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Efficient and selective microwave-assisted O-methylation of phenolic compounds using tetramethylammonium hydroxide (TMAH)

  • Fatemeh Gholipour , Mohammad Rahmani EMAIL logo and Farhad Panahi
Published/Copyright: May 27, 2019
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 8 Issue 1

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.

Keywords: microwave-assisted; O-methylation; phenolic compound; tetramethylammonium hydroxide

1 Introduction

O-Methylation of phenolic compounds is an important strategy in organic chemistry due to its high applicability in synthesis of many petrochemicals, fine chemicals, pharmaceuticals, fragrances, dyes and agrochemicals [1].

There are several reports in the literature on the synthesis of methyl aryl ethers using methylation of phenolic compounds (Figure 1).

Figure 1 Different methylating agents used for O-methylation of phenolic compounds.
Figure 1

Different methylating agents used for O-methylation of phenolic compounds.

In traditional methods toxic methylating agents such as methyl iodide [2,3] methyl bromide [3], dimethyl sulfate1 [4, 5, 6] and diazomethane [7] have been used. Methyl halides are toxic gaseous materials because they can methylate nucleic acids in living organisms [8]. To use dimethyl sulfate as alkylating agent stoichiometric amount of strong base for neutralization of acidic by-products is required [4,5]. Due to the growing awareness of environmental issues, the commercial application of these material are restricted. In order to avoid the use of mentioned methylating agents other methods have been developed. The O-methylation of phenol with methanol in the presence of a strong acid has been widely investigated [9, 10, 11, 12, 13]. This method is an acid catalyzed process and the product selectivity depends on properties of the used acid catalyst. In most cases, the conversions of phenols are not satisfactory and the C-methylation is the competition reaction and a large amount of byproduct is produced which complicated the workup and purification process. A clean, and selective methylation protocols was developed using dimethyl carbonate as a less-toxic reagent [14,15]. The reaction of phenol with dimethyl carbonate was also carried out under gas/liquid phase transfer catalysis conditions in the presence of K2CO3 coated with polyethylene glycol [16,17]. Using this reagent, anisole was obtained in high yields along with a high selectivity. However this reagent has serious limitations to be used for high boiling point phenols [18]. The vapor phase methylation of phenol with dimethyl carbonate was also performed over ion-exchanged zeolites in the continuous-flow conditions [19, 20, 21, 22]. However, this reaction is carried out at high temperature and two byproducts including o-cresol and 2,6-xylenol are produced beside to anisole.

In this situation, methylation of both phenol and p-cresol using dimethyl carbonate were performed in a continuously fed stirred tank reactor [18]. The O-methylation of phenol with dimethyl carbonate has also been reported over 1-butyl-3-methylimidazolium chloride [23,24], potassium carbonate [25], magnesium oxide [26], Mg-Al mixed oxides [27]. Unfortunately, in all of the mentioned methods for methylation using dimethyl carbonate, C-alkylation by-products are always detected. Trimethylsulfonium salts was also 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]. For example, phenyltrimethylammonium chloride, in the presence of potassium carbonate in dry dimethylformamide (DMF) afforded glaucine from its starting material in good yield [30]. The O-methylation of phenols with trimethylammonium acetate was occurred at temperature of 200-230°C and use of CaO as a base [31]. Tetramethylammonium chloride is also 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]. Other reagents used for O-methylation of phenols under microwave conditions are trimethylphosphit and trimethyl phosphate [32].

Herein 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). Using our procedure the 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.

2 Experimental

2.1 General

All phenols and naphthols and tetramethylammonium hydroxide were commercially available and purchased from Merck and Aldrich chemical companies. Tetramethylammonium hydroxide is 25% solution in water. All products were known and their spectroscopic data were compared with those of authentic compounds. Analyses of the products were done on a GC-MS (Agilent 6890 N-GC-5973 N-MSD chromatograph, using a 30 m × 0.25 mm Restek, Rtx-5SILMS column with a film layer of 0.25 μm. The initial temperature of column was 50°C for 1 min, followed by programming at 10°C/min up to 290°C and a final period at 290°C for 24 min. The temperature of the injector was 250°C. The carrier gas was helium and the operation mode was split100). Apart from the unconverted reactant 2-naphthol, the products were 2-methoxynaphthalene, and traces of 1-methyl-2-methoxynaphthalene.

