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

摘要

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.]]>