Mechanism and Origin of Selectivity in Platinum(II)‐Catalyzed Reactions of Acyclic γ,δ‐Ynones with Alkenes

Abstract

The generation of platinum‐containing carbonyl ylides from acyclic γ,δ‐ynones and the cycloaddition of these ylides with electron‐rich alkenes have been explored computationally to explain the experimentally observed regio‐ and stereoselectivity. Three pathways for the reaction of ylides with methyl vinyl ether have been investigated. Two of these involve [3+2] cycloadditions, whereas the third involves a [4+2] cycloaddition. Results indicate that the operative reaction pathway depends on the substitution pattern of the acyclic γ,δ‐ynone. For an acyclic γ,δ‐ynone without a methyl substituent at the propargylic position, we predict that both [3+2] cycloaddition pathways are more favorable than the [4+2] cycloaddition pathway, which leads to two products. On the other hand, for an acyclic γ,δ‐ynone with a methyl substituent at the propargylic position, one of the [3+2] pathways is predicted to dominate, which exclusively affords an exo product. This feature arises from the hyperconjugative stabilization effect of the methyl group. Further analysis indicates that steric interactions between the chlorine atom of the catalyst and the methyl group of the vinyl ether lead to the observed exo selectivity. In all cases, the cycloaddition is the rate determining step, whereas for chiral PtCl2(bisphosphine)‐catalyzed reactions it is also stereodetermining, the high degree of enantioselectivity arises from differences in both noncovalent dispersion interactions and extent of partial bond formation in the key transition states for the [3+2] cycloaddition.