Reactions of the nickel(0) compound NiL4 (L = PPh3) with alkyl halides RX involve initial inner-sphere halogen atom abstraction from the alkyl halides to form alkyl radicals R· and halonickel­(I) metalloradical species NiX­(PPh3)2,3. The radical pairs then undergo combination within the solvent cage to give the square planar nickel­(II) compounds NiRX­(PPh3)2. Radical intermediacy is demonstrated persuasively by observations that the relative rates vary in the orders tert-butyl > sec-butyl > n-butyl and RI > RBr > RCl, while density functional theory calculations indicate that the radical mechanism provides a lower energy pathway than do alternative, more conventional pathways. The product of the reaction of Ni­(PPh3)4 with methyl iodide, NiMeI­(PPh3)2, decomposes in solution to ethane and NiI­(PPh3)2,3, but when RX = EtI, n-BuI, sec-BuI, tert-BuI, the alkyl-nickel products undergo rapid β-hydrogen elimination to give the hydride NiHI­(PPh3)2 plus the corresponding alkene(s). Reactions also occur in which a portion of the alkyl radicals diffuses from the solvent cage and abstracts hydrogen from NiHI­(PPh3)2 to form alkanes RH and Ni­(I) species NiI­(PPh3)2. As a result, NiHI­(PPh3)2 is invariably a minor product while the major products are alkanes RH, alkenes R–H, and NiI­(PPh3)2. Hydride NiHI­(PPh3)2 is found to decompose to H2 and NiI­(PPh3)2 but is stable at low temperatures where it exhibits unusual NMR behavior because of exchange involving free PPh3 and the bis- and trisphosphine species, NiHI­(PPh3)2 and NiHI­(PPh3)3. Present in all of the reactions are paramagnetic, substitution-labile Ni­(I) metalloradical species. As a result, resonances of PPh3, ethylene, and the smaller iodoalkenes are generally broad and shifted because of exchange between free and coordinated ligands.

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.

Quinone organocatalysis is an emerging area, and this report highlights some recently developed thermal and photocatalytic reactions, with particular emphasis on photooxygenation reactions. Further, it is discussed how the orthogonal ground-and excitedstate reactivities of quinones can be utilized for the development of tandem catalytic processes.

Ground-and Excited-State Quinones: Perspectives in Organocatalysis and Visible-Light Photocatalysis -[30 refs. + subrefs.]. -(LERCH, S.; UNKEL, L.-N.; WIENEFELD, P.; BRASHOLZ*, M.; Synlett 25 (2014) 19, 2673-2680, http://dx.doi.org/10.1055/s-0034-1379363 ; Inst. Org. Chem., Univ. Hamburg, D-20146 Hamburg, Germany; Eng.) -Y. Steudel 10-302