3,4-Unsaturated systems

These systems have a high degree of stability and undergo some unique and unusual reactions. 

Everyone receives an introduction to the basic concepts of resonance in Organic Chemistry I. Organic Chemistry II requires an extension of the basic rules of resonance to other systems. In addition to constructing reasonable resonance structures, you also need to understand which structures cire more stable. 

Resonance, as you saw in Organic I, occurs in many systems, and you need to be able to recognize when it s going to affect the outcome of a reaction. In general, resonance makes a species more stable by delocalizing the electrons. Delocalization, among other things, reduces electron-electron repulsion. 

Following is an expanded set of rules for drawing resonance structures. 

This list is an expansion of the rules necessary to understand resonance for Organic Chemistry I. You may want to bookmark this list because these rules apply throughout the remainder of this book. 

Y-Unsaturated Systems - Irradiation (A, 280 nm) in methanol solution for 24 h of the amide (128) brings about Z,E isomerization as well as this process, both a 1,3- and a 1,5-acetyl migration take place. The result of the 1,3-migration is the formation of the azetine (129). The 1,5-migration from the Z-isomer results in the formation of the isoquinoline (130). The formation of this 

P-Unsaturated Systems. - 2.1.1 Isomerization. A patent has described the synthesis of photoisomerizable compounds based on the structure (47). The 

2 Hydrogen Abstraction Reactions. The deconjugation reaction of esters such as (56) leads to the corresponding p,Y-unsaturated derivatives e.g. (57)). This study used an achiral base to effect the protonation of the enol intermediate formed by photochemical hydrogen transfer. High selectivities of up to 95% 

3 Addition and Cyclization Reactions. Several chiral iV-alkylpyridinium photosensitizers (68) have been synthesized. These were used to study the conversion of (69) into the lactones (70) and (71). Low ee values were observed for this 

Quantum chemical methods have been used to study the photochemical reactivity of a series of psoralens. A review has highlighted the photochemical reactions of coumarins in the solid state. ° Spectral properties of coumarin 153 have been studied.  

Radical ion pairs are involved in the photochemical isomerism of P-ionone in the presence of pyrene. trans- -lonone is converted to its radical anion on irradiation in the presence of triphenylphosphine or triethylamine. CIDNP analysis has shown that the radical anions convert to neutral radicals by pro-tonation. Triphenylamine can be used as an electron donating sensitizer for the phototransformations of the enones (75)-(77). Triplet radical ion pairs are formed, which afford the triplet of the enone by back electron transfer, cis-Isomers of the enones are formed. The retinoid acitretin undergoes photo- 

By analogy with borazines, the initial interest in the analogous unsaturated boron-phosphorus rings focused on the fundamental issue of the extent of 

71-electron delocalisation. In practice, boron-phosphorus analogues of borazine are relatively scarce. However, stable six-membered rings of the type (RPBR )3 are obtained by salt-elimination reactions between an organoboron dihalide and a lithium organophosphide, if the substituents around boron and phosphorus provide sufficient steric protection [eqn (9.10)]. For certain combinations of more bulky substituents, unsaturated four-membered (RPBR )2 rings are formed in this cyclocondensation process. 

1 The Oxa-Di-n-methane reaction and Related Processes. Interestingly, some P,y-unsaturated ketones do not undergo 1,3-acyl shifts or the oxa-di-Ti-methane rearrangement. An example of this is compound (61), which on irradiation undergoes only a Norrish Type II hydrogen abstraction.  

The direct irradiation of the bicycloenones (62) results in a 1,3-acyl shift followed by decarbonylation. The photochemical and thermal reactivity of so-called o- and -acylcyclohexadienones has been studied in a further attempt to examine the mechanism of the photo-Fries process. 

Acetophenone-sensitized irradiation of the quinuclidinones (63) brings about an oxa-di-TT-methane rearrangement to afford (64) in 70% yield. Singh and Lahiri have reported the odd photochemistry of the enone (65, R = H, = Et). On irradiation of this compound at 300 nm in acetone, only a low yield of 

There are some important differences between the behavior and flow of water in the unsaturated zone (vadose zone) above the water table and the saturated zone below the water table. The surface tension of water or other fluids becomes important when there is a gas phase in contact with the fluid phase and the solid phase. If the total volume of a porous medium (VT) is divided into the volume of the solid portion (Vs,), 

Water movement in the unsaturated zone is dependent on the factors discussed previously — driving force, hydraulic conductivity, viscosity, and density — but is 

FIGURE 3.24 Groundwater conditions near the ground surface. Saturated and saturated zones (a), profile of moisture content vs. depth (b), pressure head and hydraulic head relationships insets = water retention under pressure heads less than (top) and greater than (bottom) atmospheric (c), profile of pressure head vs. depth (d), and profile of hydraulic head vs. depth (e). (After Freeze and Cherry, 1979.) 

