MECHANISTIC ASPECTS OF CATALYTIC HYDROGENATION OF KETONES BY RHODIUM(I)-PERALKYLDIPHOSPHINE COMPLEXES

Article abstract of DOI:10.1016/0022-328X(85)87008-X

Mechanistic aspects of the hydrogenation of ketones employing cationic rhodium(I) complexes ClO4 (NBD=norbornadiene) and ClO4 (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 r₀=k_obs0

₀ and showed an unusual negative temperature dependence of the reaction rate.The hydrogenation of the chelating substrate PhCOCONHCH2Ph followed a rate equation r₀=k_obs

with the activation parameters E_a 5.51 kcal mol^-1, ΔH excit.₃₀₈ 4.90 kcal mol^-1, ΔS excit.₃₀₈-32.0 e.u. (ClO4 catalyst); E_a 5.36 kcal mol^-1, ΔH excit.₃₀₈ 4.75 kcal mol^-1, ΔS excit.₃₀₈-30.9 e.u. (ClO4 catalyst).For the neutral complex-catalyzed hydrogenation of PhCOCONHCH2Ph, the rate equation r₀=^0.25₀

₀ was obtained with the activation parameters (E_a 3.99 kcal mol^-1, ΔH excit₃₀₈ 3.38 kcal mol^-1, ΔS excit.₃₀₈-43.0 e.u.) Several intermediate complexes in the cationic complex-catalyzed hydrogenation were alo detected spectroscopically or isolated.On the basis of these observations, a general scheme was proposed.