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

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.