenes has been investigated both experimentally and theoretically. The effect of optically active thiol catalysts to catalyse radical hydrosilylation (polarity reversal catalysis) has been studied. The use of 2,3,4,6-tetra-O-acetyl-thio-/i-D-glucopyranose as the chiral thiol (used to reduce the intermediate carbon-based radicals) furnished the hydrosilylated alkenes in low to moderate enantiomeric excesses. In addition to this work a theoretical study on the reactions of SiFt3 with ethene and propene has been undertaken using PMP2(6-31G ) and QCISD(T)(6-31G ) methods. Results indicated that the alkene-addition pathway is favoured over the alternative possible mode of reaction (H-abstraction). This is contrary to that previously suggested for the reaction of SiH3 with propene.  

Other research in the area of addition reactions onto unsaturated carbon-carbon bonds has included measurement of the rate coefficients for the addition of N03 to chloro- and trichloro-ethene, relative rate measurement for N03 addition to isoprene, TF-/rSR-measured muonium addition to vinyl aromatics and EPR studied addition of radical (65) to alkenes. In this latter study a linear dependence of the rate constant of addition with the donor/acceptor properties of the alkene partner was highlighted. 

Tetrafluorethylene reacts with nickellocene to form [6-14], equation (6-13). On the other hand, cobaltocene undergoes addition of tetrafluoro-ethylene to yield the binuclear compound [6-15], equation (6-14). 

Some metallocenes show interesting behavior upon the addition of organic halides. For example, cobaltocene reacts with organic halides to form equimolar amounts of cobalticinium halide and the 1-endo addition compound, equation (6-15). 

Generally the uncatalyzed addition of a weak acid to olefins does not readily occur. However, it is remarkable to find acetic acid adding smoothly to the double bond of vinylmetallocene. The relative rates of addition are shown in Table 6-2. 

The greater rates of addition of acetic acid to vinylmetallocenes might result from the unusual stability of the carbonium ion, which is formed by the overlap of the hag molecular orbital of the metallocene nucleus with the available /7-orbital of the positively charged a-carbon atom, equation (6-16). 

The relative rates of addition can be attributed to the increasing overlap as the size of the metal atom in the metallocene increases. 

Mitchell and Kowall described the first synthesis of l,l,2-tris(stannyl)ethenes [191], and the same group reported on the addition of hexamethylditin to 1,3-dienes [192]. Kwetkat et al. described the addition of hexamethylditin to a series of cyclic allenes and determined the stereochemistry of the products [193]. 

A neighboring hydroxyl group may participate in electrophilic addition reactions. Although this type of effect is discussed in more detail in Volume 2 some pertinent examples here will serve to illustrate this effect with alcohols. 

Winstein and Goodman obtained the epoxide (224) upon treatment of the olefin (223) to conditions appropriate for the addition of BrOH. HO-3 

Exclusive HO-5 participation was observed in the bromination of (225), although HO-6 is also possible. A five- rather than a six-membered ring is generally preferred in such reactions owing to the relatively favorable 

When the series of alkenols was brominated in trifluoroacetic acid, the relative rates for CH2=CH(CH2) OH were 1.00, 4.89, 11.2, and 14.8 when n = 1-4, respectively. The absence of anchimeric assistance was attributed to hydrogen bonding between the hydroxyl group and the solvent, which reduces the hydroxy group s nucleophilicity (Hooley and Williams ). 

1 1 mixture (71% yield) of the isomeric HO-5 participation products. No R3N-6 product was isolated. Additionally, bromination of (230) afforded an 87 % yield of a single product. Hence HO-5 participation in bromocycli-zation reactions appears to be superior to both R3N-6 participation and 0-5 amide participation. 

An addition reaction involves the combination of two molecules to form a product containing atoms from both reactants. All addition reactions occur on unsaturated organic structures—namely, alkenes, alkynes, carbonyls, and arenes. Not surprisingly, three distinct mechanisms are observed—electrophilic, nucleophilic, and radical. In the electrophilic addition pathway, the unsaturated system acts as the nucleophile supplying electrons to the electrophilic reagent. The opposite is true in the nucleophilic pathway. In either of these two cases, simple electrostatic considerations assist in predicting what will occur. 

At this point it may be useful to review our description of the bonding of simple carbonyls and olefins. In Chapter 1 we described their and it molecular orbitals of both. Recall, especially, the polarizations in the orbitals of the carbonyl group, where the LUMO is polarized toward the carbon. Also, Appendix 3 has detailed drawings of orbitals associated with olefins and the carbonyls of many representative functional groups. 

