Catalysis of Cross-Coupling and Homocoupling Reactions of Aryl Halides Utilizing Ni(0), Ni(I), and Ni(II) Precursors; Ni(0) Compounds as the Probable Catalytic Species but Ni(I) Compounds as Intermediates and Products

Both Ni(0) and Ni­(I)
compounds are believed to exhibit cross-coupling
catalytic properties under various conditions, and the compounds Ni­(PPh3)4 and NiCl­(PPh3)3 are compared
as catalysts for representative Suzuki–Miyaura and Heck–Mizoroki
cross-coupling reactions. The Ni(0) compound exhibits catalytic activities,
for cross-coupling of chloro and bromoanisole with phenylboronic acid
and of bromobenzene with styrene, yielding results which are comparable
with those of many palladium-based catalysts, but our findings with
NiCl­(PPh3)3 are at this point unclear. It seems
to convert to catalytically active Ni(0) species under Suzuki–Miyaura
reaction conditions and is ineffective for Heck–Mizoroki cross-coupling.
The paramagnetic Ni­(I) compounds NiX­(PPh3)3 (X
= Cl, Br, I) are characterized for the first time by 1H
NMR spectroscopy and are found to exhibit broad meta and para resonances at δ 9–11 and
3–4, respectively, and very broad ortho resonances
at δ 4−6; these resonances are very useful for detecting
Ni­(I) species in solution. The chemical shifts of NiCl­(PPh3)3 vary with the concentration of free PPh3, with which it exchanges, and are also temperature-dependent, consistent
with Curie law behavior. The compound trans-NiPhCl­(PPh3)2, the product of oxidative addition of chlorobenzene
to Ni­(PPh3)4 and a putative intermediate in
cross-coupling reactions of chlorobenzene, is found during the course
of this investigation to exhibit entirely unanticipated thermal lability
in solution in the absence of free PPh3. It readily decomposes
to biphenyl and NiCl­(PPh3)2 in a reaction relevant
to the long-known but little-understood nickel-catalyzed conversion
of aryl halides to biaryls. Ni­(I) and biphenyl formation is initiated
by PPh3 dissociation from trans-NiPhCl­(PPh3)2 and formation of a dinuclear intermediate, a
process which is now better defined using DFT methodologies.