Antiaromatic carbanions

Figure 3.1. Gas-phase A//add values (kcal/mol) for carbon acids that give aromatic or antiaromatic carbanions. Acyclic analogues are included for comparison. Arrow indicates acidic hydrogen.

Scheme5.71. Compounds which upon deprotonation yield antiaromatic carbanions.

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

Another group of unstable carbanions are those with antiaromatic character (Scheme 5.71). Thus, cyclopropenyl anions or oxycyclobutadienes, generated by deprotonation of cyclopropenes or cyclobutenones, respectively, will be highly reactive and will tend to undergo unexpected side reactions. Similarly, cyclopentenediones are difficult to deprotonate and alkylate, because the intermediate enolates are electronically related to cyclopentadienone and thus to the antiaromatic cyclopenta-dienyl cation. 

An ab initio study of the effect of a-substituents on the acidity of cyclopropaben-zene has shown that a-substituents stabilize the cyclopropabenzenyl anion (5) less efficiently than the cyclopropenyl anion (6).2 The attachment of induedvely/field acting substituents attached to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing the, v character of the lone pair, diminishing the antiaromatic character of the three-membered ring at the same time. 

Rings containing an odd number of carbon atoms can be aromatic or antiaromatic, if they are planar and have a conjugated p orbital on each ring atom. To have an even number of electrons in their odd number of p orbitals, these species must be ionic. They must be carbocations or carbanions. 

Another very common sigmatropic shift is the [1,2] hydride shift associated with car-bocations (Eq. 15.30). Although not usually analyzed this way, these migrations involve a cyclic, two-electron system and are Hiickel aromatic (see margin). Viewing these reactions this way nicely rationalizes why comparable [1,2] shifts are not seen in carbanions that is, they would involve a four-electron Hiickel antiaromatic transition state. 

The special stability of compounds containing such bonds can be attributed to their being isoconjugate with the cyclopropenium cation and consequently aromatic. Likewise, the failure of radicals and carbanions to undergo Wagner-Meerwein rearrangements could be attributed to the fact that the intermediate n complexes would be isoconjugate with the nonaromatic cyclopropenium radical or the antiaromatic cyclopropenium anion. 

The photocyclization of (108) to (110) is another ten-electron process, this time involving a system isoconjugate with an odd hydrocarbon anion. The first step is probably a photochemical conrotatory closure to (109). The transition state, being of anti-Huckel type in a fused 6-5 system with ten delocalized electrons, should be antiaromatic. The intermediate (109), isoconjugate with a linear C7 carbanion, was not isolated but underwent rearrangement, probably by a 1,5 sigmatropic shift via a transition state isoconjugate with the indenyl anion (111). 

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