Mechanism of formation of carbene complexes by reaction of cis-[PdCl2(PPh3)(CNR)] with secondary amines

A kinetic study is reported for the reactions of secondary aromatic amines p-YC₆H₄NHR (Y = MeO, Me, H; R = Me, Et) with the isocyanide complexes cis-[PdCl₂(p-XC₆H₄NC)(PPh₃)] (X = Me, H, Cl) leading to the carbene derivatives cis-[PdCl₂ {C(NH-p-C₆H₄X)NR-p-C₆H₄Y} (PPh₃)] in 1,2-dichloroethane at 25°C. A stepwise mechanism is proposed which involves a direct nucleophilic attack of the entering amine on the isocyanide carbon followed by proton transfers to the final carbene complexes. These take place both intramolecularly in a four-membered cyclic transition state and by the agency of one further amine molecule serving as a proton acceptor-donor in a six-membered transition state. Competition experiments with primary amines and trends in rate parameters are discussed to support the mechanism.