The presence of substituents on the benzene ring can be used to control the reaction-mode selectivity in inter- and intra-molecular cycloaddition of ethenes to the benzene ring. Electron acceptor or electron donor substituents favour the 

The photochemical reactions of 6-chloro- and 5-fluoro-l,3-dimethyluracils with naphthalene and selected substituted derivatives in solvents of varying protic and polarity characteristics are the subject of three reports within the review period. Irradiation of the chloro derivative and naphthalene in cyclohexane or benzene afforded exclusively the substituted product (35), whereas in 

The studies reported last year on the stereoisomeric control of the photo-induced Diels Alder reaction of maleic anhydride with homo-chiral anthracene derivatives such as (46) have been extended using 320-400 nm radiation, and this gives the head-to-tail anthracene dimer as well as the previously reported adduct (47) with excellent diastereoselectivity. The thermal and photochemical retroaddition process has been examined and the results suggest that this facile process may promote the anthracene as a new chiral auxiliary. 

The presence of both electron-donor (alkoxy) and electron-acceptor (cyano or carbomethoxy) substituents at the 2- and 3-positions respectively of the pyridine ring, have unlocked its photocycloaddition chemistry, and in recent years several such reactions of these heteroarenes have been described. Previously reported additions of ethenes involved acrylonitrile derivatives, but similar processes are now described with vinyl ethers as the addend. In these systems, however, it seems that the regiochemistry of addition is very sensitive to steric influences. Thus, although the dihydroazocine (48) is formed selectively from the 3-cyano- 

Electron-transfer photosensitization (2,4,6-triphenylpyrylium tetrafluorobor-ate) is reported to induce a photo-Diels Alder reaction between A-arylimines (55) and styrene or a-methylstyrene. The reaction is considered to proceed by attack of the styrene radical cation onto the arylimine and affords both dia-stereoisomers (56) and (57) in reasonable yields, although amounts of the quinoline (58) and the amine (59) are formed in some cases. 

Consider the reaction of 2,3-dimethyl-2-butene with H2 catalyzed by Pd/C. Two new C—H bonds are made, and a C=C tt bond breaks. The fact that the addition is stereospecifically syn suggests that an insertion reaction is occurring. The Pd metal is in the (0) oxidation state (i.e., d °), so it can react with H2 by an oxidative addition to give two Pd—H bonds. At this point Pd is in the (II) oxidation state. An insertion of the alkene into one of the Pd—H bonds gives Pd—C and C—H bonds. Finally, reductive elimination gives the product and regenerates Pd°, which begins the catalytic cycle anew. 

Note that curved arrows are typically not used to show electron flow in each mechanistic step. However, the catalytic cycle is explicitly drawn, and every step is one of the typical reactions of metals. 

Every late-metal hydrogenation catalyst, whether homogeneous or heterogeneous, probably uses exactly the same catalytic cycle, although some catalysts, particularly sterically encumbered ones such as Wilkinson s catalyst, require that ligand dissociation or substitution occur before the catalytic cycle gets underway, In the case of metals supported on solids such as activated C, silica, and alumina, the support may participate in the reaction in ways that need not concern you here. 

Late-metal complexes of Pd, Pt, and Rh can also catalyze hydrosilylation, hy-drostannylation, hydroboration, and diborylation reactions of it bonds. Both C=C and C=0 bonds may be hydrosilylated or hydroborated, whereas hy-drostannylation is usually carried out only on C=C bonds. (Some boranes add to C=0 and C==C bonds in the absence of catalyst, but less reactive ones, such as catecholborane ((C6H402)BH) require a catalyst. Moreover, the metal-catalyzed 

Problem 6.2. Draw a mechanism for the following diborylation reaction. 

Early studies on 4(5)-aminoimidazole (25 R=H) gave a stable urea derivative (37BJ488). Thus, treatment of a solution of 4(5)-aminoimidazole (25 R = H), made slightly acidic by addition of acetic acid, with potassium cyanate gave AMmidazol-4-yl-urea (31) (8%). TV-Imidazol-4-yl-urea (31) was similarly obtained using the dihydrochloride salt of 4(5)-aminoimidazole (25 R = H) (41MI1). 

An improved procedure for preparing urea derivatives involves reaction of isocyanates or isothiocyanates with 4(5)-aminoimidazole (25 R = H) in tetrahydrofuran solution [92JCS(P1)2779]. A THF solution of 4(5)-aminoimidazole (25 R=H) generated in situ and then treated with the appropriate reagent gave either lV-imidazole-4-yl-AT-phenylurea (56 X = 0) (32%) or AT-imidazoM-yl-A/ -phenylthiourea (56 X = S) (21%). 

As was the case for allene synthesis by copper-promoted Sn2 substitution reactions, the corresponding 1,6-addition to acceptor-substituted enynes has found sev- 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60], 

Draw the condensed structural formulas and give the names for the organic products of addition reactions of hydrogenation and hydration of alkenes. 

