Substituents Capable of -Conjugation

Substituents that are themselves chromophores usually contain n electrons. Just as in the case of n electrons, interaction of the benzene-ring electrons and the k electrons of the substituent can produce a new electron transfer band. At times, this new band may be so intense as to obscure the secondary band of the benzene system. Notice that this interaction induces the opposite polarity the ring becomes electron deficient. 

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Carbanions which have substituents capable of conjugative delocalisation of the electron pair will perforce be planar (sp2), in order to allow the maximum orbital overlap of the p orbital with those of the substituent, e.g. (4) and (10) ... 

On doing this, one finds that for substituents capable of conjugating with the double bond, Q > 0.5, whereas for groups such as Cl, OR, and alkyl, (2< 0.1, thereby reflecting the assumption that G is a measure of resonance stabilization. 

Oxoalkanoic acids and esters arise from the benzophenone-initiated photochemical addition of aldehydes to a,/ -unsaturated acids and esters [equation (35)]. " Yields are significantly diminished if the aldehyde bears an a-substituent or if the ester bears a /3-substituent capable of conjugation with the enone moiety. 

Another aspect of the geometry of the bound olefin that has been barely studied is the orientation of substituents capable of x interactions with the olefin. The most studied x substituent is the cyano group, but its linearity precludes discussion of the nature of the interaction. The structures of two complexes of diphenylethylenes (IV and XVIII, Table I) have been determined. On the basis of electronic effects one would expect a phenyl ring either to be coplanar with the olefin double bond for better conjugation or to be perpendicular to the metal-olefin plane for greatest x overlap (in the cyclopropane model). The limited evidence favors the second orientation. However, structural studies of olefins with substituent groups such as -COH, -COOR, or -N02 would be useful for the further definition of the orientation of x substituents. 

Addition of organomagnesiums is favored by strongly electron-withdrawing substituents on an aikene, even when the substituents are not capable of conjugating. Allylic reagents react readily with polyfluorinated alkenes, e.g. ... 

Where the substituent R is capable of conjugation with the P=C bond (e.g. R = CN, CO R, COR, aryl), the ylide is stabilized both against electrophilic and also against nucleophilic attack. Such stabilized ylides are much less prone to oxidation, alcoholysis or hydrolysis. They can be generated, for example, by treating appropriate phosphonium salts with alkali metal alkoxides in alcohols. This is convenient, as lithium alkyls would react with groups such as CO R or COR leading to side products. 

The —/effect of amine substituents can be outweighed by the -I-/2 electron release, provided that their unshared electrons are capable of conjugation with aromatic systems. However, the +R effect may be entirely repressed by electrostatic induction of charge displacements ( —/), when the positive ionic center is bonded directly to the... 

The base-weakening resonance in aromatic amines (3) was discussed in section II.B. Substituents capable of — / effect increase the proton removal of conjugate acids of aniline more strongly from the nearer position. Groups with +/and +i effects in the ortho and para positions result in an increase in pX values, whereas those groups with —R effects decrease these values. The low pX value of o-NHa relative to ffz-NHa (see Table 7), presumably reflects steric factors impeding... 

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