THERMAL-REACTION OF VINYLCYCLOPROPYLIDENE TO CYCLOPENTENYLIDENE - EXPLORATION OF ELECTRONIC HYPERSURFACE

The reaction of vinylcyclopropylidene to cyclopentenylidene represents a second type of carbene-carbene rearrangement. The quantum mechanical investigation of its singlet state 1(σ2) by the MINDO/3 method reveals that the reaction is initiated by a π-complex formation between the double bond and the empty p atomic orbital at the carbene site. The two electrons occupying the σ orbital do not participate in this interaction. A nonclassical carbene is formed in an intermediate stage. In this species electron density is shifted from the initial double bond toward the carbene site. The nonclassical carbene is relatively unstable toward deformation to cyclopentenylidene. Electron-donating substituents at the double bond of vinylcyclopropylidene should enhance the rate of π-complex formation. In contrast to vinylcyclopropylidene are the corresponding hetero analogues, vinylazirinylidene and vinyloxiranylidene, which should be less reactive under nucleophilic attack, because the p AO at the carbene site is partially filled with electron density supplied by its electron-rich neighbor atom. © 1978, American Chemical Society. All rights reserved.