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.