2.2 General procedure for O-methylation of phenolic compounds using TMAH under MW irradiation

A mixture of 1 mmol phenol derivates (0.144 gr 2-naphthol, 0.094 gr phenol, 0.173 gr 4-bromophenol, …) and 1 mmol tetramethylammonium hydroxide (0.34 mL TMAOH 25%) and 6 mL ethanol was placed in a sealed teflon container (screw cap type, 50 mL) and subjected to microwave irradiation in a microwave oven (Anton Paar 3000) at 120°C for 27 min. After cooling, the products were analyzed by GC–MS.

To isolate the product, after completion of the reaction the solvent was removed to obtain a concentrated mixture and ethyl acetate (20 mL) was added to the reaction mixture and it was washed with water (2 x 10 mL). Then the organic layer was dried over Na2SO4 and after removing solvent the crude product was obtained which purified by column chromatography.

2-Methoxynaphthalene (2a): 1H NMR (300 MHz, CDCl3): δ 7.78-7.55 (m, 1H), 7.41-7.31 (m, 1H), 7.24 (ddd, J = 8.1, 6.9, 1.3 Hz, 1H), 7.09-6.98 (m, 1H), 3.83 (s, 1H).

Anisole (2K): 1H NMR (300 MHz, CDCl3): δ 7.32-7.08 (m, 1H), 6.93-6.68 (m, 1H), 3.68 (s, 2H).

3 Results and discussion

In order to find optimized conditions for our protocol, a reaction model was selected and different reaction conditions were checked (Table 1).

First, we checked the reaction in water as a green solvent and no product was observed (Table 1, entry 1). Also, no product was detected using glycerol as solvent (Table 1, entry 2). Interestingly, in ethanol solvent about 70% of 2-methoxynaphthalene (Figure 2a) was produced (Table 1, entry 3). Other solvents including dimethylformamide (DMF) and tetrahydrofuran (THF) were checked and no superiority related ethanol was observed thus ethanol was selected as solvent for our protocol (Table 1, entries 4 and 5). Then, the reaction time to obtain maximum yield of product was changed and 27 min was sufficient for high conversion (Table 1, entries 6-8). Temperature was other parameter which we investigated and 120°C as optimal temperature was selected. The optimized condition for conversion of 2-naphtol to 2-methoxynaphthalene using TMAH as a new methylating agent was selected as entry 3 of Table 1

Figure 2 Products of O-methylation of different phenolic compounds using TMAH reagent under MW irradiation. a) Reaction conditions: phenolic compound (1 mmol), TMAH (1 mmol), solvent (6 mL). b) All yields refer to GC yield. c) Isolated yields.
Figure 2

Products of O-methylation of different phenolic compounds using TMAH reagent under MW irradiation. a) Reaction conditions: phenolic compound (1 mmol), TMAH (1 mmol), solvent (6 mL). b) All yields refer to GC yield. c) Isolated yields.

After optimization study in order to show the generality of this process in methylation of other substrates several phenolic compounds containing different functional groups were checked and results are given in Figure 2.

Table 1

Optimization of reaction condition.a

EntrySolventTemp (°C)Time (min)Yield (%)b
1H2O120270
2Glycerol120270
3Etdanol1202770
4DMF1202770
5tdF1202750
6Etdanol1203071
7Etdanol1203569
8Etdanol1202560
9Etdanol1102760
10Etdanol80270
11Etdanol1302771

  1. a Reaction condition: 2-naphtol (1 mmol), TMAH (1 mmol), solvent (6 mL)

    b GC yield.

