Oxidative Addition of Aryl Carboxylates to Ni(0) Complexes Involving Cleavage of the Acyl-O Bond

Article abstract of DOI:10.1021/ja00531a016

Reactions of aryl carboxylates RCOO-p-C6H4X (R=CH3, C2H5, n-C3H7; X=H, CH3, OCH3, CN) with bis(1,5-cyclooctadiene)nickel, Ni(cod)2, in the presence of phosphine ligands yield olefin (R(-H)), p-XC6H4OH, and nickel carbonyl complex(es) when the R group has a β hydrogen, whereas CH4, C2H6, nichel carbonyl complex(es), and nickel phenoxide are formed when the R group is CH3.The formation of the products is accounted for by assuming oxidative addition of the ester to nickel involving the cleavage of the acyl-O bond of RCOO-p-C6H4X followed by decarbonylation of the acylnickel complex and decomposition of the alkylnickel complex: RCOO-p-C6H4X + NiLn -> RCONiLnOC6H4X -> RNiLnOC6H4X + CO.The intermediate alkyl(phenoxo)nickel-type complex NiCH3(OC6H5) was in fact isolated in the reaction of phenyl acetate with Ni(cod)2 in the presence of 2,2'-bipyridine.The rate of the reaction is first order with respect to the concentration of the zerovalent nickel complex and the pseudo-first-order rate constant increases with with the increase in the basicity of the phosphine ligand added and with the increase of the electron-withdrawing ability of X.On the basis of these results a mechanism involving a nucleophilic attack at the carbonyl carbon by nickel is proposed.The activation parameters for the reaction of C2H5COOC6H5 with the mixture of Ni(cod)2 and PPh3 are ΔH<*>=21+/-2 kcal/mol, ΔS<*>=-8.8+/-2.9 eu.