Diboron-Promoted Reduction of Ni(II) Salts: Precatalyst Activation Studies Relevant to Ni-Catalyzed Borylation Reactions

The activation and reduction of nickel­(II) salts under conditions relevant to Ni-catalyzed borylation reactions is reported. Methanolic solutions of NiCl2·6H2O reacted with >2 equiv of (iPr)2NEt were converted to polymeric Ni­(OMe)2, which was characterized by IR spectroscopy, magnetic susceptibility measurements, and verified by independent synthesis from NaOMe. When diboron reagents such as bis­(neopentylglycolato) diboron (B2(npg)2) were exposed to methanolic solutions of Ni­(II) salts and (iPr)2NEt, nickel metal was deposited along with the evolution of hydrogen gas. A direct relationship between yield of nickel metal and equivalents of B2(npg)2 relative to [Ni] was also observed, reaching >99% yield at 8 equiv. Ni(0) coordination complexes were also isolated when a phosphine, phosphite, and/or diene ligand was present, all starting from NiCl2·6H2O, including the following: Ni­[P­(OPh)3]4 (74% yield), Ni­[P­(OiPr)3]4 (54% yield), Ni(PPh3)4 (75% yield), (dppp)2Ni + Ni­(1,5-cod)2 (dppp = 1,3-bis­(diphenyl­phosphine)­propane) (91% yield), Ni­(1,5-cod)2 (1,5-cod = 1,5-cyclooctadiene) (69% yield), and (dppf)­Ni­(1,5-cod) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene) (84% yield). The high yields observed indicated the efficient reduction of Ni­(II) to Ni(0) and a likely route for precatalyst entry into the Ni-borylation catalytic cycle. These in situ reduction conditions were also successfully applied to a previously developed Ni-catalyzed alpha-arylation reaction where the requisite Ni­(1,5-cod)2 precatalyst was substituted for NiCl2·6H2O and catalytic diboron. Comparable yields to the original report were observed under these conditions, further demonstrating that Ni(0) active species can be efficiently accessed with diboron reagents under protic conditions from Ni­(II) salt hydrates.