The most characteristic reaction of alkenes is the addition of atoms or groups of atoms to the carbon atoms in a double bond. Addition occurs because double bonds are easily broken, providing electrons to form new single bonds. 

The addition reactions have different names that depend on the type of reactant we add to the alkene, as Table 11.7 shows. 

Name of Addition Reaction Reactants Catalysts Product 

In a reaction called hydrt nation, H atoms add to each of the carbon atoms in a double bond of an alkene. During hydrogenation, the double bonds are converted to single bonds in alkanes. A catalyst such as finely divided platinum (Pt), nickel (Ni), or palladium (Pd) is used to speed up the reaction. The genaal equation for hydrogenation can be written as follows  

Addition mechanisms are broadly defined to be heterolytic, homolytic, or cyclic, which are processes that involve ionic or radical intermediates or are concerted, respectively. Concerted reactions will be discussed in Chapter 11. The emphasis here will be heterolytic (ionic) additions, although we will also consider some aspects of radical reactions. Addition reactions may be categorized further as being electrophilic or nucleophilic. In an electrophilic addition, a compoimd with a multiple bond reacts with an electrophilic reagent to produce an intermediate that subsequently reacts with a nucleophile. In nucleophilic addition, the unsaturated substrate reacts with a nucleophile to produce an intermediate that subsequently reacts with an electrophile to produce the final product.  

3 We generally think of electrophilic and nucleophilic additions as being heterolytic processes, but there can be electrophilic and nucleophilic character to radical additions also. Alkyl or aryl substituents on a carbon entered radical make the radical more nucleophilic than a methyl radical, while electron-withdrawing substituents make it more electrophilic. For a discussion, see Zipse, H. He, J. Houk, K. N. Giese, B. /. Am. Chem. Soc. 1991,113,4324. Perfluoroalkyl radicals are strongly electrophilic see, for example, Avila, D. V. Ingold, K. U. Lusztyk, J. Dolbier, W. R. Pan, H.-Q. /. Am. Chem. Soc. 1993,115,1577. 

Perspectives on Structure and Mechanism in Organic Chemistry, Second Edition By Felix A. Carroll Copyright 2010 John WUey Sons, Inc. 

Polymers with double bonds in the backbones or in the pendant groups can undergo numerous addition reactions. Some are discussed in this section. 

Hydro chlorination of natural rubber is often accompanied by cyclizatimi [35, 36]  

Polysilanes with alkene substituents add HCl and HBr in the presence of Lewis acids [58]. The products are the corresponding chlorine and bromine containing polymers with little degradation of the polysilane backbone  

Chlorinations of rubber, however, are fairly complex, because several reactions occur simultaneously. These appear to be (1) additions to the double bond (2) substitutions (3) cyclizations and (4) cross-linkings. As a result, the additions of halogens to the double bonds are only a minor portion of the overall reaction scheme [37, 38]. In CCI4, the following steps are known to occur  

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene [42]. The free-radical mechanisms, on the other hand, cause cross-linking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene, however, with addition of as much as 30% by weight of chlorine without cyclization [39, 40]. Also, chlorination of polyalkenamers both cis and trans yields soluble polymers. X-rays show that the products are partly crystalline [43, 44]. The crystalline segments obtained from 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

Irradiation of the enantiomorphous co-crystals obtained from mixed solutions of 9-methylbenz[c]acridine and diphenylacetic acid is reported to yield the photodecarboxylation-addition product (44) as essentially a racemic mixture, along with diphenylmethane and 1,2-tetraphenylethane. The lack of selectivity in the formation of (44) from the co-crystals is considered to arise from the molecular arrangement of the reactants in the lattice, and the same products are also formed from irradiation of the acridine and the acid in acetonitrile solution. 

Although the earliest examples of the use of US as a substitute for phase transfer catalysts in organic addition reactions were reported more than two decades ago and a number of such reactions have since been improved as a result [1-7], the sole analytical application exploiting this potential is a method for the determination of paracetamol where the drug is derivatized by hydrolysis to p-aminophenol, which reacts with o-cresol in an alkaline medium to form the Indophenol Blue dye, according to the following reaction  

The method was developed for determining the analyte in suppositories, so extraction from a toluene solution to an aqueous phase was required prior to hydrolysis and the 

One of the main advantages of the use of US for enhancing processes implemented in a continuous fashion over that of microwave energy is the small temperature rise involved in the former case, which avoids the presence of undesirable air bubbles in the dynamic system and hence of parasitic signals at the detector. 

A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction. The various types of addition reactions are  

What product is formed when X2(x = Br,Cl)is added to alkenes in the dark Explain the mechanism of the reaction. 

