title: 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.
authors.affiliation: Department of Chemistry, Organic Chemistry Royal Institute of Technology 100 44 Stockholm, Sweden. pbrandt@kth.se
authors.forename: Peter
authors.initials: P
authors.lastname: Brandt
authors.forename: Per-Ola
authors.initials: PO
authors.lastname: Norrby
authors.forename: Adrian M
authors.initials: AM
authors.lastname: Daly
authors.forename: Declan G
authors.initials: DG
authors.lastname: Gilheany