Chromium–Salen-Mediated Alkene Epoxidation: A Theoretical and Experimental Study Indicates the Importance of Spin-Surface Crossing and the Presence of a Discrete Intermediate

Abstract

The mechanism of alkene epoxidation by chromium(v) oxo salen complexes has been studied by DFT and experimental methods. The reaction is compared to the closely related Mn‐catalyzed process in an attempt to understand the dramatic difference in selectivity between the two systems. Overall, the studies show that the reactions have many similarities, but also a few critical differences. In agreement with experiment, the chromium system requires a change from low‐ to high‐spin in the catalytic cycle, whereas the manganese system can proceed either with spin inversion or entirely on the high‐spin surface. The low‐spin addition of metal oxo species to an alkene leads to an intermediate which forms epoxide either with a barrier on the low‐spin surface or without a barrier after spin inversion. Supporting evidence for this intermediate was obtained by using vinylcyclopropane traps. The chromium(v) oxo complexes can adopt a stepped shape or form a more concave surface, analogous to previous results on manganese salen complexes.