Reactivity of diaryloxy palladium complex with TMEDA (N,N,N′N′-tetramethylethylenediamine) ligand toward carbon monoxide and carbon dioxide

The reactivity of the diaryloxy palladium complex toward carbon monoxide was investigated relevant to the mechanism of the palladium-catalyzed oxidative carbonylation of phenol to produce diphenyl carbonate (DPC). (TMEDA)Pd(OC6H4-p-t-Bu)(2) (1) reacted at high CO pressures (10-80 atm) at 100 degreesC to give the di(p-tert-butyl)phenyl carbonate. The yield of the carbonate increased with the increase in the CO pressure and with the addition of triphenylphosphine. The C-13{1H} NMR and IR spectroscopic studies at room temperature under high CO pressure (50 atm) revealed that CO inserts into one of the Pd-O bonds in 1. Thus, the formation of palladium phenoxide followed by carbonylation and subsequent reductive elimination is considered to be one of the possible DPC formation routes in the oxidative carbonylation of phenol; the reductive elimination is slower than the carbonylation. The reactivity of 1 with carbon dioxide was also examined relating to the DPC synthesis from carbon dioxide and phenol. The reaction of I with CO2 led to the formation of a palladium carbonate complex, (TMEDA)Pd(eta(2)-CO3) (2), whose molecular structure was determined by X-ray crystallography.