Results in Figure 2 show that different phenolic compounds bearing both electron-withdrawing and electron-donating groups can O-methylated effectively using TMAH under our optimized conditions. O-Methylation of 1-naphtol gave corresponding product in 83% yield (Figure 2b). Good yields of products from O-methylation of nitro-phenols were obtained (Figures 2c and 2d). Also, halogen-substituted phenols undergo in the reaction and give satisfactory yields of corresponding ethers (Figures 2e-g). Both hydroxyl group of pyrocatechol effectively methylated under optimized conditions and 64% of 1,2-dimethoxybenzene was produced (Figure 2h). An acceptable yield of product was obtained from 4-benzylphenol using TMAH reagent under MW conditions (Figure 2i). 2,6-Dimethylphenol as a sterically hindered reagent was used and interestingly 88% yield of 2-methoxy-1,3-dimethylbenzene (Figure 2j) was detected. Using this methodology it is possible to convert phenol to anisole (Figure 2k) as a solvent in organic chemistry in high yield of 91%. Pyridin-4-ol as a heterocyclic phenol (Figure 2l) was employed in this protocol to evaluate its efficiency in synthesis of heterocyclic methyl ethers. The tolerance of cyano, aldehyde and ester functional groups related to the optimized reaction conditions was also checked and corresponding anisoles were produced in high yields without any change in molecular functionality (Figures 2m-o). Overall, this method is very useful and practical for selective O-methylation of phenolic compounds in order to synthesis diverse anisole derivatives quickly.

In view point of reaction mechanism (Figure 3), it seems that, phenolic compound in the presence of TMAH lose a proton so it converted to a phenoxide ion with counter ion of tetramethylammonium ion. In this situation, under reaction conditions phenoxide attack to one the methyl group of tetramethylammonium to afford the O-methylated product.

Figure 3 The reaction mechanism of methylation of phenolic component in the presence of TMAH.
Figure 3

The reaction mechanism of methylation of phenolic component in the presence of TMAH.

As observable in Figure 3 the byproducts of this reaction are water and trimethylamine. Because trimethylamine is soluble in water to concentration of 40% in equilibrium with TMAH, this unpleasant smelling compound is trapped and the bad smell of this byproduct is not feeling after completion of reaction at room temperature.

In order to show the merit and applicability of this new methodology in O-methylation of phenolic compounds a table of comparison was provided. Table 2 demonstrates that this developed procedure has some modifications in view pints of reaction yields and conditions compared to previous reports.

Table 2

Different methods for O-methylation of phenolic compounds.

Methylating AgentBaseSolventSolid CatalystTemperature (°C)YieldRef.
Methyl iodideKOHDMSO–5585[3]
Metdyl bromideKOHDMSO–5573[3]
Dimetdyl sulfateNaOHWater–10047[4]
Dimetdyl sulfateKOH–Zeolite56[1]
Silica gel
Dimetdyl sulfate–CyclohexaneAlumina3036[5]
Diazometdane––Silica gel3099[7]
MetdanolKOHH2SO4Alumina26078[10]
Metdanol––Cs/SiO240092[11]
Metdanol––Zeolite Y20015[12]
Dimetdyl carbonate––ZSM520026[19]
Dimetdyl carbonate–[BMIm]Cl–12099[23]
Dimetdyl carbonate––Mg/Al oxide20099[26]
Trialkylsulfonium saltK2CO3CH2Cl2–3055[28]
Acetone82
Tetrametdyl ammonium chlorideK2CO3Dimetdoxy etdane–14587[29]
Cs2CO3
TMAH–Etdanol–12091This work

4 Conclusion

In conclusion, we have illustrated a novel protocol for efficient and selective O-methylations of phenolic compounds to evaluate the potential of TMAOH as a new methylating agent. Using this methodology phenolic compounds can be converted to aryl methyl derivatives in high yields. This procedure was accomplished in ethanol as a green solvent and under microwave irradiation. The side products of this reaction are trimethylamine and water which can be recovered from the reaction. TMAOH can be used successfully as methylating agent for O-methylation of various phenols and naphthols.

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Received: 2018-10-12
Accepted: 2019-03-11
Published Online: 2019-05-27
Published in Print: 2019-01-28

