Catalytic Hydrogenation Using Abnormal N-Heterocyclic Carbene Palladium Complexes: Catalytic Scope and Mechanistic Insights

Palladium complexes containing abnormally bound C4-bound dicarbene ligands have been exploited for catalytic alkene hydrogenation. Comparison to normally C2-bound homologues indicates that the carbene bonding mode critically influences the catalytic activity. Good catalytic performance in the hydrogenation of cis-disubstituted olefins and non-isomerizable terminal olefins under mild conditions (RT, 0.1 MPa H-2) only occurs when the carbene is abnormally bound to the palladium center. Detailed mechanistic investigations using dynamic light scattering in connection with time-dependent analysis of conversions, and also performance of substoichiometric catalytic experiments provide evidence that the catalysis is heterogeneous and that the abnormally bound carbene ligand has the role of an activator.