Isocyanide insertion into the palladium-carbon bond of complexes containing bidentate nitrogen ligands: A structural and mechanistic study

Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), tert-butyl isocyanide (TIG) and tosylmethyl isocyanide (TosMIC) into the Pd-Me bond of complexes (NN)Pd(Me)Cl (NN = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2'-bipyrimidine (bpm)) afforded quantitatively(NN)Pd(C(=N-R)Me)Cl(R = 2,6-Me2C6H3, t-Bu, CH(2)Tosyl). The course of the reaction has been shown to proceed via the intermediates [(NN)Pd(CN-R)(Me)]Cl(NN = bpy, phen; R = 2,6-Me2C6H3, t-BU, CH(2)Toysl), which have been characterized at 250 K by NMR and IR spectroscopies and conductivity measurements. The mechanism oft;he insertion reaction involves substitution of the halide by the isocyanide followed by a rate-determining methyl migration to the precoordinated isocyanide. Kinetic measurements of the isocyanide insertion reaction, which provided the reaction rate constants of the isocyanide association (k(1)), isocyanide dissociation (k(-1)), and methyl migration reaction (k(2)), have demonstrated that the migration rate of the methyl group to the precoordinated isocyanide increases with increasing electrophilicity of the isocyanide. Methyl migration in the intermediate complexes [(NN)Pd(CN-R)(Me)]Cl(NN = bpy, phen; R = 2,6-Me2C6H3, t-Bu, CH(2)Tosyl) also occurs in the solid state. The complexes (NN)Pd(C(=N-R)Me)X(NN = bpy, phen; R = t-Bu, CH(2)Tosyl; X = Cl, Br) show a fluxional behavior due to a site exchange of the nitrogen donor atoms. Spin saturation transfer H-1 NMR experiments showed that the mechanism of this process involves Pd-N bond breaking and subsequent isomerization via a Y-shaped intermediate.