Catalytic ionic hydrogenations of ketones using molybdenum and tungsten complexes

Abstract

Ketone complexes [CpM(CO)₂(PR₃)(η¹-Et₂CO)]⁺BAr′₄⁻ (R = Ph or Me; M = Mo or W) were prepared from hydride transfer from Cp(CO)₂(PR₃)MH to Ph₃C⁺BAr′₄⁻ [Ar′ = 3,5-bis(trifluoromethyl)phenyl] in the presence of 3-pentanone. These ketone complexes are catalyst precursors for hydrogenation of Et₂CO under mild conditions (23 °C, <4 atm H₂). Analogous catalytic hydrogenations are obtained from reaction of the PCy₃ complexes Cp(CO)₂(PCy₃)MH with Ph₃C⁺BAr′₄⁻. The proposed mechanism involves displacement of the ketone by H₂, producing a cationic metal dihydride [CpM(CO)₂(PR₃)(H)₂]⁺. Proton transfer from the dihydride gives a protonated ketone, followed by hydride transfer from the neutral metal hydride CpM(CO)₂(PR₃)H to produce the alcohol complex [CpM(CO)₂(PR₃)(Et₂CHOH)]⁺. The free alcohol product is released from the metal through displacement by H₂ or ketone, completing the catalytic cycle. In most cases, conversion of the ketone or alcohol complexes to the dihydride is the turnover-limiting step of the catalytic cycle, with ketone and alcohol complexes being observed during the reaction. For reactions using the W–PCy₃ system, the dihydride [CpW(CO)₂(PCy₃)(H)₂]⁺ is observed as the resting state of the catalytic process. Proton transfer is slow and becomes turnover-limiting in this case. The Mo catalysts are more active than W, and the dependence on phosphine is PCy₃ > PPh₃ > PMe₃. The turnover rates are slow, with the fastest initial rate of about 2 turnovers per hour found for the Mo–PCy₃ system. This ionic hydrogenation mechanism does not require coordination of the ketone to the metal for the hydrogenation, thus differing from traditional mechanisms where coordination of a ketone to a metal precedes insertion of the ketone into a M–H bond.