Carbanions inversion barrier

Furthermore, the repulsion between the electron pairs of fluorine atoms is responsible for the pyramidal structure of the carbanion derived from fluoroform. The inversion barrier of the anion is 100 kcal/mol, while that of CH3 is only 2 kcal/ mol. As the acidity of fluoroform is lO" times higher than that of methane, the role of the pyramidal form in stabihzing the carbanion CF3 is essential. 

Carbanions have been less studied, apart from CH3, 167,168 but included in a more recent set of calculations169 on several carbanions (CH3-, C2H5-, and ethynyl anion) are calculations on C2H3-. For reliable calculations on this type of molecule, diffuse orbitals must be added to the basis set. Several different basis sets were used, but the geometry of the neutral parent molecule was used in some of the calculations. The main aim of this paper was to investigate the electron density and difference densities, electron affinities, and proton affinities. The inversion barrier in the vinyl anion was ca. 142 kJ mol-1 which was in good agreement with that found by Lehn et a/.148 in an earlier calculation. 

Niemeyer, H. M. On the inversion barriers of pyramidal carbanions. Tetrahedron 1977, 33, 2267-2270. 

Carbanions a to the sulfonyl group can be easily prepared due to their high acidity in comparison with a-Aio carbanions. Although a-sulfonyl carbanions have a more sp -hybridized character than a-thio carbanions [24,56], a-sulfonyl carbanions have been shown by Gais and coworker to have a somewhat higher inversion barrier to racemization than a-thio carbanions [35]. For example, when an enantiomerically pure trifluoromethyl sulfone was first deprotonated with n-BuLi and subsequently protonated with trifluoroacetic acid, the sulfone was recovered with 90% ee [Eq. (12)]. 

As in other areas of organic chemistry, computational investigations have provided important insights into the nature of carbanions, and the results complement the experimental studies. For example, ab initio calculations suggest that the inversion barrier of a methyl anion is ca. 2.2kcal/mol and that the inversion barrier of the ethyl anion is 3.3 kcal/mol. These values contrast with the <0.2 kcal/mol inversion barrier of the resonance-stabilized cyanomethyl anion on the one hand and the ca. 15 kcal/mol barrier for inversion of the cyclopropyl anion via a highly strained transition structure on the other hand. By comparison, the inversion barrier of ammonia is about 5.5 kcal/mol. ... 

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