Deciphering the Mechanism of the Nickel-Catalyzed Hydroalkoxylation Reaction: A Combined Experimental and Computational Study

The [Ni(0)(cod)(2)]/(PP)-P-boolean AND-catalyzed hydroalkoxylation of butadiene to form butenyl ethers is studied mechanistically, where (PP)-P-boolean AND = 1,4-bis(diphenylphosphino)butane (dppb) and 1,2-bis(diphenylphosphinomethyl)-benzene (dppmb). Experimental studies suggest the intermediacy of [(dppb)Ni(0)(butadiene)] and [(PP)-P-boolean AND)Ni(II)(ally1)] intermediates and rule out the involvement of Ni-H species. The related species [((PP)-P-boolean AND)Ni(0)(1,4-diphenylbutadiene)], 1, and [((PP)-P-boolean AND)Ni(II)(crotyl)(C1)] complexes 2 ((PP)-P-boolean AND = dppmb) and 3 ((PP)-P-boolean AND) = 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 Rdppmb)Ni(II)(eta-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.