The mechanism of this reaction begins by recognizing the fact that X2 is polarized on contact with the double bond due to the negatively charged ir electron cloud of the bond. One end of the halogen molecule becomes positively 

The positively charged end is attracted to the negative electron cloud. This results in the cleavage of the X-X bond the negatively charged X leaves as shown  

Through a nucleophilic attack, the X adds to this species to give the dihalide  

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

Addition to alkenes can be sensitized by both electron-donors and electron-acceptors, and it is most likely that the reactive species is the alkene radical anion or the alkene radical cation, respectively. 1,1-Diphenylethylene can be converted to the Markownikov addition product with methanol (2.48) using the electron-donating sensitizer t-methoxynaphthalene no added proton acid is needed. Using 

The anti-Markownlkov orientation of addition in the presence of electron-acceptor sensitizers applies also to intramolecular reaction, and 5,5-dipheny pent-4-en-1-ol gives a tetrahydrofuran (2.SI) when irradiated in solution with 9,10-dicyanoanthracene, whereas its thermal reaction under proton-acid catalysis leads to 2,2-diphenyltetrahydropyran by Markownikov addition. Sometimes an added sensitizer is not required, if the alkene itself can act as a good electron-donor or electron-acceptor, and this is likely to be the reason why 1-lo-methoxyphenyl)propene adds photochemically to acetic acid (2.52), whereas l-phenylpropene does not. 

A further mechanism for photoaddition that applies to cyclohexenes or cydoheptenes begins with formation of the highly reactive trans isomer of the cycloalkene. In this way 1-methylcydoheptene gives an ether on irradiation in methanol (2.53), and I-methylcydohexene an acetate with acetic acid (2.54). In both cases a 

The restriction of this mechanism to cydoalkenes with a six- or seven-membered ring enables selective reaction to be achieved with a non-conjugated diene such as limonene (2.56), for which thermal reaction would occur at both double bonds. 

The initial report of intramolecular benzene-tertiary amine addition by Bryce-Smith et al. might have been expected to trigger a flurry of research activity similar to that generated by intramolecular benzene-olefin photoaddition reactions. However. a detailed account of the formation of adducts 3 and 4, including isolated yields and complete structural characterization, never appeared. Neither have additional examples of arene-tertiary amine addition reactions been reported. 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor.  

Exciplex fluorescence is observed for the (Af,AI-dimethylami-noethyl)styrcne 37 and several of its p-substituted derivatives however, Aoyama et al. have questioned the involvement of exciplexes as intermediates in the photoaddition process. p-Cy-ano and p-methoxy substituents shift the exciplex fluorescence to a longer and shorter wavelength, respectively, as observed for the analogous intermolecular exciplexes. Increasing sol- 

Our investigation of the mechanism of intramolecular adduct formation employed the technique of arene-amine exciplex quenching by primary amines, which had been developed in earlier investigations of exciplex quenching. These experiments provided evidence for the occurrence of adduct formation via proton transfer in the fluorescent exciplex. In the case of 39, activation parameters for exciplex formation and proton 

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

In Chapter 6, elimination reactions were presented. In the context of elimination reactions, the formation of double bonds was noted regardless of the elimination mechanism discussed. Continuing from the concept of using elimination reactions to form sites of unsaturation, one may reason that addition reactions can be used to remove sites of unsaturation. Thus, elaborating upon addition reactions, this chapter provides an introduction to relevant mechanisms applied to both carbon-carbon double bonds (olefins) and carbon-oxygen double bonds (carbonyls). 

Carbon and oxygen atoms become active by the breaking of the Jt bond in the ketone carbonyl group. Even though ketones are less reactive than aldehydes, they undergo all the same addition reactions as aldehydes. 

Longifolene could not be hydrogenated over a Ni or Pd catalyst in neutral medium 43). However, it can be reduced with Pd catalyst in acetic acid (43) or with Adam s PtOa catalyst in either ethyl acetate or acetic acid (44). The product of hydrogenation consists of an approximately 1 1 mixture of two longifolanes (54) and (55) (44). In contrast, camphene is readily hydrogenated to isocamphane (56) the other epimer arising from endo addition is formed in trace quantities only (43). 

Longifolene on hydroboration followed by oxidation with alkaline hydrogen peroxide furnishes, in 74% yield, essentially pure longifolol (57) as a result of endo attack (46). On the other hand, camphene, under the same treatment, is known (47) to give almost exclusively endo-iso-camphanol (58) by way of exo approach of the reagent. Oxidation of longifolyl-borane with air or silver oxide results in transannular products which are discussed in a later section. 

Insertion and addition are definitely two of the most fundamental reaction modes for both carbenes and silylenes. While the most convenient method for trapping carbenes involves their addition to double bonds to give stable cyclopropane derivatives, the corresponding method for trapping silylenes is generally complicated by the fact that most of the silacyclopropanes are unstable species and therefore undergo various secondary processes. In practice, with the exception of alkynes and coqjugated dienes, addition reactions are rarely employed to intercept silylenes. 

