ABSTRACT
In the past two decades, dimethyl carbonate (DMC) has emerged as a nontoxic green alternative methylating reagent [4] to replace dimethyl sulfate or methyl halide, and a number of methods have been reported for DMC-mediated processes [5, 6]. For example, methylation of 2-naphthol has been reported using potassium carbonate as catalyst in the presence of tetramethylammonium chloride as methylating reagent with 87% yield [7]. Quantitative yield of 1,6-dimethoxynaphthalene is recovered when the reaction is performed using sodium hydroxide as catalyst using dimethyl sulfate in ethanol [8]. On the other hand, DMC has been used as a green methylating reagent with a wide range of catalysts such as uorine-containing metal oxides [9], potassium carbonate [10], ionic liquids [11], potassium carbonate combined with PEG 1000 [12], Hβ [13], NaX [14], 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) [15, 16], KBr/SiO2 [17], tetrabutylammonium bromide as phase transfer catalyst [18], tetrabutylammonium bromide as catalyst under semicontinuous process [19], calcined Mg-Al hydrotalcites [20], and C,N-chelated organotin(IV) compounds [21]. NaX has also been used as catalyst for the methylation of phenol using DMC in the presence of dimethyl sulfoxide (DMSO) as solvent [22]. However, in most of the cases, DMC was used as solvent and reagent, which reduces the overall cost and workup procedure. The green metric analysis of LDH-Met is compared with the reported catalytic systems in methylation of phenol (Table 9.1).