FIGURE 3.25 Graphs of hydraulic conductivity vs. volumetric water content showing characteristic curves for different sediments from the VOC-arid site integrated demonstration at the Hanford site. 

The addition of a small quantity of electrolyte (such as minerals dissolved from adjacent soil particles) increases surface tension. A small quantity of soluble organic compound (alcohol, soap, or acid) decreases the surface tension. The addition of glycerine to water reduces surface tension and thus makes it possible to stretch water him into bubbles, as with a child s bubble-blowing game. 

7-Uiisaturated Systems - As was mentioned earlier in this Chapter, Norrish Type I processes can occur with P,y-unsaturated enones. This process is also observed in the formation of a ketene on irradiation at X, 230 nm of cyclopenten-3-one in an argon matrix. The presence of the ketene intermediate was detected by IR spectroscopy. Irradiation at 300 nm of the enones 

The enones (138) and (139) undergo efficient conversion into (140) and (141), respectively, on direct irradiation through Pyrex in a benzene solution. The reaction is a good example of a 1,3-acyl migration in a P,y-unsaturated enone and is a route to the protoilludanoid skeleton. Irradiation using quartz filtered light affords a complex mixture of products. 

The enones (118) undergo both Norrish Type I fission and an oxa-di-K-methane process on direct irradiation in benzene solution.The decarbonyl-ated product (119) predominates and the oxa-di-7i-methane product (120) is minor under these conditions. However, the oxa-di-K-methane product (120) becomes the dominant reaction mode when irradiation is carried out under acetone sensitisation. The decarbonylation product (119) arises by Norrish Type I fission, decarbonylation and intramolecular SET to afford the zwit-terion (121) which is trapped by water. Elimination of methanol then affords the final product. 

It is clear that some information is lost as a result of our approximation, specifically regarding the individual bonds formed by sp carbon atoms, but the total of the F values in a molecule is expected to be generally reasonably accurate. The salient feature of ethylenic double bonds, namely, the inward displacement of tt orbital centroids on the C=C axis revealed by direct calculations, and its important role in energy calculations, can now be put in a clear perspective and efficiently tested for large collections of molecules. 

Unsaturated hydrocarbons, such as alkenes, polyenic material, as well as aromatic molecules, are our target. Carbonyl compounds are considered in Chapter 16. 

The aj,i parameters are deduced from Eq. (10.41), but attention must be given to the fact that tr and tt populations vary in inverse directions, Eq. (6.3), a circumstance that affects aj,i and thus 

The CC and CH bonds of ethane (Example 10.1), and the final selection See = 69.633 and 8ch = 106.806 kcal/mol, are used to get the CC and CH bonds found in unsaturated hydrocarbons by retaining both the contribution of Fkh Eq. (11.12), and the effect of charge variations described by Eq. (10.37). The reference CC double bond of ethylene and the reference CC bonds of benzene, however, roughly estimated along the lines described in Example 10.1, are deduced from the appropriate CH bond energies and the energy of atomization of the corresponding molecule, AE, obtained from experimental data. 

Examples 10.1 and 11.1-11.4 are used, along with a number of similar examples presented below, to generate all the required C sp )—C sp ), C sp )—C sp ), and C sp )—H bonds, including those formed by aryl carbons. Conjugation must therefore also be considered. 

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

The hydrogen abstraction, in this instance, presumably occurs at the P-carbon of the enone (139). The resultant biradical ring closes to afford the observed products. The ic-ic excited triplet state is involved in the conversion of the enone (141) into the spiroketone (142). The reaction again involves hydrogen abstraction by the ft-carbon followed by cyclization within the biradical. Photocyclization of enamides such as (143) has led to a new synthetic approach to the yohimbine an l reserpine type alkaloids. The photocyclization of the enone (144) provides a route for the synthesis of 3.10-dimethoxyprotoberberines.  

Previous work by Jeger and his coworkers has studied the photoreactivity of y.5-epoxy enones. More recent work by Ishii et aJ has examined the photochemical reactivity of the related nitriles (145) and (146). The direct irradiation of (145) brings about isomerization by ring opening of the cyclopropane to afford (147) as the major product. This is accompanied by the cyclopropenyl nitrile (148) presumably formed via a carbene intermediate. Triplet sensitized irradiation of (145) only brings about trans-cis isomerization. The epoxide (149) is also photoreactive and direct irradiation affords the products (150) - (156) either by the intermediacy of an ylide (157) or a carbene (156).  