In Table 7, the studies on silylene addition to c-bond systems are summarized. They are classified primarily according to the types of silylenes. The addition reactions of monomeric SiF2 formed by the high-temperature Si-SiF4 reactions are omitted in this table because they have already been summarized in Table 3. In the following sections, the addition reactions are discussed according to the type of molecules the silylenes are adding to, namely, olefins, dienes, alkynes, and aromatic compounds. 

There is no report on a definite product pattern of SiH2 addition to olefins. However, Gaspar and coworkers have studied recoil Si reactions in a 

3- Methyl derivative of 7 Difluoro derivative of 6 Difluoro derivative of 7 

PH3-C2H4 mixture and have observed a product which was tentatively identified as SiH3CH2CH2PH2. A trace amount of SiH3CH2CH3 was also observed. They proposed a mechanism as shown in equations (95) and (96) which involved the addition of SiH2 to the double bond followed by the interaction of the intermediate silacyclopropane with a PH3 molecule. 

The reaction sequence for a typical vinyl polymer has four steps. In the first step, a free radical must be produced from the initiator such as those shown in Figs. 2.18 and 2.19. These radical formation reactions are typically first order in rate and are promoted by the elevated temperature of the reaction. For some free radical initiators, light can also promote the reaction. Then a sequence of events in the reaction mixture occurs, including initiation of a chain, followed by propagation, and finally termination of the chain. Termination of the chain will be discussed later. The schematic steps to produce an addition polymer from bulk or solvent polymerization are detailed in Fig. 2.19. The radical produced from the initiator reacts with the monomer in Step 2 to produce a new free radical by opening the double bond of a 

Termination (Step 4) can normally occur by either combination or disproportionation reactions, as shown in Fig. 2.20. For a combination termination, two chains with active centers combine to form a new chain without an active center. The molecular weight of the chain produced is the sum of both radicals. If these are two 

Acetal Loss of molecular weight - acid hydrolysis and oxidation 

PA Loss of molecular weight by hydrolysis with water contamination 

We saw in the last chapter that we are in the middle of a four-phase program to get you to the point where you can study without explicit instructions. Let s review where we are in this process. Here are the four phases  

With substitution reactions, you were given all of the information so that you could see how to go through the process of analyzing each and every factor based on the mechanisms. 

With elimination reactions, you were told how to find the information you need to understand elimination reactions (mechanisms, factors, etc.), and you recorded the information as you went along (in the form of charts and other drawings). 

In this chapter, you will be asked to draw the mechanisms and record the important factors by yourself, without too much help. 

Finally, when you have finished this chapter, you will record all of the information at the end of Chapter 8. which records every mechanism that you encounter. At that point, you will have the tools you will need to study every reaction that you see in the rest of the course. You will know how to look at the mechanisms. You will know how to look for the factors that determine regiochemistry and stereochemistry. You will keep a record (in Chapter 8) of every reaction you learn, and you will know what you should focus on when you study. 

While the usual eonsequence of hydration of enamines is eleavage to a secondary amine and an aldehyde or ketone, numerous cases of stable carbinolamines are known (102), particularly in examples derived from cyclic enamines. The selective terminal hydration (505) of a cross-conjugated dienamine-vinylogous amide is an interesting example which gives an indication of the increased stabilization of the vinylogous amide as compared to simple enamines, which is also seen in the decreased nucleophilicity of the conjugated amino olefin-carbonyl system. 

Extension of the hydration reaction to hydrogen peroxide has shown that stable peroxides are formed from enamines and the imonium salts derived from secondary amines and ketones (506,507). 

The application of this addition to aminomethylene ketones provides a convenient synthesis of monoamides of pimelic acid (508). It should be noted that the corresponding oxidation of hydroxy methylene cyclohexanone leads to ring contraction and formation of cyclopentanoic acid. 

While carboxylate anions do not add to the imonium function of ketone derived enamines, such as morpholinocyclohexene, when these are combined with carboxylic acids (38), the addition of thiophenol or benzyl mercaptan leads to a-aminothioethers (509,510). 

Addition of hydrogen sulfide results in formation of monomeric gem dithiols or trimeric thioketals (511,512). The initially reported thione formation (513-515), analogous to hydration of morpholinocyclohexene 

An alternative cyclopropane synthesis via an active methylene compound can also be enhanced by sonication [110]. The number of examples quoted in the literature is low but in the case of ethyl cyanoacetate and dibromoethane sonicated with potassium carbonate and polyethylene glycol in ethylene dichloride the expected cyclopropane is generated in 85 % yield (Eq. 3.19). 

