EPR Evidence for the Origin of Nonlinear Effects in an Enantioselective Cu(II)-Catalyzed Spiroannulation

Abstract

Herein, we describe an enantioselective Cu(II)-catalyzed spiroannulation of N-Boc-iminooxindoles with allylsilanes where a significant positive nonlinear effect (NLE) is observed. EPR spectroscopic studies of the copper(II) species present under synthetically relevant conditions reveal explicit spectroscopic evidence based on analysis of the metal center for the species responsible for the positive NLE in a metal-catalyzed system. EPR spectroscopy indicates that formation of a heterochiral ML2 species under scalemic conditions enriches the effective enantiopurity of the catalytically active species, leading to the asymmetric amplification observed in the spiroannulation. Mathematical analysis of the positive NLE reveals a high thermodynamic preference toward formation of the heterochiral ML2, which has a low relative reactivity when compared to the homochiral ML2.