Effects of bulky ligands and water in Pd-catalyzed oxidative carbonylation of phenol

Abstract

A diaryloxy Pd complex with a bulky 6,6′-dimethyl-2,2′-bipyridyl (6,6′-Me 2bpy) ligand reacted with pressurized CO (5 MPa) at 25 °C to produce a diaryl carbonate, whereas a diaryloxy Pd complex with an unsubstituted 2,2′-bipyridyl (bpy) ligand hardly reacted. 1H and 13C NMR studies revealed that CO inserts into one of the Pd–O bonds in the latter complex to form a Pd aryloxycarbonyl complex, but that the subsequent reductive elimination of diaryl carbonate is slow. This is consistent with the much higher catalytic activity of the Pd-(6,6′-Me 2bpy) system for the oxidative carbonylation of phenol compared to the Pd–bpy system. To verify the steric effects of the ligands, the catalytic performance was also examined using 2,2′-bioxazolyl ligands with various substituents. Introducing bulky substituents at the 4,4′-position effectively promoted the catalytic reaction. The TONs of DPC increased in the following order: methyl < benzyl < iso-butyl < tert-butyl. The methylene-bridged bioxazolyl ligand with tert-butyl groups gave the highest TON (54 mol-DPC/mol-Pd in 3 h), which is higher than the TON for the 6,6′-Me 2bpy ligand. The addition of molecular sieve 3A to the reaction system further improved the TON and suppressed phenyl salicylate formation. The addition of the molecular sieve also prevented CO 2 formation, probably due to suppression of the reaction between CO and water, in addition to suppression of the hydrolysis of DPC.