Alkyl Methyl Carbonates as Methylating Agents. The O-Methylation of Phenols

The O-methylation reaction of a variety of phenols (ArOH: Ar = Ph, p-CH₃C₆H₄, p-ClC₆H₄, o- and p-CH₃COC₆H₄, and 2-naphthyl) can be conducted in a highly selective manner by using asymmetric alkyl methyl carbonates CH₃OCOOR (R = n-Pr, 3b; n-Bu, 3d; CH₃O(CH₂)₂O(CH₂)₂, 3e) as alkylating agents. For example, at 150 °C, phenol can be quantitatively converted into anisole in 4.5 h, using 2-(2-methoxyethoxy)ethyl methyl carbonate 3e in the presence of K₂CO₃ as a catalyst. Compared to the methylation reactions using dimethyl carbonate which require sealed pressurized reaction vessels, asymmetric alkyl methyl carbonates allow much simpler and safer alkylations at ambient pressure. The selectivity towards O-methylation is scarcely affected by the temperature (in the range of 120-150 °C), while it depends on the nature and on the amount of the solvent. DMF and triglyme (triethylene glycol dimethyl ether) have proven to be the better reaction media.

alkyl carbonates - O-alkylation - methyl selectivity - anisoles - methylation