Insertion of isocyanides into the palladium—carbon bond of C2-palladated heterocycles. Synthesis of trans-[PdCl{C(R N)NR}(PPh 3) 2] complexes (R N = 2-pyridyl, 2-pyrazyl; R = alkyl or aryl group)

Abstract

The titles complexes trans-[PdCl{C(R N)NR}(PPh 3) 2] (R N = 2-pyridyl (2-py), R = p-C 6H 4OMe, Me; R N = 2-pyrazyl (2-pyz), R = p-C 6H 4OMe) can be prepared by reaction of the N-protonated compounds, cis-[PdCl 2(R NH)(PPh 3] (R NH = 2-pyridylium (2-pyH) or 2-pyrazylium (2-pyzH) group), with PPh 3, followed by addition of the isocyanide CNR and deprotonation with triethylamine, in a molar ratio Pd/PPh 3/CNR/NEt 3 of 1/1/1/1.1. The reaction sequence involves the successive formation of the cationic intermediates trans-[PdCl(R NH)(PPh 3) 2] +, trans-[Pd(R NH)(CNR)(PPh 3) 2] 2+ and trans-[Pd(R N)(CNR)(PPh 3) 2] +, which were isolated and characterized as perchlorate salts for R N = 2-pyridyl. In the final step the coordinated isocyanide of trans-[Pd(R N)(CNR)(PPh 3) 2] + undergoes migratory insertion into the Pd-R N σ bond, promoted by the chloride ions progressively displaced by the entering neutral ligands from cis-[PdCl 2(R NH)(PPh 3)]. The resulting products were characterized by conventional spectroscopic techniques and, for R N = 2-pyridyl and R = p-C 6H 4OMe, also by ligand substitution reaction at the palladium center and by protonation and coordination of the p-methoxyphenylimino(2-pyridyl)methyl group with strong mineral acids (HClO 4, HCl) and ZnCl 2, respectively.