Molecular mapping of the catalytic cycle of the cobalt-catalyzed hydromethoxycarbonylation of 1,3-butadiene in the presence of pyridine in methanol

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 [MeOH ₂]⁺[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 [MeOH ₂]⁺[Co(CO)₄]^- to 1 is the only pathway to 2-butenylcobalt tetracarbonyl, CH₃CH=CHCH ₂Co(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-C₄H ₇)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. © 2011 American Chemical Society.