Insertion of Isocyanides across the Pd_C Bond of Phosphinoquinoline Allyl Palladium Complexes Bearing η1- and η3-Coordinated Allyl Groups. A Synthetic and Mechanistic Study

We undertook an exhaustive study on the structural characteristics and reactivity toward the insertion of isocyanides across the Pd_C bond of palladium complexes bearing coordinated chloride, bidentate phosphoquinolines as spectator ligands, and differently substituted allyl groups. It was shown that the allyl hapticity is influenced not only by the nature of the spectator ligand but also by the position of the methyl substituents on the allyl fragment. Thus, in the presence of chloride, the η1-η3 configuration is always assumed when the distorted 8-(diphenylphosphino)-2-methyl quinoline (DPPQ-Me) acts as ancillary ligand, whereas the unsubstituted 8-(diphenylphosphino)quinoline ligand (DPPQ) generally favors the _2-_1 configuration with the exception of the complex with the 2-methyl-substituted allyl fragment, which adopts the κ1-η3 configuration with chloride as the counterion. We have determined the reactivity toward the insertion of the isocyanide on the Pd-C bond, and we have observed decidedly different rate laws when η3 or η1 -allyl complexes were investigated. In particular, the rate law of the reaction involving the _1-allyl derivatives displays a second-order dependence on complex and isocyanide concentration, whereas the insertion on η3 -allyl complexes is better described by a first-order process. We have interpreted such experimental results on the basis of a general preequilibrium mechanism in which the observed rates of the studied reactions are governed by the magnitude of the equilibrium constant. Finally, we have determined the solid-state structure of the insertion product palladium(chloro)(8- (diphenylphosphino)quinoline)(2,6-dimethyl-N-(4-methylpent-3-enylidene)benzenamine) (complex 1Bd).