Spin Surface Crossing in Chromium-Mediated Olefin Epoxidation with O2

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Abstract
[Cp*Cr(μ-Cl)Cl]2 reacted with dioxygen (O2) to produce Cp*Cr(O)Cl2 (1), which has been structurally characterized. Although 1 oxidized PPh3 and 1,4-cyclohexadiene catalytically, it did not epoxidize olefins. DFT calculations have been performed on the system to characterize the potential energy surface for the epoxidation of ethylene and, in particular, the consequences of the crossing from the doublet surface of the starting materials to the quartet surface of the product (i.e. a chromium(III) epoxide adduct). These calculations suggested that “spin-blocking” was not a significant problem and that the reaction of CpCr(O)Cl2 (3) with ethylene should have a lower activation barrier. On the basis of this computational prediction, 3 was prepared; it was found to epoxidize olefins stoichiometrically.