Computational Study of Enantioselectivity in the Asymmetric Allylation of Aldehydes with Chiral Pt(II) Phosphinite Complexes.

Abstract

Computational investigations on the enantioselectivity observed in the allylation of cinnamaldehyde have been carried out using four closely related chiral platinum catalysts with ascorbic acid-based bisphosphinite ligands. Enantioselectivity depends on the substitution of hydrogen on the hydroxyl group by benzyl group(s) in the ascorbic acid framework. The key intermediate is assumed to be one, where both the aldehyde (via oxygen) and the η1 allyl group are coordinated to the metal center, and C-C bond formation between the terminal allylic carbon and carbonyl carbon is the rate-determining step. A screening process has been adopted to select potentially relevant reactant adducts from a total of several hundred theoretically possible conformers by molecular mechanics (MM) calculations. Only 50 transition state structures (TSs) lead to the product based on a two-layer QM/MM study employing B3LYP and UFF methods. Out of these 50 TSs, around 10 structures were selected for the final step QM calculation at B3LYP/LANL2TZ(f)(Pt),6-311G(d,p)//B3LYP/LANL2DZ(Pt),6-31G(d,p) level of theory. This computational approach predicts chirality induction correctly in most of the seven catalysts examined. The successful prediction of enantioselectivity carried out using the MM, QM/MM, and QM approaches suggests that the described simplified procedure could be useful in other catalyst systems also.