Acylcobalt complex cycle

A second example is the reaction of the acylcobalt complex with hydrogen equivalents and the successive reductive elimination of the aldehyde (step 7). This reaction was studied in great detail and has been controversial [93]. For this very last step of the hydroformylation cycle several pathways may be imagined. The two most plausible pathways are shown in eq. (4). 

Mirbach declared the reaction of the acylcobalt complex with HCo(CO)4 to be the minor pathway, whereas the reaction with H2 dominates the catalytic cycle in the hydroformylation of 1-octene and cyclohexene. On the other hand, Marko found that the hydridocobalt complex reacts 12 times faster with the acylcobalt complex than does hydrogen. Undoubtedly, under industrially applied reaction conditions the hydrogenolysis of the acylcobalt complex is effected exclusively by hydrogen. Yet, the remaining hydridocobalt species is still in question besides HCo(CO)3, the cobalt cluster HCo3(CO)g is discussed [94]. 

An example is the hydroformylation reaction of cyclohexene catalyzed by the unsaturated compound HCo(CO)3 which is formed under reaction conditions from the precursor HCo(CO)4. Following the usual mechanism (see, e. g., [18]), the catalytic cycle is depicted in Scheme 1. Since the oxidative addition of H2 to the acylcobalt complex is the rate-determining step in this case the rate equation follows eq. (2) (cf. Section 2.1.1) ... 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

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