An Oxidation of Benzyl Methyl Ethers with NBS that Selectively Affords Either Aromatic Aldehydes or Aromatic Methyl Esters

Abstract

Either mono-or di-bromination of benzyl methyl ethers can be achieved by controlling the amount of NBS and the temperature. Elimination of methyl bromide from the monobrominated intermediates produces aromatic aldehydes, whereas hydrolysis of the dibrominated intermediates affords aromatic methyl esters in good yields.cushman@purdue.edu . § On leave from Faculty of Pharmacy, Al-Azhar University, Cairo 11884, Egypt.Supporting Information Available. Preparation of methyl ethers, characterization data, copies of NMR spectra for all new and known compounds. This material is available free of charge via the internet at http://pubs.acs.org. This article describes a method to selectively convert benzyl methyl ethers to either aromatic aldehydes or aromatic methyl esters by reaction with either one or two equivalents of NBS in carbon tetrachloride. The conversion of benzyl methyl ethers to the corresponding methyl esters by NBS has not been previously reported. NIH Public AccessInitially, NBS oxidative cleavage of dichlorobenzyl methyl ether in refluxing CCl 4 was studied, utilizing excess NBS. The reaction mixture was illuminated by a normal 60-watt light bulb (Table 1, entry 5). The purification method was optimized by adding dilute NaOH to remove the unreacted NBS and the reaction by-products (succinimide and HBr). As reported, the corresponding aldehyde was obtained in low yield. Interestingly, the major product was the corresponding methyl ester, which may be formed through reaction of the dibromobenzyl intermediate 8 with NaOH (Scheme 1). Next, the conditions for each oxidation type were examined. To determine whether higher temperatures are essential for bromide elimination and methyl ether cleavage to produce the aldehyde, the reaction was conducted at room temperature (entry 3). Interestingly, the only isolable product was the methyl ester, in excellent yield, with no detectable aldehyde.The initial step in the proposed mechanism is formation of a monobromo intermediate 4 (Scheme 1) that can either break down, at higher temperature, into an aldehyde, or undergo an immediate second free-radical bromination. The relatively unstable dibromomethoxyl intermediate 8 may react with 0.1 M NaOH to afford the corresponding ester (Scheme 1). The reported 3 moderate yields of aldehydes may be due to the use of excess NBS, leading to formation of dibromomethoxymethyl intermediates. These intermediates may decompose at high reaction temperature. Unlike the reported N-benzylsuccinimide intermediates 3 obtained by conducting the reaction at high temperature, dibromomethoxymethyl intermediates are hypothesized to be formed under very mild conditions.With respect to the proposed reaction mechanism, the optimal NBS stoichiometry is informative. In the case of aldehyde formation, using 1.0 equivalent of NBS afforded a slightly better yield than reported. 3 In the case of ester formation, 2.0 equivalents of NBS are necessary for optimal yield. Using more than the optimal NBS equivalent(s) resulted in lower yields.With the o...

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