Catalytic Reduction of Carbonyl Functional Groups in 2-Propanol by Molybdocene Hydrides

The catalytic reduction of acetophenone in 2-propanol by the organometallic complex [Cp₂Mo(μ-OH)₂ MoCp₂](OTs)₂ (I) proceeds through a molybdocene hydride intermediate. Activation parameters for the reduction of acetophenone to 1-phenylethyl alcohol in 2-propanol-d₈ are 19 kcal mol^-1 and −26 cal mol^-1 K^-1 for ΔH^⧧ and ΔS^⧧, respectively. These parameters suggest 2-propanol and I are involved in a C−H activation step to form a molybdocene monohydride that may be the active species in the hydrogenation catalysis. When the catalysis reaction was performed with (CH₃)₂CHOD as the solvent, 1-deuterio-1-phenylethyl alcohol was almost quantitatively formed. This is consistent with a mechanism where a Cp₂Mo hydride, formed in the C−H activation of (CH₃)₂CHOD, is converted to the deuteride by the solvent's alcoholic deuteron. The labeling experiment further supports the a molybdocene hydride as the catalytic species.