Iron vs. ruthenium--a comparison of the stereoselectivity in catalytic olefin epoxidation

The synthesis and the full characterization of a new ruthenium(II) complex with the pentadentate bispidine ligand L(1) is reported and shown to be a very active catalyst for olefin epoxidation. The selectivity in the epoxidation of cis- and trans-beta-methylstyrene with the formation of cis and trans products, each, was determined and compared with that of the iron bispidine complex of L(1). There is a significant difference in selectivity between the two catalysts in the epoxidation of cis-beta-methylstyrene but the epoxidation of trans-beta-methylstyrene is highly stereoselective with both catalysts. Based on these results, electrochemical and labeling studies, a radical pathway for the epoxidation and isomerization is proposed, and this is supported by computational data. DFT indicates that, with both catalysts, the epoxidation is based on a stepwise mechanism, which, in the first step leads to a radical-based intermediate. This exists in two configurations, which interconvert with a relatively low energy barrier. The product ratio depends on the relative energies of the two configurations of the radical intermediate and the height of the energy barriers to the cis- and trans-epoxide products. For the Fe-based system there is, as expected, the additional complication of the availability of various spin levels, and multi-state reactivity is observed. The computed structures and energies are in agreement with the observed data.