Three-membered rings. Part I. Semiempirical calculations of the molecular conformation of compounds of the type PhCH·X·CH2(X = CH2, O, S, NH, or CCl2)

Abstract

Semiempirical CNDO/2 calculations have been performed on the series of compounds phenylcyclopropane, phenyloxiran, phenylthiiran, phenylaziridine, and 1,1-dichloro-2-phenylcyclopropane. From the energies obtained, it is concluded that for all these molecules the most stable conformation is that in which the planes of the phenyl group and of the three-membered ring are orthogonal (or very close to orthogonal) to one another. These results indicate that inter-ring conjugation is present. The energy barrier to rotation about the phenyl–threemembered ring bond is ⩽ 13.4 kJ mol^–1 for the first four compounds, which suggests that interconversion between one orthogonal form and another, equivalent to the first, is rapid at room temperature. Instead, 1,1-dichloro-2-phenylcyclopropane, because of the steric and electrostatic effects of the chlorine atoms, gives an energy barrier of 61.9 kJ mol^–1. For this reason interconversion in this molecule is slow.