Deciphering the mechanism of the nickel-catalyzed hydroalkoxylation reaction: A combined experimental and computational study

The [Ni(0)(cod)b₂]/P^∪P-catalyzed hydroalkoxylation of butadiene to form butenyl ethers is studied mechanistically, where P^∪P = 1,4-bis(diphenylphosphino)butane (dppb) and 1,2-bis(diphenylphosphinomethyl)-benzene (dppmb). Experimental studies suggest the intermediacy of [(P^∪P)-Ni(0)(butadiene)] and [(P^∪P)Ni(II)(allyl)] intermediates and rule out the involvement of Ni-H species. The related species [(dppb)Ni(0)(1,4-diphenylbutadiene)], 1, and [(P^∪P)Ni(II)(crotyl)(Cl)] complexes 2 (P^∪P = dppmb) and 3 (P^∪P = dppb) have been synthesized and characterized on the basis of VT NMR spectroscopy and X-ray crystallographic studies. Compounds 2 and 3 are shown to be catalytically competent for the hydroalkoxylation reaction. Computational studies on [(dppmb)Ni(0)(butadiene)] indicate a facile protonation that forms a cationic allylic intermediate [(dppmb)Ni(II)(η-C₄H₇)]OMe. C-O bond formation then occurs via external attack by the solvent-stabilized methoxide nucleophile. Hydroalkoxylation proceeds with modest computed barriers of ca. 18 kcal/mol, and the butenyl ether product formation is only marginally exergonic. Overall, the results are consistent with initial kinetic control leading to the major branched isomer followed by a reversible isomerization process operating under thermodynamic control.