Selenite-catalyzed oxidative carbonylation of alcohols to dialkyl carbonates

The oxidative carbonylation of alcohols to prepare corresponding dialkyl carbonates was investigated using alkali metal methylselenite [MSeO₂(OCH₃)] and alkali metal selenites (M₂SeO₃, M = Li, Na, K, Cs) as catalysts. Of various alcohols tested, methanol and 2-methoxyethanol (MEG) are found to be highly reactive, affording corresponding dialkyl carbonates in yields of more than 30%, which are significantly higher than those achieved with Cu-based catalysts under similar reaction conditions. XRD and GC-Mass analysis of the spent catalyst recovered from the M₂SeO₃-catalyzed reaction of methanol revealed that M₂SeO₃ was converted into elemental selenium and MHCO₃ along with the formation of several selenium-containing by-products including malodorous dimethyldiselenide, 1,2-dimethoxydiselane, and O,Se-dimethyl carbonoselenoate. On the contrary, the oxidative carbonylation of MEG with K₂SeO₃ was highly selective toward the formation of bis(2-methoxyethyl) carbonate (BMEC), without producing any selenium-containing by-products. A plausible mechanism for the MSeO₂(OCH₃)- and M₂SeO₃-catalyzed oxidative carbonylation of MEG was suggested on the basis of mechanistic and experimental results.