Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

We report here the synthesis and characterization of a series of NNN pincer-nickel complexes of the type ((NNN)-N-R2)-NiCl2(CH3CN) (R = Pr-i, Bu-t, Cy, Ph, and p-F-C6H4) based on bis(imino)pyridine ligands. In solution, these complexes are found to be equilibrium mixtures containing one and two pincer ligands, respectively. While the crystal structure of the former was reported by us recently for R = Pr-i, we report the crystal structure of the latter in this study for R = p-F-C6H4. The considered NNN pincer-Ni complexes have been successfully employed to accomplish the catalytic beta-alkylation of several secondary alcohols with a variety of benzyl alcohols at 140 degrees C with high yields and unprecedented turnovers. A maximum of 92% yield of the beta-alkylated product at 18 400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyophenyoethane-1-ol in the presence of 0.005 mol % of ((NNN)-N-Ph2)NiCl2 (CH3CN) and 5 mol % of (NaOBu)-Bu-t at 140 degrees C after 24 h. The reaction exhibits zero-order dependence of rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed beta-alkylation of alcohols and, hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.