Investigation of the structure and stability of α-substituted cyclopropyl carbanions by semiempirical quantum-chemical methods

The cyclopropyl carbanions C₃H₄X^– (X=H (I), F (II), Cl (III), CF₃ (IV), COCH₃ (V), CN (VI), NO₂ (VII), which are formed by the removal of a proton at the position, were calculated by the semiempirical MINDO/3 and MNDO methods with full optimization of the geometry. The MINDO/3 method, in agreement with the results from nonempirical methods and experimental data, leads to a nonplanar configuration at the anionic center; the angle between the CX bond and the CCC plane was 58–62°. In all probability the conclusion about a planar structure for all the investigated carbanions except (II), which emerges from the MNDO calculations, is unreliable. By calculation of the change of energies in the isodesmic reactions C₃H₅^– + C₃H₅X C₃H₄X^– + C₃H₆ it was found that substitution of hydrogen by the polar substituent X stabilizes the carbanion. On the basis of an analysis of the changes in the length of the cyclic and extracyclic bonds in the transition from the neutral molecule to the carbanion it was established that the introduction of substituents N0₂, COCH₃, and CN into the carbanion gives rise to predominant donation of electron density along the system through the ring to the substituent while the introduction of a halogen leads to transfer of electron density from the substituent to the anionic center.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 590–594, September–October, 1985.