Cyclopropyl- und Alkyl-Carbanionen mit Akzeptor-Substituenten in α-Stellung. Eine MO-Studie zur Konfiguration der Anionen und Acidität der entsprechenden CH-Säuren / Cyclopropyl and Alkyl Carbanions with Acceptor Substituents in α-Position. An MO Study of the Configuration of the Anions and the Acidity of the Corresponding CH Acids

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Abstract
A-Hydrogens a-Hydrogens of R-C ≡ C-and N ≡ C-substituted cyclopropanes are more acidic than their open chain analogs, as expected from the higher acidity of cyclopropane versus propane. In the case of the carbonyl substituted species, however, this order is reversed. Furthermore, in H/D exchange reactions of optically active compounds, only cyclopropyl cyanides retain their configuration; aliphatic and cyclopropyl ketones exchange and racemize essentually at the same rate. This behaviour has been rationalized qualitatively in the following manner: RC = C-and N ≡ C-substituents stabilize a negative charge mostly inductively; on the other hand, with carbonyl groups, delocalisation of the negative charge (resonance) is more important than the inductive effect. Thus the unfavourable configuration (I-strain) of the planar cyclopropyl enolate anions is in agreement both with the low acidity of the corresponding cyclopropanes and with the loss of optical activity in H/D-exchange reactions. In order to put this explanation on a more quantitative basis, we calculated the relative proton affinities of isopropyl-and cyclopropyl-carbanions by means of the ab-initio STO-3G method. In the case of X = H, CF3 and C ≡ N, the cyclic anion (CH2)2C⊖X is more stabilized by the substituent X than in the anion (CH3)2C⊖X; with X = CHO, COOH and NO2 the order is reversed. Cyclopropyl anions should have a pyramidal configuration with X = H, CF3, C ≡ N and NO2, which indicates a strong inductive effect also of the nitro group. In summary: STO-3G calculations support the qualitative explanations of the experimental results with regard to the acidities of substituted cyclopropanes.
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