Stabilities of substituted oxametallacycle intermediates: Implications for regioselectivity of epoxide ring opening and olefin epoxidation
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abstract
Total energy density functional theory (DFT) calculations were performed to determine the relative stabilities of methyl substituted isomers of oxametallacycle intermediates potentially formed during ring-opening reactions of propylene oxide (PO) and isobutylene oxide (IO) on metal dimers. The relative stabilities of different oxametallacycle isomers may determine the regioselectivity of reactions involving these intermediates. In the ring opening of PO and IO, two oxametallacycle isomers may be formed: one in which the methyl groups are attached to the bridging carbon of the O-C-C backbone and one in which the methyl groups are attached to the surface-bound carbon. DFT calculations consistently indicate that oxametallacycles incorporating methyl groups attached to the bridging C atom are more stable by approximately 2-7 kcal/mol for all 14 metals studied. However, the relatively small thermodynamic stability differences between isomers suggest that kinetic barriers and the manipulation of these by surface modification may control the regioselectivity of reactions involving oxametallacycles in some cases. Computed oxametallacycle stabilities are compared with regioselectivities of epoxide ring opening and olefin oxidation reactions previously reported on metal surfaces and catalysts in order to examine which examples are consistent with the formation of the thermodynamically preferred oxametallacycle intermediate.
