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)], 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.