Abstract
Abstract In this study, an efficient method for O-methylation of phenolic compound using tetramethylammonium hydroxide (TMAOH) as a new alkylating agent under microwave irradiation was developed. Ethanol was selected as a green reaction media to accomplish this protocol under environmentallybenign conditions. Less than half hour time is needed to obtain good to excellent yields of products at temperature of about 120°C. Using this procedure, phenolic compounds could be converted to the corresponding aryl methyl derivatives quickly. The results indicated that different phenolic compounds bearing both electron-withdrawing and electron-donating groups are O-methylated effectively using TMAOH under microwave irradiation. This novel protocol has many improvements in view point of reaction yields, selectivity and conditions compared to previous studies. Trimethylamine and water were the side products of O-methylation reaction which can be recovered easily from the product mixture. This methodology showed high selectivity for O-methylation and no C-methylated products were observed.
Highlights
This reaction is carried out in the presence of methanol and potassium carbonate while the conversion was not satisfactory. Quaternary amines such as phenyltrimethylammonium salts was employed for O-methylation of phenolic compounds in alkaloids chemistry [29]
Tetramethylammonium chloride is reported to be used as a methylating agent for the methylation of phenols in the presence of K2CO3 or Cs2CO3 as base and dimethoxymethane as a solvent at 145°C under microwave conditions [29]
The optimized condition for conversion of 2-naphtol to 2-methoxynaphthalene using tetramethylammonium hydroxide (TMAH) as a new methylating agent was selected as entry 3 of Table 1
Summary
By [13, Lemma 1.w3a] seaocbhtaeinigeedninsphaicgeh oyfieAld,sAa∗lohnagswdiitmh eanhsiigohnsoelneec.tiLvietty.{θi}di= deno values of A. Trimethylsulfonium salts was used for methylation of phenols [28] This reaction is carried out in the presence of methanol and potassium carbonate while the conversion was not satisfactory. In this situation, quaternary amines such as phenyltrimethylammonium salts was employed for O-methylation of phenolic compounds in alkaloids chemistry [29]. Tetramethylammonium chloride is reported to be used as a methylating agent for the methylation of phenols in the presence of K2CO3 or Cs2CO3 as base and dimethoxymethane as a solvent at 145°C under microwave conditions [29]. We report a green and fast process for O-methylation of phenolic compounds using tetramethylammonium hydroxide (TMAH) in ethanol as green solvent at 120°C under microwave conditions without using base (Figure 1). O-methylation of phenolic compounds was achieved under more environmental conditions without using stoichiometric amount of base and using TMAOH as an abundant available and cheap methylating agent
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