Comprehensive Theoretical Study on the Mechanism of Regioselective Hydroformylation of Phosphinobutene Catalyzed by a Heterobinuclear Rhodium(I)−Chromium Complex

Abstract

A concerted mechanism for the hydroformylation of phosphinobutene catalyzed by the heterobinuclear complex (CO)4Cr(μ-PH2)2RhH(CO)(PH3) was elucidated by density functional theory (DFT), with particular emphasis on the catalytic cycle, the regioselectivity, the cooperativity of Cr with Rh, and the interpretation of experimental observations. Four possible mechanisms were investigated, and the results were compared. It is found that the introduction of the Cr(CO)4 moiety remodels the mechanism. The Rh−Cr-catalzyed hydroformylation mechanism includes the following: (a) formation of the chelate acyl species through a chelate associative mechanism including olefin addition, olefin insertion, and carbonyl insertion steps, (b) CO addition to the chelate acyl species with the formation of a monodentate acyl species, and (c) the conversion of the monodentate acyl species to the product aldehyde through H2 coordination, H2 oxidative addition, and aldehyde elimination. Carbonyl insertion is predicted to be the rate-limiting step, with a free energy barrier of 86.76 kJ/mol in benzene solution at 353.15 K and 27.15 atm. The favorability of the branched product is predicted to be nearly 100% both kinetically and thermodynamically. The chromium serves as an orbital reservoir in olefin addition and insertion steps via the variation of the orbital interaction between Rh and Cr atoms. The catalytic activity of the Rh(I)−Cr bimetallic complex is higher than that of the monometallic Rh catalysts. These could explain satisfactorily the reported experimental observations.