Organometallic compounds of Group III: XLIV. Molecular hydrogen and aluminum hydride transfers mediated by nickel(0) complexes: the intermediacy of nickel hydrides in the Ziegler nickel effect

Abstract

In investigating the possible mechanistic similarity between dihydrogen and aluminum hydride transfers to unsaturated hydrocarbons, the catalytic action of nickel(0) complexes on such transfers was investigated in detail. For dihydrogen transfers the tendency of dihydroaromatics to disproportionate into tetrahydro-aromatic and aromatic hydrocarbons was evaluated for 1,2- and 1,4-dihydrobenzenes, 1,2- and 1,4-dihydronaphthalenes and 9,10-dihydroanthracene toward (Cod) 2Ni, Bpy(Cod)Ni and (Et 3P) 4Ni. The catalyzed disproportionation, which proceeded with decreasing rate in the order, 1,4-C 6H 8 > 1,2-C 6H 8 > 1,2- and 1,4-C 10H 10 » 9,10-C 14H 12, was interpreted in terms of the formation of intermediate allylic nickel hydrides. The study of aluminum hydride transfer to unsaturated hydrocarbons was carried out by letting organoaluminum alkyls or hydrides interact with nickel(0) complexes. Three different reactions were observed: (1) nickel-catalyzed olefin formation from R 3Al; (2) nickel-catalyzed hydroalumination from R 2AlH and unsaturated hydrocarbons; and (3) stoichiometric reaction of nickel(0) compounds and LiAlH 4 or R 2AlH to form aluminum nickelides (LiAlH 2Ni). The rates, stereochemistry, regiochemistry and deactivation of the thermal and nickel-catalyzed hydroaluminations of alkenes and alkynes were compared, in order to learn about the most probable catalytic carriers in such nickel catalysis. The foregoing lines of evidence on how nickel(0) interacts with organoaluminum compounds are brought together in formulating a new, comprehensive mechanism for the Ziegler Nickel Effect. In this novel molecular view, the catalytic carriers in the Nickel Effect are dialkylaluminum nickel hydrides. Seen in this light, dihydrogen and aluminum hydrogen transfers have more than just a formal similarity; both processes proceed by nickel hydrides, which are formed by way of oxidative insertions, into a C-H and an Al-H bond, respectively.