Microwave-assisted methylation of dihydroxybenzene derivatives with dimethyl carbonate

Abstract

Using a focused microwave reactor, methylation with dimethyl carbonate (DMC) of 1,2-and 1,4-dihydroxybenzene derivatives, found in the product spectrum of lignin depolymerisation, leads to the respective aromatic bis-methyl ethers with excellent isolated yields. Stoichiometric as well as catalytic amounts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are effective for the bis-methylation of these dihydroxybenzenes at relatively mild temperatures (160-190 C). Conversion of resorcinol (1,3-dihydroxybenzene) under similar conditions leads to a mixture of 1,3-dimethoxybenzene and methyl 2,4-dimethoxybenzoate. The unusual reactivity of resorcinol's phenyl ring towards DMC can be explained by the synergic effect of its two strongly activating ortho/para directing groups.

Methylation with Dimethyl Carbonate/Dimethyl Sulfide Mixtures: An Integrated Process without Addition of Acid/Base and Formation of Residual Salts

“…ChemSusChem ring). [41] This may be because the concentration of the basic species was low throughout our methylation process, and thus strongly activating resorcinolate dianions were not formed, preventing the electrophilic aromatic substitution from occurring. However, the methylation of two trihydroxybenzenes (phloroglucinol and pyrogallol) demonstrated their contrasting reactivities.…”

“…This was because the hydroxy substituents in phloroglucinol, which all occupy 2-and 4-positions with respect to the unsubstituted positions on the benzene ring, have synergistic ortho-/para-directing effects for the aromatic ring to undergo electrophilic aromatic substitutions. [41] The noncarboxymethylation of phloroglucinol may have been due to the steric inaccessibility of the carbonyl moiety of DMC. [41]…”

“…Homogeneous and heterogeneous catalytic systems have been used for DMC methylation, and amine bases are commonly used as homogeneous organocatalysts. [24,33,41] It has been proposed that some homogeneous organocatalysts also serve as nucleophilic activators, by generating a stronger base and/or more reactive methylating agents from DMC. For example, Glasnov et al described a methylation process that used tributylamine (n-Bu 3 N) as a catalyst in a DMC-DMF solution, and suggested that it proceeded via the in-situ formation of the basic organic salt [n-Bu 3 NMe] + [OCO 2 Me] À .…”

Conservative evolution and industrial metabolism in Green Chemistry

“…An almost quantitative conversion of catechol and hydroquinone toward the corresponding bis-methylated products were achieved in excellent isolated yields at 170°C. 56…”

One-Pot Synthesis of Al-P-O Catalysts and Their Catalytic Properties for O-Methylation of Catechol and Methanol

“…Recently, vapour-phase selective O-methylation of catechol to synthesize guaiacol, which is an economically and environmentally friendly route for industrial applications, has attracted extensive attention from scientific researchers [16][17][18][19][20]. Methanol is believed to be a very promising candidate for use as a methylation agent for selective O-methylation of catechol due to its low toxicity and low cost.…”