Isocyanide Insertion into the Palladium−Carbon Bond of Complexes Containing Bidentate Nitrogen Ligands:  A Structural and Mechanistic Study

Abstract

Insertion of the isocyanides 2,6-dimethylphenyl isocyanide (DIC), tert -butyl isocyanide (TIC) and tosylmethyl isocyanide (TosMIC) into the Pd−Me bond of complexes (N⌒N)Pd(Me)Cl (N⌒N = 2,2‘-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2‘-bipyrimidine (bpm)) afforded quantitatively (N⌒N)Pd(C( N R)Me)Cl (R = 2,6-Me 2 C 6 H 3, t -Bu, CH 2 Tosyl). The course of the reaction has been shown to proceed via the intermediates [(N⌒N)Pd(CN−R)(Me)]Cl (N⌒N = bpy, phen; R = 2,6-Me 2 C 6 H 3, t -Bu, CH 2 Tosyl), which have been characterized at 250 K by NMR and IR spectroscopies and conductivity measurements. The mechanism of the 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 [(N⌒N)Pd(CN−R)(Me)]Cl (N⌒N = bpy, phen; R = 2,6-Me 2 C 6 H 3, t -Bu, CH 2 Tosyl) also occurs in the solid state. The complexes (N⌒N)Pd(C( N R)Me)X (N⌒N = 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 1 H NMR experiments showed that the mechanism of this process involves Pd−N bond breaking and subsequent isomerization via a Y-shaped intermediate.