Mechanistic aspects of catalytic hydrogenation of ketones by rhodium(I)-peralkyldiphosphine complexes

Abstract

Mechanistic aspects of the hydrogenation of ketones employing cationic rhodium(I) complexes [Rh((i-Pr) 2P(CH 2) 4P(i-Pr) 2)(NBD)]ClO 4 (NBD = norbornadiene) and [Rh(CyDIOP)(NBD)]ClO 4 (CyDIOP = 2,3- O-isopropylidene-2,3-dihydroxy-1,4-bis(dicyclohexylphosphino)butane) and a neutral complex, “Rh-(CyDIOP)Cl” were studied. The cationic complex-catalyzed hydrogenation of the poor coordinating simple ketone substrates followed a rate equation r 0 = k obs[Rh][ketone] 0[H 2] 0 and showed an unusual negative temperature dependence of the reaction rate. The hydrogenation of the chelating substrate PhCOCONHCH 2Ph followed a rate equation r 0 = k obs[Rh][H 2] with the activation parameters E a 5.51 kcal mol −1, Δ H ‡ 308 4.90 kcal mol −1, Δ S ‡ 308 −32.0 e.u. ([Rh((i-Pr) 2P(CH 2) 4P(i-Pr) 2)(NBD)]ClO 4 catalyst); E a 5.36 kcal mol −1, Δ H ‡ 308 4.75 kcal mol −1, Δ S ‡ 308 − 30.9 e.u. ([Rh(CyDIOP)(NBD)]ClO 4 catalyst). For the neutral complex-catalyzed hydrogenation of PhCOCONHCH 2Ph, the rate equation r 0 = [Rh] 0.25[ketone] 0[H 2] 0 was obtained with the activation parameters ( E a 3.99 kcal mol −1 Δ H ‡ 308 3.38 kcal mol −1, Δ S ‡ 308 −43.0 e.u.). Several intermediate complexes in the cationic complex-catalyzed hydrogenation were also detected spectroscopically or isolated. On the basis of these observations, a general reaction scheme was proposed.