Aziridine synthesis can be achieved via nitrene addition to alkenes making use of a sulfonoxycarbamate [112]. Treatment of this reagent vith potassium carbonate in the presence of an alkene together with a PTC leads to the aziridine (Eq. 3.20). The reaction time is reduced from 2-3 h to 15 min when sonication is applied. 

Addition of cyanide ion to a carbonyl compound leads to a cyanohydrin, a process with many applications including the synthesis of amino adds via an aminonitrile. The dired reaction between an aldehyde, KCN and NH4CI in acetonitrile leads to a mixture 

Method PhCHO PhCH(OH)CN PhCH(NH2)CN PhCH(OH)COPh 

The kinetics of spinel growth generally follow a parabolic-rate law when the 

Inert markers have been used to obtain additional information regarding the mechanism of spinel formation. A thin platinum wire is placed at the boundary between the two reactants before the reaction starts. The location of the marker after the reaction has proceeded to a considerable extent is supposed to throw light on the mechanism of diffusion. While the interpretation of marker experiments is straightforward in metallic systems, giving the desired information, in ionic systems the interpretation is more complicated because the diffusion is restricted mainly to the cation sublattice and it is not clear to which sublattice the markers are attached. The use of natural markers such as pores in the reactants has supported the counterdiffusion of cations in oxide spinel formation reactions. A treatment of the kinetics of solid-solid reactions becomes more complicated when the product is partly soluble in the reactants and also when there is more than one product. 

The foregoing treatment assumes that at least one of the reactants is a single crystal and the reactant/product geometry is well-defined. Many technologically important ceramic reactions, on the other hand, are usually carried out between polycrystalline powders. The reaction kinetics in these cases depend on several physical factors such as particle size, packing density, porosity, and so on. Jander (1927) and Carter (1961) have proposed models for powder reactions making several simplifying assumptions. 

Efforts have been made to eliminate diffusion-control of solid-solid reactions by using superlattices of nanometric dimensions as reactants. Formation of Cu MOgSeg from the superlattices of Cu, Mo and Se is one such example (Fister et al., 1994). The results reveal that superlattice reactant geometry could be used to kinetically trap the ternary phases which are thermodynamically unstable with respect to the binary phases. 

It is twenty years since the first reports appeared describing 

2-methyl-1,3-dioxole are similarly interpreted in terms of polarity 

Wender and co-workers continue to make elegant synthetic use of the intramolecular meta photocycloaddition of phenyl-ethenyl non-conjugated bichromophoric systems and this year describe three such applications of the reaction. The synthesis of ( )-silphinene (40), the first member of a new family of tri-quinane natural products, has been achieved in three steps and 

Intramolecular ortho-cycloaddition of benzene-ethylene systems 

In contrast tetrafluoropyridyl-prop-2-enyl is stable under the same conditions and the thio ether (64) undergoes C-S bond cleavage to give pentafluorothiophenol and cyclohexylpentafluorophenyl sulphide. The photorearrangement of (65) to (66) has previously been 

This section has described several synthetically important 1,2- and 1,3-elimination processes. In both. ases, substitution can be a competitive reaction, but proper choice of reaction conditions will maximize ormation of the elimination products. Several interesting disconnections that are possible with substrates. apable of undergoing 1,2- and 1,3-eliminations are shown. 

Acids and bases have figured prominently in all of the sections of this chapter. The concept of an alkene Linctioning as a base has also been explored. In this section, the ir bond of an alkene or an alkyne will be used a base, both with protonic acids, Lewis acids, and with other electrophilic centers. When an alkene donates. n electron pair (acts as a base) to a proton, the ir bond breaks and forms a new C—H bond to one carbon of nc old C=C unit. The other carbon of that unit becomes an electron deficient center (a carbocation), which. acts further, usually by substitution or elimination. When the alkene donates the electrons to a Lewis acid,, c resulting complex will react to give other products. In all cases, recognizing the Lewis basicity of alkenes and alkynes will help explain the addition reactions discussed in this section. 

Chapter 2. Acids, Bases, Functional Group Exchanges 

Those involving free and metal stabilized ions as intermediates. 

Those involving symmetrically bridged ions as intermediates. 

The substitution reaction of nitric acid with glycerol produces the explosive nitroglycerine. Alfred Nobel s (1833-1896) discovery in 1866 that this very sensitive material could be made into a safe explosive by absorbing it into diatomaceous earth or wood meal led to his development of d)mamite. 

concentrated H2SO4 reacts with alcohols to form alkyl hydrogen sulfates. The reaction with lauryl alcohol is an important industrial reaction. 

The neutralization reaction of an alkyl hydrogen sulfate with NaOH then produces the sodium salt of the alkyl hydrogen sulfate. 