A study of the sensitized photochemical behaviour of the aromatic retinoids (177) has shown that they undergo (4 + 2)-cyclodimerisations. The photo reactions of the pentaenals (178) has been reported. 

Wagner and Nahm have observed the photoaddition of a remote double bond to the benzene ring of acetophenone derivatives affording (179) and (180) from the irradiation of (181) and (182) respectively. In another publication they have reported further on the process and have demonstrated that the final product from the reaction is a secondary photoproduct. Thus the irradiation of (181) has been shown by n.m.r. spectroscopy to afford an initial photoproduct (183). This is thermally labile and.is converted into the triene (184) which undergoes a photochemical cyclization to yield the stable product (179).  

Mihailovic and coworkers report that the enone (141) undergoes 

The irradiation of phenylcyclohexenyl ketone (146) under argon in methanol results in its conversion to the cyclic ketone (147) by the well known Nazarov reaction. A careful study of this reaction by n.m.r. spectroscopy in deuteriated solvent shows that the enol (148), a long suspected intermediate, is the initial photochemical product. This is transformed readily by a thermal reaction into the final product (147). If non-protic solvents are used for the irradiation two new products (149) and (150) are formed. The structure of the stable compound (149) was determined by X—ray crystallography. The formation of the compound (149) is 

Considerable interest is being shown in the highly enantio- 

The photochemical rearrangement of (157a) into the bicyclic compound (158) in acetone of methanol is reported to be a di-n-methane process involving interaction with the benzyl group on the D-carbon atom. The compound (lS7b) does not follow this 

Scheffer and his coworkers have continued their investigations of the influence which phase exerts on the outcome of photochemical hydrogen abstraction reactions of enones. This work is exemplified by the difference in behaviour of the enone 

Two reports have dealt with the photochemical behaviour of 

In the absence of oxygen rebonding will afford the mixture of 

Type I processes. Analogous cis-trans-isomerization occurs with 

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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),... 

Vinyllithium [917-57-7] can be formed direcdy from vinyl chloride by means of a lithium [7439-93-2] dispersion containing 2 wt % sodium [7440-23-5] at 0—10°C. This compound is a reactive intermediate for the formation of vinyl alcohols from aldehydes, vinyl ketones from organic acids, vinyl sulfides from disulfides, and monosubstituted alkenes from organic halides. It can also be converted to vinylcopper [37616-22-1] or divinylcopper lithium [22903-99-7], which can then be used to introduce a vinyl group stereoselectively into a variety of a, P-unsaturated systems (26), or simply add a vinyl group to other a, P-unsaturated compounds to give y, 5-unsaturated compounds. Vinyllithium reagents can also be converted to secondary alcohols with trialkylb o r ane s. 

Substitutions. The cyanamide anion is strongly nucleophilic and reacts with most alkylating or acylating reagents (4) addition to a variety of unsaturated systems occurs readily (4). In some cases, a cyanamide salt is used in others, base catalysis suffices. Ethyl iodide reacts with sodium hydrogen cyanamide [17292-62-5] to form a trisubstituted isomelamine. 

Many of these reactions occur in the course of synthesis of fully or partly unsaturated products after initial ring closure, giving rise to more unsaturated systems, e.g. in the pyrido[2,3-pipemidic acids (Section 2.15.4.1) and their derivatives, e.g. (16a) -> (17) (74JAP(K)7444000). Examples are also found in the pyrido[3,2-[Pg.205]

Nucleophilic attack on ring atoms of large heterocycles is largely confined to saturated systems, saturated parts of partially unsaturated systems, and to carbonyl functions and the like. These reactions are not fundamentally different from those of corresponding acyclic systems, except for transannular reactions. 

Michael addition of aromatic or heterocyclic aldehydes (via cyanohydrins) to o. unsaturated systems. Also addilion of aliphatic aldehydes catalyzed by thiazoFium yKds... 

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

There appears to be no difficulty in introducing three or even four trigonal centers into the D ring, provided the unsaturated system includes the 14-posi-... 

The photochemical addition of trifiuoroiodomethane to unsaturated systems has been thoroughly investigated by Haszeldine. Little use has been made of this reaction in the steroid field. Irradiation of the enol ether (64) in trifiuoroiodomethane containing pyridine in a quartz vessel furnishes in 60 %... 

Preparation of the oxime presents no problem, since there is little tendency for hydroxylamine to add to the double bond of the unsaturated system. A variety of acid catalysts has been used e.g., POCI3, BF3, p-ACNHC6H4SO2CI) to effect the rearrangement, and phosphorous oxychloride is the usual choice. 

