Air-Stable alpha-Diimine Nickel Precatalysts for the Hydrogenation of Hindered, Unactivated Alkenes

摘要

Treatment of a mixture of air-stable nickel(II) bis(octanoate), Ni(O2CC7H15)(2), and alpha-diimine ligand, (DI)-D-iPr or (Cy)ADI ((DI)-D-iPr = [2,6-Pr-i(2)-C6H3N=C(CH3)](2), (Cy)ADI = [C6H11N=C(CH3)](2)) with pinacolborane (HBPin) generated a highly active catalyst for the hydrogenation of hindered, essentially unfunctionalized alkenes. A range of tri- and tetrasubstituted alkenes was hydrogenated and a benchtop procedure for the hydrogenation of 1-phenyl-1-cyclohexene on a multigram scale was demonstrated and represents an advance in catalyst activity and scope for the nickel-catalyzed hydrogenation of this challenging class of alkenes. Deuteration of 1,2-dimethylindene with the in situ-generated nickel catalyst with (DI)-D-iPr exclusively furnished the 1,2-syn-d(2)-dimethylindane. With cyclic trisubstituted alkenes, such as 1-methyl-indene and methylcyclohexene, deuteration with the in situ generated nickel catalyst under 4 atm of D-2 produced multiple deuterated isotopologues of the alkanes, signaling chain running processes that are competitive with productive hydrogenation. Stoichiometric studies, titration, and deuterium labeling experiments identified that the borane reagent served the dual role of reducing nickel(II) bis(carboxylate) to the previously reported nickel hydride dimer [((DI)-D-iPr)NiH](2) and increasing the observed hydrogenation activity. Performing the catalyst activation procedure with D-2 gas and HBPin generated both HD and DBPin, establishing that the borane is involved in H-2 activation as judged by H-1 and B-11 nuclear magnetic resonance spectroscopies.