Impact of the Substitution Pattern in N–heterocyclic Olefin-Substituted Phosphines on Palladium Catalysis

Abstract

In recent years, N-heterocyclic olefins (NHOs) have been recognized for their strong electron-donating properties. These properties have been utilized in the design of phosphine ligands (NHOPs) to enhance the electron density in palladium complexes, thereby promoting palladium-catalyzed cross-coupling reactions. In this study, the impact of the substituent in the olefin backbone was investigated. NHOP ligands with methyl and phenyl moieties have been prepared and their catalytic performance compared with the hydrogen-substituted analogs. The increased ligand bulk in the backbone results in a reduced catalytic activity in selected Pd-catalyzed Buchwald–Hartwig aminations. The unfavorable steric profile, as revealed by the crystal structures of the ligands and the observation of various decomposition products, is proposed as the reason for the diminished activity.