Caution in the Use of Nonlinear Effects as a Mechanistic Tool for Catalytic Enantioconvergent Reactions: Intrinsic Negative Nonlinear Effects in the Absence of Higher-Order Species

Abstract

Investigation of the dependence of product enantiometric excess (ee) on catalyst ee is a widely used tool to probe the mechanism of an enantioselective reaction; in particular, the observation of a nonlinear relationship is usually interpreted as an indication of the presence of one or more species that contain at least two units of the chiral entity. In this report, we demonstrate that catalytic enantioconvergent reactions can display an intrinsic negative nonlinear effect that originates purely from the kinetic characteristics of certain enantioconvergent processes and is independent of possible aggregation of the chiral entity. Specifically, this intrinsic negative nonlinear effect can arise when there is a kinetic resolution of the racemic starting material, and its magnitude is correlated with the selectivity factor and the conversion; the dependence on conversion provides a ready means to distinguish it from a more conventional nonlinear effect. We support our analysis with experimental data for two distinct enantioconvergent processes, one catalyzed by a chiral phosphine and the other by a chiral Pd/phosphine complex.