NUCLEOPHILIC-ATTACK AT THE CENTRAL ALLYL CARBON-ATOM IN [(ETA-3-ALLYL)ML2]+ COMPLEXES (M = PD, PT) - EXPERIMENTAL FACTS AND NEW THEORETICAL INSIGHTS

The reaction of ketene silyl acetals with [(eta3-allyl)PtL2]+OAc- leads to formation of platina-(II)cyclobutanes upon the nucleophilic attack of the enolate at the central allyl carbon atom (C(c)). As a result, from a new theoretical analysis for complexes of the type [(eta3-allyl)ML2]+ (M = Pd, Pt), the C(c)-centered MO (C3-pi(perpendicular-to)*) is not destabilized as usually assumed. Rather, it competes with another MO, (d(pi)-nC3) (centered on metal and terminal C(t) carbon atoms), to be the first LUMO of the system. On this basis, the attack of a nucleophile at the C(c) atom is almost as probable as attack at the C(t) atoms. The initial MO picture of the precursor complex can only suggest the regioselectivity of the reaction, saying nothing about the attractive/repulsive interactions encountered by the incoming nucleophile in the pathway toward metallacyclobutane. Within the limits imposed by the EHMO method, a simple qualitative description is offered of the intended correlations among metal-carbon and carbon-nucleophile bonding/antibonding MO's during the formation of the metallacyclobutane product. An energetic barrier, which varies as a function of the sigma-donor strength of the nucleophile, is observed, and its origin is described.