Article

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

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Abstract

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 CH3 OCOOR (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.

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... The polarity of solvent was significantly affecting on selectivity of methylation where dimethylformamide (DMF) and triglyme showed higher rate of methylation. [122] The 1,8-Diazabicyclo [5.4.0]undec-7-ene DBU and microwave accelerated green chemistry in methylation of phenols with dimethyl carbonate as methylating agent and DMF as solvent was studied by Shieh et al. [123] and reported 80-fold enhancement in the rate of reaction. ...
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Reaction of phenols in dimethyl carbonate in the presence of cesium carbonate at 120-160° C gave aryl methyl ethers in good yields, whereas the reaction of aliphatic alcohols gave the corresponding alkyl carbonates. This method provides a useful synthetic method for preparation of various aryl methyl ethers without using toxic methyl iodide or dimethyl sulfate. O-Methylation of the aromatic hydroxy group of estradiol was carried out in 2 steps without protection of the alcoholic hydroxy group in the same molecule.
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A stereoelectronic effect in bimolecular nucleophilic substitution at a benzylic center has been observed in the rates of reaction of thiourea in dimethyl-d6 sulfoxide with a series of sulfonium perchlorates with differing degrees of constraint in the orientation of the reacting C-S+ bond with respect to the plane of the aromatic ring. The substrates and their relative rates (at 37°C, unless otherwise noted) are as follows: (a, highly constrained) 2-ethyl-1,3-dihydrobenzo[c]thiophenium perchlorate (4b), 1.00, and 2-isobutyl-1,3-dihydrobenzo[c]thiophenium perchlorate (4c), 2.8; (b, partly constrained) 2-ethyl-3,4-dihydro-1H-2-benzothiopyranium perchlorate (7), 45 (at 82°C); (c, unconstrained) S,S-dibenzyl-S-ethylsulfonium perchlorate (5b), 8.1 × 103. An (8 × 103)-fold rate difference was also observed between S,S-dibenzyl-S-ethylsulfonium tetrafluoroborate (5a) and 2-ethyl-1,3-dihydrobenzo[c]thiophenium tetrafluoroborate (4a) with potassium thiocyanate-18-crown-6 in acetonitrile-d3. Analysis of these rate differences, taken with inspection of molecular models which show that the observed rate differences are not adequately accounted for by simple nonbonding or angle strain effects, clearly indicates a dihedral angle dependent factor consistent with π-overlap between p-orbitals on the aromatic ring and the carbon atom undergoing substitution. The "orbital-overlap factor" is estimated to increase the reactivity of the benzylic center in 5b (4 × 103)-fold relative to that of the corresponding ethyl group (in 21). A single-crystal X-ray analysis on 4c showed the salt crystallizes in space group P21/n, unit cell dimensions a = 17.382 (1) Å, b = 9.1731 (6) Å, c = 8.7828 (3) Å, and β = 91.7 (2)°, with Z = 4. On the basis of 2054 unique data with F2 > 2σ, full matrix refinement converged at R = 0.044 for 232 variables. There is a dihedral angle of 3.8° between the C2,S,C9 plane and the plane of the aromatic ring consistent with the observed low reactivity of the benzylic center and the present analysis. The entropies of activation for benzylic substitution of the cyclic and acyclic substrates (4b, 4c, and 5b) with thiourea in Me2SO-d6 are very similar (-12 ± 1 cal mol-1 K-1), in contrast to the great ΔS† difference reported for an analogous α-carbonyl system by Bartlett and Trachtenberg; the origin of this difference is discussed.
Article
Poly(ethylene glycol)-400 (PEG-400) was used to catalyze the allylation of phenoxide with allyl bromide in the two-phase reaction system. n-Decane was employed as the aprotic organic solvent phase, and a mixture of H2O and PEG-400 was used as the protic solvent phase. The initial O-allylation rate increased dramatically with the increase of PEG-400 content in the protic solvent phase. The reaction rate in n-decane/PEG-400 was about 60 times the reaction rate in n-decane/H2O. The extraction constant of allyl bromide and the reaction rate constant for the allylation of phenoxide in a H2O-PEG-400 homogeneous phase were determined to assist in identifying the mechanism of the two-phase reaction. It was found that the reaction occurred both in the aprotic organic solvent phase and in the protic solvent phase when the amount of PEG-400 in the protic solvent phase was less than 60%. However, the reaction only took place in the H2O-PEG-400 phase when the amount of PEG-400 in the protic solvent phase was greater than 60%.