Regiospecific Electroacetoxylation of 4-MethylphenyI Acetate to Form 4-Acetoxybenzyl Acetate. A Significant Procedure for Vanillin Synthesis Involving Novel Etherification Methods of Aryl Bromides

Electroacetoxylation of 4-methylphenyl acetate (1) was carried out in AcOH-t-BuOH (9:1 v/v) m m the presence of copper(II) acetate by using carbon electrodes to give the side-chain-oxidized products in 88% selectivity (90% conversion), i.e., 4-acetoxybenzyl acetate (2a, 69%), 4-acetoxybenzaldehyde (4a, 8%), and 4-acetoxybenzylidene diacetate (3, 3%). The electrolysis products 2a, 3, and 4a, either by platinum oxide catalyzed oxygen oxidation or by acid-catalyzed hydrolysis, were smoothly converted to 4-hydroxybenzaldehyde (4b) whose bromination provided 3-bromo-4-hydroxybenzaldehyde (5a, 90%) as well as 3, 5-dibromo-4-hydroxybenzaldehyde (5b, 4.5%). Treatment of 5awith either ROH-NaOH-CaO/DMF-CuCl2 or ROH-BaO-DMF-CuCl2 resulted in vanillin (6a, 94%) and ethylvanillin (6b, 93%), respectively. On the other hand, acid-catalyzed hydrolysis of 2a gave 4-hydroxybenzyl alcohol (11, 89%), and acid-catalyzed alcoholysis of 2a furnished 4-hydroxybenzyl ethers 8a (100%) and 8b (99%). The oxygen oxidation of both 8 and 11 can produce 4b in good yield. 4-Hydroxy-3-methoxy(or ethoxy)benzyl ether (10), another key precursor for the vanillin synthesis, was prepared from both 8 and 11 by bromination followed by etherification with ROH-BaO-DMF-CuCl2 in good yield.