© 2019 Gholipour et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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  74. Synthesis of vitamin E succinate catalyzed by nano-SiO2 immobilized DMAP derivative in mixed solvent system
  75. Extraction of phytosterols from melon (Cucumis melo) seeds by supercritical CO2 as a clean technology
  76. Production of uronic acids by hydrothermolysis of pectin as a model substance for plant biomass waste
  77. Biofabrication of highly pure copper oxide nanoparticles using wheat seed extract and their catalytic activity: A mechanistic approach
  78. Intelligent modeling and optimization of emulsion aggregation method for producing green printing ink
  79. Improved removal of methylene blue on modified hierarchical zeolite Y: Achieved by a “destructive-constructive” method
  80. Two different facile and efficient approaches for the synthesis of various N-arylacetamides via N-acetylation of arylamines and straightforward one-pot reductive acetylation of nitroarenes promoted by recyclable CuFe2O4 nanoparticles in water
  81. Optimization of acid catalyzed esterification and mixed metal oxide catalyzed transesterification for biodiesel production from Moringa oleifera oil
  82. Kinetics and the fluidity of the stearic acid esters with different carbon backbones
  83. Aiming for a standardized protocol for preparing a process green synthesis report and for ranking multiple synthesis plans to a common target product
  84. Microstructure and luminescence of VO2 (B) nanoparticle synthesis by hydrothermal method
  85. Optimization of uranium removal from uranium plant wastewater by response surface methodology (RSM)
  86. Microwave drying of nickel-containing residue: dielectric properties, kinetics, and energy aspects
  87. Simple and convenient two step synthesis of 5-bromo-2,3-dimethoxy-6-methyl-1,4-benzoquinone
  88. Biodiesel production from waste cooking oil
  89. The effect of activation temperature on structure and properties of blue coke-based activated carbon by CO2 activation
  90. Optimization of reaction parameters for the green synthesis of zero valent iron nanoparticles using pine tree needles
  91. Microwave-assisted protocol for squalene isolation and conversion from oil-deodoriser distillates
  92. Denitrification performance of rare earth tailings-based catalysts
  93. Facile synthesis of silver nanoparticles using Averrhoa bilimbi L and Plum extracts and investigation on the synergistic bioactivity using in vitro models
  94. Green production of AgNPs and their phytostimulatory impact
  95. Photocatalytic activity of Ag/Ni bi-metallic nanoparticles on textile dye removal
  96. Topical Issue: Green Process Engineering / Guest Editors: Martine Poux, Patrick Cognet
  97. Modelling and optimisation of oxidative desulphurisation of tyre-derived oil via central composite design approach
  98. CO2 sequestration by carbonation of olivine: a new process for optimal separation of the solids produced
  99. Organic carbonates synthesis improved by pervaporation for CO2 utilisation
  100. Production of starch nanoparticles through solvent-antisolvent precipitation in a spinning disc reactor
  101. A kinetic study of Zn halide/TBAB-catalysed fixation of CO2 with styrene oxide in propylene carbonate
  102. Topical on Green Process Engineering
https://doi.org/10.1515/gps-2019-0028
Keywords for this article
microwave-assisted; O-methylation; phenolic compound; tetramethylammonium hydroxide
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  3. Flow-mode biodiesel production from palm oil using a pressurized microwave reactor
  4. Reduction of free fatty acids in waste oil for biodiesel production by glycerolysis: investigation and optimization of process parameters
  5. Saccharin: a cheap and mild acidic agent for the synthesis of azo dyes via telescoped dediazotization
  6. Optimization of lipase-catalyzed synthesis of polyethylene glycol stearate in a solvent-free system
  7. Green synthesis of iron oxide nanoparticles using Platanus orientalis leaf extract for antifungal activity
  8. Ultrasound assisted chemical activation of peanut husk for copper removal
  9. Room temperature silanization of Fe3O4 for the preparation of phenyl functionalized magnetic adsorbent for dispersive solid phase extraction for the extraction of phthalates in water
  10. Evaluation of the saponin green extraction from Ziziphus spina-christi leaves using hydrothermal, microwave and Bain-Marie water bath heating methods
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  13. Synergistic effect between CO2 and H2O2 on ethylbenzene oxidation catalyzed by carbon supported heteropolyanion catalysts
  14. Hydrocyanation of 2-arylmethyleneindan-1,3-diones using potassium hexacyanoferrate(II) as a nontoxic cyanating agent
  15. Green synthesis of hydratropic aldehyde from α-methylstyrene catalyzed by Al2O3-supported metal phthalocyanines
  16. Environmentally benign chemical recycling of polycarbonate wastes: comparison of micro- and nano-TiO2 solid support efficiencies
