Alkenes example: alkene

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

Because electrophilic attack on benzene is simply another reaction available to a cai bocation, other cai bocation precursors can be used in place of alkyl halides. For example, alkenes, which are converted to caibocations by protonation, can be used to alkylate benzene. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

This section lists examples of the reaction of RH RR (R,R = alkyl or aryl). For the reaction C=CH -> C=C-R (R = alkyl or aryl), see Section 209 (Alkenes from Alkenes). For alkylations of ketones and esters, see Section 177 (Ketones from Ketones) and Section 113 (Esters from Esters). 

Four kinds of adsorbed species are postulated to result from the adsorption of alkenes on noble metals, Jt-com plcxcd double bonds, Jt-complexed jt-allyl species, mono-O-bonded alkanes, and di-o-bonded alkanes. Distinguishing among these is difficult and has been the subject of much speculation. For example, alkenes may adsorb either by Jt-complexing or by di-C-adsorption, and 7t-allyl species are thought to occur readily on Pd but not so readily on Pt.55... 

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

The formation of alkenes and alkene-related polymerization products can seriously reduce the yields of desired alkane products from secondary alcohols, which can undergo elimination reactions. For example, reduction of 2-octanol at 0° with boron trifluoride gas in dichloromethane containing 1.2 equivalents of tri-ethylsilane gives only a 58% yield of n-octane after 75 minutes (Eq. II).129 The remainder of the hydrocarbon mass comprises nonvolatile polymeric material.126... 

Tetrasubstituted Alkenes. Tetrasubstituted alkenes lacking electron-withdrawing substituents undergo facile ionic hydrogenation to alkanes in very good yields. Simple examples include 2,3-dimethyl-2-butene,208,214 1,2-dimethyl-cyclopentene, 1,2-dimethylcyclohexene,229 and A9(10)-octalin.126,204,212... 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

Quaternary ammonium dichloroiodates [10] are considerably more convenient to use than iodine monochloride for electrophilic iodination reactions and their use in stoichiometric amounts is well documented (see e.g. [II] and references cited therein). Thus, for example, alkenes undergo addition reactions to yield the a-chloro-p-iodoalkanes [11] and enaminones are converted into the corresponding a-iodoenaminones (75-90%) (Scheme 2.4) [12]. 

The product of an addition reaction depends on the symmetry of the reactants. A symmetrical alkene has identical groups on either side of the double bond. Ethene, CH2 = CH2, is an example of a symmetrical alkene. An alkene that has different groups on either side of the double bond is called an asymmetrical alkene. Propene, CH3CH=CH2, is an example of an asymmetrical alkene. 

The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

Hydroformylation reactions have been shown to be amenable to use in tandem or domino reaction sequences. In one elegant example, alkene 36 was subjected to rho-dium(I)-catalyzed hydroformylation, and the resulting aldehyde underwent smooth intramolecular allylboration (Scheme 5.14) [19]. This produced a new terminal alkene which underwent a second hydroformylation to provide, after workup,lactols 37 in 80% yield and with excellent diastereoselectivity. 

In contrast to the intermolecular cyclopropanation, the dirhodium tetraprolinates give modest enantioselectivities for the corresponding intramolecular reactions with the do-nor/acceptor carbenoids [68]. For example, the Rh2(S-DOSP)4-catalyzed reaction with al-lyl vinyldiazoacetate 32 gives the fused cyclopropane 33 in 72% yield with 72% enantiomeric excess (Eq. 4) [68]. The level of asymmetric induction is dependent upon the substitution pattern of the alkene cis-alkenes and internally substituted alkenes afford the highest asymmetric induction. Other rhodium and copper catalysts have been evaluated for reactions with vinyldiazoacetates, but very few have found broad utility [42]. 

Substitution of methyl for hydrogen at the a carbon of an oxime causes an upheld shift of the nitrogen resonance. Although these shifts are similar in direction, they are about twice the magnitude of those observed for the corresponding carbons of alkenes. Examples are... 

The sulfur-nitrogen halide (NSC1)3 is also a useful reagent in organic chemistry. For example, alkenes or alkynes react readily with (NSC1)3 to produce 1,2,5-thiadiazoles and the reaction with A-alkylpyrroles gives bis-1,2,5-thiadiazoles. 

The reaction of an alkene (in excess) with a polyhydride can strip hydrogen from the metal. The open sites formed in this process can now react, whether by cyclometalation or by coordinating the alkenes. If the alkene has accessible C—H bonds (e.g. propene) these may be broken by the metal. Indeed dehydrogenation may proceed until a stable 16-electron compound has been formed. In most cases this hydrogen stripped from the ligand is passed to the excess of alkene. Examples of these types of reaction have been described by Wilkinson (Scheme 5).131... 

A wide variety of alcohols and alkenes can be utilized in this process. Numerous functional groups in either the alcohol or the alkene can be accommodated during alkoxymercuration. For example, alkenes differing in their nucleophilicity as much as vinylic ethers419 and a,(3-unsaturated aldehydes or ketones420,421 react smoothly (equations 259 and 260). 

To name alkenes, we follow the basic IUPAC rules, but add the suffix -ene to indicate the main functional group is an alkene. The location of the double bond is given by a locant number, which identifies the first carbon in the double bond. The main functional group always gets the lowest number. Figure 11.9 illustrates some example alkene structures and names. 

Finally it should be mentioned that many other compounds containing C-C- and C-O double bonds afford cyclobutanes on irradiation in the presence of alkenes. Examples of monocyclic compounds include endiones,... 

Hofmann elimination usually gives the least substituted alkene. Example i 2 3 4 150 °C... 

Other nucleophiles may also intercept the cation for example, alkenes can be treated with HCl to form alkyl chlorides, with HI to form alkyl iodides, and with H2S to form thiols. 

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