Insertions of Allenes into Palladium−Carbon Bonds of Complexes Containing Bidentate Nitrogen Ligands. Structural and Mechanistic Studies

The insertion reactions of the allenes propadiene and 1,2-heptadiene in the Pd−C bond of complexes (N⌒N)Pd(R)X (N⌒N = 8-PQ, p-An-BIAN, i-Pr-DAB, p-An-DAB, i-Pr-PyCa; R = Me, C(O)Me, C(O)Ph, C(O)i-Pr; X = Cl, Br) have been investigated. An X-ray crystal structure determination of (8-PQ)Pd{(1−3-η)-2-methylpropenyl}Cl exhibited the unexpected monodentate coordination of the nitrogen ligand. The monodentate coordination in apolar solvents and bidentate coordination in polar solvents was demonstrated by means of NOE NMR experiments. Kinetic measurements revealed that the reactions are first order in the palladium concentration and occur via an allene concentration independent and dependent pathway. Reactions of complexes containing flexible bidentate nitrogen ligands were retarded by additional free bidentate nitrogen ligand indicating that initial dissociation of a nitrogen donor is an important step in the reaction. We have strong indications that the migration of the R group to the precoordinated allene is the rate-determining step. Instead of mass-law retardation by excess X- (X = Cl-, Br-), an enhancement of the reaction has been observed in case of the complexes (8-PQ)Pd(Me)Cl, (8-PQ)Pd(Me)Br, and (i-Pr-DAB)Pd(C(O)Me)Cl. Flexible bidentate nitrogen ligands greatly enhance the reaction, owing to the easy formation of an accessible site on the metal center. The insertion of allenes into the Pd−C bonds of complexes containing rigid bidentate nitrogen ligands probably proceeds via initial allene association followed by either halide or nitrogen dissociation and subsequent migration of the R group to the precoordinated allene.