Catalytic hydroxylation of benzene and cyclohexane using in situ generated hydrogen peroxide: New mechanistic insights and comparison with hydrogen peroxide added directly

Hydrogen peroxide is considered an ideal "green" oxidant due to its high oxidizing ability and lack of toxic by-products. Herein, we report on an oxidation procedure that couples metallic palladium-catalyzed in situ hydrogen peroxide generation from dihydrogen and dioxygen with a second vanadium or iron catalyst that utilizes the hydrogen peroxide for the hydroxylation of benzene and cyclohexane. Studies indicate that the slow step in the overall reaction is the formation of usable hydrogen peroxide, and the mechanism of hydroxylation by the second catalyst is not affected by the presence of metallic palladium. The reported procedure, which resembles monoxygenases, allows the direct use of dioxygen in catalytic oxidations. Comparisons between the in situ method of hydrogen peroxide generation and hydrogen peroxide added via syringe pump show that the in situ method is more selective. Additionally, new insight into the mechanism of vanadium-catalyzed benzene hydroxylation is reported. Mechanistic investigations include the observation of a high NIH shift, the use of a radical cation rearrangement probe, and the first use of H₂¹⁸O enrichment studies. Based on these, an electron transfer mechanism resulting in a radical cation intermediate is proposed.