Sodium salts of the alkyl hydrogen sulfates that contain about 12 carbon atoms are excellent detergents. They are also biodegradable. (Soaps and detergents were discussed in Section 14-18.) 

An addition reaction involves an increase in the number of atoms or groups attached to carbon. The molecule becomes more nearly samrated. 

Here also, all elements of the reactants (propene and hydrogen) are incorporated in the final product (propane). The reaction is a 100% atom economical reaction. 

Similarly, cycloaddition reactions and bromination of olefins are 100% atom economical reactions. 

In the sensitized photooxidation of the didehydro ester (72), the two peroxides (107) and (108) were isolated after column chromatography. The endocyclic peroxide (107) underwent base-catalyzed conversion to the ketoalcohol (109) (Olive and Mousseron-Canet, 1969 Mousseron-Canet et ai, 1970). 

In a sensitized photooxidation, retinaldehyde (2) gave, via the cyclic peroxide (110), the compound (111), which fragmented very rapidly to give the lactone (112) on contact with atmospheric oxygen (Mousseron-Canet et aL, 1966 Lemere/fl/., 1970). 

Irradiation of retinyl palmitate (113) with a 150-W high pressure mercury lamp in the presence of thiobenzophenone (114) gave the 1 1 adduct (115) (Yoshioka er a/., 1968). 

When exposed to UV light, retinyl acetate (9) in hexane underwent dimerization to give the adduct (116) (Mousseron-Canet et aL, 1968 Giannotti, 1968). 

This (4 + 2) adduct (116) has a structure similar to that of one of the components (117) of kitol, a naturally occurring mixture of dimers of retinol (1) (Tsukida and Ito, 1980). 

A reaction takes place when the positive end of the HCl molecule approache.s the double bond in ethylene. The pi bond breaks, and the electrons it contained move as indicated by the curved arrows shown in the following equation, forming a sigma bond between the H atom of the HCl molecule and one of the C atoms. As this new bond forms, two things happen  

Because there cannot be more than one bond to the H atom, the original bond between H and Cl breaks. Both of the electrons originally shared by H and Cl go with the Cl atom. The resulting intermediate species are shown in square brackets in the following equation (dashed lines represent bonds that are being formed)  

The C atom on the right bears a positive charge after the valence electron it raiginally shared (in the pi bond with the other C atom) is removed from it completely. A species such as this. 

The C atom forming the new sigma bond to the H atom changes from sp -hybridized to sp -hybridized  

The carbon atom bearing the positive charge is still sp -hybridized at this point. 

The C atom on the right bears a positive charge after the valence electron it originally shared (in the pi bond with the other C atom) is removed from it completely. A species such as this, in which one of the carbons is surrounded by only six electrons, is called a carbocation. Although carbon must obey the octet in any stable compound, some reactions involve transient, intermediate species in which a caibon atom may be electron deficient—having only three electron pairs around it. 

Student Annotation When an electrophile approaches another species and accepts electrons from it to form a bond, this is called electrophilic attack. 

Interestingly, silylated propargylic zinc reagents, such as 342 may be better viewed as the allenic zinc reagent 343 that reacts with an aldehyde via a cyclic transition state affording only the onti-homopropargylic alcohol 344 with 90% yield 

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Can the useful material lost in the purge streams be reduced by additional reaction If the purge stream contains significant quantities of reactants, then placing a reactor and additional separation on the purge can sometimes be justified. This technique is used in some designs of ethylene oxide processes. 

Employ additional reaction and separation of waste streams to allow increased recovery. 

Additional reaction and separation of waste streams. Sometimes it is possible to cany out further reaction as well as separation on waste streams. Some examples have already been discussed in Chap. 4. 

Benzene can undergo addition reactions which successively saturate the three formal double bonds, e.g. up to 6 chlorine atoms can be added under radical reaction conditions whilst catalytic hydrogenation gives cyclohexane. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

However, the term saturated is often applied to compounds containing double or triple bonds which do not easily undergo addition reactions. Thus ethanoic acid is termed a saturated carboxylic acid and acetonitrile a saturated nitrile, whereas a Schiff base is considered to be unsaturated. 

This method follows the ASTM D 1159 and D 2710 procedures and the AFNOR M 07-017 standard. It exploits the capacity of the double olefinic bond to attach two bromine atoms by the addition reaction. Expressed as grams of fixed bromine per hundred grams of sample, the bromine number, BrN, enables the calculation of olefinic hydrocarbons to be made if the average molecular weight of a sufficiently narrow cut is known. 

One may justify the differential equation (A3.4.371 and equation (A3.4.401 again by a probability argument. The number of reacting particles VAc oc dc is proportional to the frequency of encounters between two particles and to the time interval dt. Since not every encounter leads to reaction, an additional reaction probability has to be introduced. The frequency of encounters is obtained by the following simple argument. Assuming a statistical distribution of particles, the probability for a given particle to occupy a... 

