INTERPLAY BETWEEN CONJUGATIVE AND STERIC EFFECTS IN CYCLOPROPYLARENES

The X-ray crystal structures of three cyclopropylarenes are reported. The data suggest that, for the series phenyl → a-naphthyl → 9-anthryl, increasing steric interactions force a distortion from the normally preferred bisected conformation to the perpendicular conformation. In the bisected conformation, orbital alignment between the aromatic 7r-system and the cyclopropyl HOMO is maximal and electron donation from the cyclopropyl to the arene can be detected by an asymmetry in the lengths of the vicinal and distal C—C bonds of the cyclopropane ring. In the perpendicular conformation, the 7r-system of the arene is orthogonal to the HOMO, but aligned with the LUMO of the cyclopropyl group. Consequently, the cyclopropyl group can only act as an electron acceptor. Within experimental error, there was no apparent asymmetry in the lengths of the vicinal and distal C—C bonds, suggesting no significant electronic interaction between the arene and the cyclopropyl group in the perpendicular conformation. © 1990, American Chemical Society. All rights reserved.