Carbanions charge distribution

The very important reactive intermediate, the enolate ion, is an example of a Z-substituted carbanion. The charge distribution and HOMO obtained by SHMO calculation are shown below ... 

In the case of cyclopropyl methyl ketone, disconnection of either the 1,2- or 1,3-carbon-carbon bond of the cyclopropane ring results in the preferred charge distribution shown in (4), namely, the carbanion site is adjacent to the meso-merically stabilising carbonyl group, and the carbocation site may be viewed as a halide-carrying carbon. The reagent equivalent may therefore be 5-chloro-pentan-2-one. 

Carbanions of a-haloalkyl aryl sulfones, sulfonates and sulfonamides react with quinoxalines according to two general pathways vicarious nucleophilic substitution of hydrogen and/or bis-annulation to bis(azirino)quinoxaline derivatives. The charge distribution in the anionic (T-adducts and reaction conditions influence the direction of these reactions. ... 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

Figure 16. Net atomic charge distribution for the benzyl carbanion. The top two figures are for the metal complex, (NHs)2LiCH2C6H5, and indicate a net charge on the organic group of about —0.5 electron. The bottom two figures are for the isolated benzyl carbanion (83).

CNDO/INDO estimates of the net atomic charge distribution for the benzyl and fluorenyl carbanions are given in Figures 16 and 17 (25). The electrostatic potential energy distribution at 2.0 A above the mean plane for these distributions of point charges are shown in Figures 18 and 19. The electrostatic model predicts that the lithium atom would be located over the potential energy minima in the two carbanions— that is, on a normal to the fluorenyl plane that intersects the plane just inside the 9 position, and a normal to the mean benzylic plane that intersects the plane about 0.4 A for C(7) on the C(l)—C(7) bond. In fact, the observed position of the lithium atom is about 1.5 A from the pre-... 

Figure 17. CNDO II charge distribution for isolated fluorenyl carbanion (83)...

Figure 18. Potential energy surface at 2.0 A above the plane of the fluorenyl carbanion calculated from the CNDO II atomic charge distribution of Figure 17. Contour lines are drawn at levels of 0.02 eV (83).

The ET-sensitized photoamination of 1,1-diarylethylenes with ammonia and most primary amines yields the anti-Markovnikov adducts. Photoamination of unsymmetrically substituted stil-benes yields mixtures of regioisomers 15 and 16. Modest re-gioselectivity is observed for p-methyl or p-chloro substituents however, highly selective formation of adduct 15 is observed for the p-methoxy substituent (Table 5). Selective formation of 15 was attributed to the effect of the methoxy substituent on the charge distribution in the stilbene cation radical. This re-gioselectivity has been exploited in the synthesis of intermediates in the preparation of isoquinolines and other alkaloids." Photoamination of 1-phenyl-3,4-dihydronaphthalene yields a mixture of syn and anti adducts 17 and 18 (Scheme 5)." Use of bulky primary amines favors formation of the syn adduct (Table 5), presumably as a consequence of selective anti protonation of the intermediate carbanion. 

In conclusion, the structure and charge distribution for a 1,1-diphenylalkyl carbanion and the corresponding dimers can be represented by structures 2, 3 and 4 below, in which the a-carbon is sp -hybridized (3). The delocalized system (2) including the Ci carbons of the phenyl rings and the a-carbon would be expected to be sensibly planar however, at any given time, one or both of... 

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