The two reaction schemes of Figures 3-13 and 3-15 encompass a large proportion of all organic reactions. However, these reactions do not involve a change in the number of bonds at the atoms participating in them. Therefore, when oxidation and reduction reactions that also change the valency of an atom ate to be considered, an additional reaction scheme must be introduced in which free electron pairs are involved. Figure 3-16 shows such a scheme and some specific reaction types. 

The search in the Theilhcimer reaetion database [62] provides 161 reactions for this query. If the search is performed without any additional bond spheres (covering only atoms of the inner sphere with a dark gray bac kgroiind in Figure 10.3-42 as well as the added atom groups on the precursor side), 705 reactions arc obtained in the Theilhcimer database. The result of this search is less precise than that of the first search. Additionally, reactions forming any kind of C-0 bonds (c.g., making an ether bond instead of an ester bond) arc found. However, in both searches too many hits arc obtained in order to detect suitable reactions in a reasonable... 

The Michael Addition Reaction consists in the addition of the sodio-derivative of ethyl acetoacetate, ethyl malonate or ethyl cyanoacetate to an olefine group... 

The basic premise for making bromosafrole has been to mix sa-frole with Hydrobromic Acid (a.k.a. hydrogen bromide, HBr). That s it. The HBr does what is called a Markovnikov addition reaction whereby the HBr sees the allyl double bond of safrole and preferentially attaches its hydrogen to the gamma carbon and its bromine to the middle beta carbon (don t ask). 

Within the last decade remarkable progress has been made with highly stereoselective addition reactions to C = C and C = 0 double bonds using chiral reagents. These reagents include ... 

The target molecule above contains a chiral center. An enantioselective synthesis can therefore be developed We use this opportunity to summarize our knowledge of enantioselective reactions. They are either alkylations of carbanions or addition reactions to C = C or C = 0 double bonds ... 

Finally a general approach to synthesize A -pyrrolines must be mentioned. This is tl acid-catalyzed (NH4CI or catalytic amounts of HBr) and thermally (150°C) induced tea rangement of cyclopropyl imines. These educts may be obtained from commercial cyan> acetate, cyclopropyl cyanide, or benzyl cyanide derivatives by the routes outlined below. Tl rearrangement is reminiscent of the rearrangement of 1-silyloxy-l-vinylcyclopropancs (p. 7 83) but since it is acid-catalyzed it occurs at much lower temperatures. A -Pyrrolines constitut reactive enamines and may be used in further addition reactions such as the Robinson anei lation with methyl vinyl ketone (R.V. Stevens, 1967, 1968, 1971). 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

Phenyl-1,4-hcxadicnc (122) is obtained as a major product by the codimerization of butadiene and styrene in the presence of a Lewis acid[110]. Pd(0)-catalyzed addition reaction of butadiene and aiiene (1 2) proceeds at 120 C to give a 3 1 mixture of trans- and c -2-methyl-3-methylene-l,5.7-octatriene (123)[lll]. 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Alky]-5-imino-3-methy -A2-l,2,4-thiadiazoIines react exotherm ally at 0°C with dibenzoy] or dimethoxy carbonylacetylenes in tetrahydrofuran to give the 2-alkylaminothiazoles in high yields (1564). The cycio addition reaction of 2-pyridyl isothiocyanates with 1-azirines results in the formation of 2-pyridylaminothiazoles (1565). 

Tautomerism of the A-2-thiazoline-5-thiones has not been investigated intensively. A recent report shows that 2-phenylthiazo e-5-thiols exist in the thiol form in both polar and nonpolar solvents (563). This behavior is in contrast with that of corresponding thiazolones. Addition reactions involve only the exocyclic sulfur atom, and thiazole-5-thiols behave as typical heteroaromatic thiols towards unsaturated systems, giving sulfides (1533) (Scheme 80) (563),... 

Like pyridines (334), thiazoles undergo addition reactions with dimethyl acetylenedicarboxylate leading to 2 1 molar adducts, the structure of which has been a matter of controversy (335-339). 

Other ring-expansion reactions have already been mentioned in regard to addition reactions leading to pyrrolothiazoles (Section III. 3), which sometimes rearrange to 1,4-thiazines (333, 497). 

Alkenes are commonly described as unsaturated hydrocarbons because they have the capacity to react with substances which add to them Alkanes on the other hand are said to be saturated hydrocarbons and are incapable of undergoing addition reactions... 

The relationship between reactants and products m addition reactions can be illustrated by the hydrogenation of alkenes to yield alkanes Hydrogenation is the addition of H2 to a multiple bond An example is the reaction of hydrogen with ethylene to form ethane... 

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