Activation of Epoxides by a Cooperative Iron–Thiolate Catalyst: Intermediacy of Ferrous Alkoxides in Catalytic Hydroboration

This paper describes a cooperative iron–thiolate catalyst Cp*Fe­(1,2-Ph2PC6H4S)­(NCMe) (Cp*– = C5Me5–, [1(NCMe)]) for regioselective hydroboration of aryl epoxide by pinacolborane (HBpin). The critical catalytic step involves the direct addition of epoxide to the catalyst rather than activation of the B–H bond of HBpin. Through iron–thiolate cooperation, [1(NCMe)] opens the aryl epoxide rings affording ferrous–alkoxide compounds. Notably, the ferrous–alkoxide intermediate (4) was structurally characterized after its isolation from the reaction of [1(NCMe)] with trans-2,3-diphenyloxirane. The more Lewis acidic hydroboranes such as H3B·THF and 9-BBN (BBN = bora­bicyclo­nonane) can also be captured by [1(NCMe)]. The resulting iron–borane adducts [1H­(BH2)] and [1H­(BBN)] feature an agnostic Fe···B–H interaction. DFT calculations indicate that the addition of HBpin across the iron–thiolate sites is endergonic by 12.9 kcal/mol, whereas it is exergonic by 20.2 kcal/mol with BH3 and 4.6 kcal/mol with 9-BBN. Combining the experimental data with theoretical studies, a mechanism of the substrate activation by [1(NCMe)], followed by HBpin addition, is proposed for the catalysis.