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  24. Green cyclic acetals production by glycerol etherification reaction with benzaldehyde using cationic acidic resin
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  45. Microwave-assisted synthesis of pyrrolidinone derivatives using 1,1’-butylenebis(3-sulfo-3H-imidazol-1-ium) chloride in ethylene glycol
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  54. Assessment of aqueous extract of Gypsophila aretioides for inhibitory effects on calcium carbonate formation
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  57. A reactive coupling process for co-production of solketal and biodiesel
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  62. Green synthesis of copper oxide nanoparticles using Juglans regia leaf extract and assessment of their physico-chemical and biological properties
  63. Deep eutectic solvents (DESs) as powerful and recyclable catalysts and solvents for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones
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  65. Efficient and selective microwave-assisted O-methylation of phenolic compounds using tetramethylammonium hydroxide (TMAH)
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  69. Selenium supplementation during fermentation with sugar beet molasses and Saccharomyces cerevisiae to increase bioethanol production
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  71. Investigating the effectiveness of classical and eco-friendly approaches for synthesis of dialdehydes from organic dihalides
  72. Pyrolysis of palm oil using zeolite catalyst and characterization of the boil-oil
  73. Azadirachta indica leaves extract assisted green synthesis of Ag-TiO2 for degradation of Methylene blue and Rhodamine B dyes in aqueous medium
  74. Synthesis of vitamin E succinate catalyzed by nano-SiO2 immobilized DMAP derivative in mixed solvent system
  75. Extraction of phytosterols from melon (Cucumis melo) seeds by supercritical CO2 as a clean technology
  76. Production of uronic acids by hydrothermolysis of pectin as a model substance for plant biomass waste
  77. Biofabrication of highly pure copper oxide nanoparticles using wheat seed extract and their catalytic activity: A mechanistic approach
  78. Intelligent modeling and optimization of emulsion aggregation method for producing green printing ink
  79. Improved removal of methylene blue on modified hierarchical zeolite Y: Achieved by a “destructive-constructive” method
  80. Two different facile and efficient approaches for the synthesis of various N-arylacetamides via N-acetylation of arylamines and straightforward one-pot reductive acetylation of nitroarenes promoted by recyclable CuFe2O4 nanoparticles in water
  81. Optimization of acid catalyzed esterification and mixed metal oxide catalyzed transesterification for biodiesel production from Moringa oleifera oil
  82. Kinetics and the fluidity of the stearic acid esters with different carbon backbones
  83. Aiming for a standardized protocol for preparing a process green synthesis report and for ranking multiple synthesis plans to a common target product
  84. Microstructure and luminescence of VO2 (B) nanoparticle synthesis by hydrothermal method
  85. Optimization of uranium removal from uranium plant wastewater by response surface methodology (RSM)
  86. Microwave drying of nickel-containing residue: dielectric properties, kinetics, and energy aspects
  87. Simple and convenient two step synthesis of 5-bromo-2,3-dimethoxy-6-methyl-1,4-benzoquinone
  88. Biodiesel production from waste cooking oil
  89. The effect of activation temperature on structure and properties of blue coke-based activated carbon by CO2 activation
  90. Optimization of reaction parameters for the green synthesis of zero valent iron nanoparticles using pine tree needles
  91. Microwave-assisted protocol for squalene isolation and conversion from oil-deodoriser distillates
  92. Denitrification performance of rare earth tailings-based catalysts
  93. Facile synthesis of silver nanoparticles using Averrhoa bilimbi L and Plum extracts and investigation on the synergistic bioactivity using in vitro models
  94. Green production of AgNPs and their phytostimulatory impact
  95. Photocatalytic activity of Ag/Ni bi-metallic nanoparticles on textile dye removal
  96. Topical Issue: Green Process Engineering / Guest Editors: Martine Poux, Patrick Cognet
  97. Modelling and optimisation of oxidative desulphurisation of tyre-derived oil via central composite design approach
  98. CO2 sequestration by carbonation of olivine: a new process for optimal separation of the solids produced
  99. Organic carbonates synthesis improved by pervaporation for CO2 utilisation
  100. Production of starch nanoparticles through solvent-antisolvent precipitation in a spinning disc reactor
  101. A kinetic study of Zn halide/TBAB-catalysed fixation of CO2 with styrene oxide in propylene carbonate
  102. Topical on Green Process Engineering
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