Anhydrous hydrogen fluoride adds across carbon-carbon double and triple bonds and to other unsaturated systems, but wide variations of solvent, catalyst, temperature, and conditions are required with different substrates. 

Addition of Halogen Fluorides to Unsaturated Systems by Andrew E. Feiring... 

The first example of a cycloaddition reaction of a multiple bond to a diene was reported in 1917 Surprisingly, it was found that benzal azine adds to 2 equivalents of several unsaturated systems, when offered in excess, to yield bicyclie compounds. This reaction was named criss-cross" cycloaddition [190], Exploitation of the preparative potential of criss-cross cycloaddition began only in the early 1970s, when hexafluoroacetone azine became available on a larger scale [191,192] The study of this reaction proved to be an impetus tor the development of azine chemistry [183, 193]... 

BiCl3, microwave irradiation, 2 min, THF, 70-96% yield. a,/3-Unsaturated systems were not effectively cleaved under these conditons. ... 

Heats of formation, molecular geometries, ionization potentials and dipole moments are calculated by the MNDO method for a large number of molecules. The MNDO results are compared with the corresponding MINDO/3 results on a statistical basis. For the properties investigated, the mean absolute errors in MNDO are uniformly smaller than those in MINDO/3 by a factor of about 2. Major improvements of MNDO over MINDO/3 are found for the heats of formation of unsaturated systems and molecules with NN bonds, for bond angles, for higher ionization potentials, and for dipole moments of compounds with heteroatoms. 

The simplest description of an excited state is the orbital picture where one electron has been moved from an occupied to an unoccupied orbital, i.e. an S-type determinant as illustrated in Figure 4.1. The lowest level of theory for a qualitative description of excited states is therefore a Cl including only the singly excited determinants, denoted CIS. CIS gives wave functions of roughly HF quality for excited states, since no orbital optimization is involved. For valence excited states, for example those arising from excitations between rr-orbitals in an unsaturated system, this may be a reasonable description. There are, however, normally also quite low-lying states which essentially correspond to a double excitation, and those require at least inclusion of the doubles as well, i.e. CISD. 

The technique of catalytic hydrogenation can be applied almost universally to unsaturated systems, and therein lies its chief advantage (7). By appropriate selection of catalyst, pressure, and temperature, a remarkable variety of substrates can be made to undergo hydrogenation, many of them under hydrogen pressure not exceeding 50 psi (see Appendix 3 for description and use of low-pressure hydrogenation apparatus). 

A83543A aglycon 595 f. acetic acid 5 acetoxonium ion 538, 762 acetoxymercuration 390 acetylene coupling 268, 275 f. active ester 624 P-acylamino a,p-unsaturated system 111... 

The reactivity of the unsaturated system of oxepin gives rise to a variety of cycloaddition reactions. A number of 3,6-bridged oxepins have been reacted with peracids to give 2,3-ep-oxyoxepins i,129 154.213.218 At higher temperature, the 2,3,6,7-diepoxy derivatives 2 are formed.164... 

All other methods for obtaining bacteriochlorins are by partial synthesis from porphyrins, chlorins or chlorin derivatives. The bacteriochlorin is thereby produced by reactions which occur at the peripheral /J-C — C double bonds of the higher unsaturated systems. The main problem of these syntheses is the lack of regio- and stereoselectivity because three or four C —C double bonds, which are present in the starting products may be attacked, so that rcgio-and stcreoisomcric products can be formed. 

A number of studies have found that increasing nucleophilieity of the attacking radical favors abstraction over addition to an unsaturated system (benzene ring or double bond).41 ns 119 Bertrand and Siirzur149 surveyed the literature on the reactions of oxygcn-ccntcrcd radicals and observed that the ratio of abstraction to addition increased as shown in Figure 1.10. 

The addition of radicals and, in particular, propagating radicals, to unsaturated systems is potentially a reversible process (Scheme 4.46). Depropagation is cntropically favored and the extent therefore increases with increasing temperature (Figure 4.4). The temperature at which the rate of propagation and depropagalion become equal is known as the ceiling temperature (rc). Above Tc there will be net depolymerization. 

Additions include the attachment of two univalent atoms or groups (called addends) to an unsaturated system, e. g., to olefins, carbonyl groups, aromatic systems, carbenes, etc. (Rule 2.1). For example, the addition of hydrocyanic acid to the car-... 

B. Radical Addition of Sulfonic Acid Derivatives to Unsaturated Systems. 189... 

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