An SCF-MO study of the relative barriers to inversion and to ring opening in three-membered ring carbanions

Abstract

The barriers to inversion of configuration at the centre X and to ring opening in a series of isoelectronic three-membered ring systems (1)–(4) have been investigated at the single determinantal MNDO and ab initio SCF-MO levels of theory. The MNDO-method-predicted barriers to inversion at carbanionic centres are too low compared with experiment and with the ab initio calculations. In the specific case of cyclopropyl carbanion, the MNDO method incorrectly predicts the C^– centre to be planar. The two theoretical methods give more comparable results for inversion at nitrogen and oxygen centres. At the highest level of theory employed (RHF-MP4‖6-31 + G), loss of configuration in (1a)–(3a) is predicted to occur via ring opening to give an enolate anion, rather than via direct inversion at C^–. The reverse is true in cyclopropyl anion (4), where direct inversion at the carbon centre has the lower barrier. These results lead to the prediction that oxirane and aziridine anions should be configurationally stable species, whereas thiirane carbanions are predicted to ring-open rapidly to give a thioenol.