Molecular Mapping of the Catalytic Cycle of the Cobalt-Catalyzed Hydromethoxycarbonylation of 1,3-Butadiene in the Presence of Pyridine in Methanol

Abstract

High-pressure in situ IR and NMR investigations of the hydromethoxycarbonylation of 1,3-butadiene (1) to methyl 3-pentenoate (2) in methanol in the presence of Co2(CO)8 and pyridine under carbon monoxide has revealed that the reaction starts by the methanol- and/or pyridine-assisted disproportionation of Co2(CO)8, followed by the establishment of equilibria involving the ionic species [Co(Py)6]2+{[Co(CO)4]−}2, [Co(Py)6]2+[MeO]−[Co(CO)4]−, [PyH]+[Co(CO)4]−, and [MeOH2]+[Co(CO)4]−. The addition of HCo(CO)4 (3) to pyridine or methanol results in the formation of [PyH]+[Co(CO)4]− and [MeOH2]+[Co(CO)4]−, respectively, and 3 is not detectable by either IR or NMR. The ionic 1,4-addition of [MeOH2]+[Co(CO)4]− to 1 is the only pathway to 2-butenylcobalt tetracarbonyl, CH3CH═CHCH2Co(CO)4 (4), via the protonation of 1 followed by the reaction of the C4 carbocation with the counteranion tetracarbonylcobaltate. In the absence of carbon monoxide, 4 could lose a coordinated carbon monoxide to form (η3-C4H7)Co(CO)3 (7) in a reversible reaction. In the presence of carbon monoxide, 4 is converted to the acylcobalt tetracarbonyl species 5 via CO insertion into the Co–carbon bond of 4 followed by the reaction with CO. The pyridine-assisted methanolysis of 5 leads to the formation of the product methyl 3-pentenoate (2) and pyridinium tetracarbonylcobaltate. The key intermediates of the catalytic cycle were isolated and characterized.