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

Treatment of a mixture of air-stable nickel(II) bis(octanoate), Ni(O₂CC₇H₁₅)₂, and α-diimine ligand, ^iPrDI or ^CyADI (^iPrDI = [2,6-ⁱPr₂-C₆H₃N=C(CH₃)]₂, ^CyADI = [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 ^iPrDI exclusively furnished the 1,2-syn-d₂-dimethylindane. With cyclic trisubstituted alkenes, such as 1-methyl-indene and methylcyclohexene, deuteration with the in situ generated nickel catalyst under 4 atm of D₂ 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 [(^iPrDI)NiH]₂ and increasing the observed hydrogenation activity. Performing the catalyst activation procedure with D₂ gas and HBPin generated both HD and DBPin, establishing that the borane is involved in H₂ activation as judged by ¹H and ¹¹B nuclear magnetic resonance spectroscopies.