Cyclopropyl anions inversion barrier

The barrier to the inversion imposed by the isocyano group permitted the evaluation of the effect of gegenion, solvent and temperature on this cyclopropyl anion. As is evident from Table 27 only at — 5°C is the loss of configuration very rapid. This would be consistent with the view that there is a great deal of ionic character associated with the lithium-carbon bond ... 

In agreement with Walborsky s results and a recent X-ray structure determination of a-cyano-benzyllithium the calculations nicely show that H2C-CN (0.5 kcal mol and Me2CCN (0 kcal mol have almost no inversion barrier while the a-cyano cyclopropyl anion has a sizeable one (9.3 kcal mol ). Whether in the case of the CHO and COOH substituted cyclopropyl anions a pyramidal configuration with a very small barrier (0.7 and 1.9 kcal mol respectively) exists, is not clear. The barrier may be an artifact of the small basis set, and the enolates may well be planar. In any case, the calculations are again in agreement with the experimental result according to which kgx/ rac case of the optically active cyclopropyl phenyl ketone ( —)-(/ )-252 is unity (see Section IV.B.l.a). 

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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