Synthesis and reactivity of phenoxycarbonyl palladium complex: relevant to the mechanism of oxidative carbonylation of phenol

Phenoxycarbonyl palladium complex was synthesized and its reactivity was investigated relevant to the mechanism of the palladium-catalyzed oxidative carbonylation of phenol to produce diphenyl carbonate (DPC). The phenoxycarbonyl palladium complex PdCl(CO2Ph)(PPh3)2 (1) was synthesized by oxidative addition of phenyl chloroformate to Pd(PPh3)4. Complex 1 could be isolated as single crystals and characterized by X-ray crystallography. The thermolysis of 1 resulted in DPC formation, although degradation of the PPh3 ligand to PhCl and PhCO2Ph simultaneously occurred. PdCl2(PPh3)2 was a major newly formed palladium species. An efficient DPC formation was observed for the reaction of 1 with phenyl chloroformate. On the other hand, the reaction of 1 with sodium phenoxide (one equivalent) proceeded at −20 °C causing the instant formation of a new species assignable to Pd(OPh)(CO2Ph)(PPh3)2 (2) as judged by NMR (1H, 13C{1H}, and 31P{1H}) spectroscopy; the nucleophilic attack by phenoxide preferentially took place on the palladium center rather than on the carbonyl group. When the reaction mixture was heated, DPC was produced probably via the reductive elimination from 2. These results as well as the previous finding that diaryl carbonate is formed from palladium diaryloxide by carbonylation and subsequent reductive elimination suggest that Pd(OPh)(CO2Ph)L2 is the final intermediate toward DPC: the reductive elimination requires a relatively high temperature.