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HCo(CO)3-Catalyzed Propene Hydroformylation. Insight into Detailed Mechanism

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journal contribution
posted on 10.11.2003, 00:00 by Chun-Fang Huo, Yong-Wang Li, Matthias Beller, Haijun Jiao
The entire catalytic cycle of propene hydroformylation using HCo(CO)3 as an active catalyst has been systematically investigated at the B3LYP density functional level of theory. It is found that the most stable π-complex HCo(CO)32-H2CCHCH3) has a CC double bond coordination in the equatorial position, and the subsequent olefin insertion (alkylation) process is reversible, in agreement with the experimental finding. The hydride migratory insertion is accompanied by Co(CO)3 pseudorotation, leading to the Co···H−C agostic stabilized (iso)propyl complex (C3H7)Co(CO)3 with the alkyl group at the axial position and thus does not take place on a Cs symmetry potential energy surface. The regioselectivity is mainly determined by the relative stability of the alkyl cobalt tetracarbonyl complexes (C3H7)Co(CO)4 from the exothermic and irreversible CO addition to the alkyl cobalt tricarbonyl complexes (C3H7)Co(CO)3, which is therefore a thermodynamic controlled process. The CO insertion process (carbonylation) proceeds via two Co(CO)3 pseudorotated transition states and a Co···H−C agostic stabilized intermediate. The resulting most stable complex (C3H7CO)Co(CO)3 with the acyl group in the axial site has a η2-OC interaction at the equatorial site, and the computed characteristic vibrational modes agree well with the available experimental data. In contrast to the generally accepted conclusion, H2 coordination to the acyl complex rather than oxidative addition is the rate-determining step after HCo(CO)3 generation. This finding is supported by the high stability of the acyl complex toward further H2 addition, as found experimentally.