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

The [Ni­(0)­(cod)₂]/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)], <b>1</b>, and [(P^∩P)­Ni­(II)­(crotyl)­(Cl)] complexes <b>2</b> (P^∩P = dppmb) and <b>3</b> (P^∩P = dppb) have been synthesized and characterized on the basis of VT NMR spectroscopy and X-ray crystallographic studies. Compounds <b>2</b> and <b>3</b> 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