2,4,6-Trimethylphenol (1a) was selectively oxidized to 3,5-dimethyl-4-hydroxybenzaldehyde (2a) by molecular oxygen in the presence of a copper(II) chloride-oxime or copper(II) chloride-amine catalyst in alcohol at ambient temperature. When an acid was present in the catalyst system, the formation of both 2,6-dimethyl-p-benzoquinone (3) and a formaldehyde dialkyl acetal was accelerated. It is likely that 2a is produced by way of the formation of a quinonemethide intermediate, followed by a repeated 1,6-addition of alcohol on the intermediate to form a 4-(alkoxymethyl)-2,6-dimethylphenol (5a), and then 3,5-dimethyl-4-hydroxybenzaldehyde dialkyl acetal, which would be hydrolyzed to 2a. When the reaction time was shortened, 5a was isolated in good yield. Oxidations of the other p-methyl-substituted phenols were also examined using the present oxidation system.
2,4,6-三甲基
苯酚(1a)在
铜(II)
氯化物-
肟或
铜(II)
氯化物-胺催化剂的存在下,利用分子氧在
酒精中于室温下可选择性地氧化为
3,5-二甲基-4-羟基苯甲醛(2a)。当催化剂体系中存在酸时,2,6-二甲基
对苯醌(3)和
甲醛二烷基
缩醛的形成会加速。2a很可能是通过形成醌亚甲基中间体,随后在中间体上重复进行1,6-加成反应,生成4-(烷氧基甲基)-
2,6-二甲基苯酚(5a),然后形成
3,5-二甲基-4-羟基苯甲醛二烷基
缩醛,最终
水解为2a。当反应时间缩短时,5a可以高收率地分离出来。通过本氧化体系对其他对位甲基取代
苯酚的氧化反应进行了研究。