FORMATION OF CYCLOPROPANES FROM ELECTROPHILIC TRANSITION-METAL CARBENE COMPLEXES AND ALKENES - AN EHMO STUDY OF THE REACTION-MECHANISM

The mechanism of cyclopropane formation by reaction of strongly electrophilic transition-metal carbene complexes with alkenes has been studied by means of Extended Huckel calculations. The least-energy pathways for the reactions of ethylene with (CO)(5)Cr=CH2, (CO)(5)Cr=C(Ph)H, and [Cp(CO)(2)Fe=CH2](+), respectively, were determined. For all the reactions, a smooth transformation of the starting compounds to the products with an early transition state was observed. The formation of an intermediate could not be detected. The least-energy pathway involves initial nucleophilic attack of one carbon of the alkene (C-alpha) on the carbene carbon. Ring-closure occurs by electrophilic attack of the alkene C-beta atom on the carbene carbon from the direction opposite to the metal ('backside ring-closure'). C(carbene)-C-alpha and C(carbene)-C-beta bond formation are concerted but non-synchronous. The calculations also indicate that the alternative mechanism involving intermediary formation of a metallacyclobutane ('frontside ring-closure') is energetically unfavorable. Substituting a phenyl group for a hydrogen atom in the carbene ligand of (CO)(5)Cr=CH2 modifies the preferred direction of alkene approach, which for (CO)(5)Cr=C(Ph)H is in a sector approximately 'above' and 'below' the C(carbene)-H bond. Based on these results, a mechanistic model to explain the stereoselectivity of carbene transfer from strongly electrophilic carbene complexes to substituted alkenes is proposed.