Thermo-analytical determination of intermediates in the copper catalysed rearrangement of o-toluic acid to meta-cresol
作者:Abraham C. Sunil、Ernst H. G. Langner、Charlene Marais、Barend C. B. Bezuidenhoudt
DOI:10.1039/c3cy00119a
日期:——
The intermediates formed during the conversion of o-toluic acid 2 to m-cresol 1 in the presence of Cu(II) according to the Keading process, together with the temperatures at which crucial changes occur, have been investigated by means of X-ray diffractometry [which confirmed the formation of the paddlewheel copper complex tetrakis(μ2-2-methylbenzoato)bis(2-methylbenzoic acid)copper(II) 5 with two apical o-toluic acid ligands], differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), NMR spectroscopy, infrared spectrometry (IR) and Maldi-Tof mass spectrometry. The major chemical change observed at 164 °C might be ascribed to the dissociation of the apical o-toluic acid ligands from 5 to give tetrakis(μ2-2-methylbenzoato)copper(II) 6 accompanied by water loss. O–C bond formation between the carboxylate oxygens and ortho-carbons of the toluic acid moieties in adjacent paddlewheels of the stepped polymeric 6 is proposed to explain the formation of copper(I) 2-methyl-6-[(2-methylphenyl)carbonyl]oxy}benzoate 8via intermediate 7 and initiates the decomposition observed by DSC at 236 °C. Decarboxylation of 8 at 249.5 °C gives 3-methylphenyl 2-methylbenzoate 4, which can be hydrolysed to o-toluic acid 2 and the target compound, m-cresol 1.
在Keading过程的条件下,Cu(II)催化下,将邻甲基苯酸2转化为中甲基酚1的过程中形成的中间体,以及发生关键变化的温度,已经通过X射线衍射法、差示扫描量热法(DSC)、热重分析法(TGA)、核磁共振光谱(NMR)、红外光谱(IR)和MALDI-TOF质谱进行了研究。观察到的主要化学变化发生在164°C,这可能归因于峰位邻甲基苯酸配体从5中解离,形成四甲基(μ2-2-甲基苯甲酸)铜(II)6,并伴随水的 loss。提出在阶梯聚合物6的相邻舵轮中,羧酸根氧与邻位碳原子之间形成O–C键,以解释通过中间体7形成铜(I)2-甲基-6-[(2-甲基苯基)羰基]氧}苯甲酸酯8,并启动了在236°C时DSC观察到的分解。8在249.5°C的脱羧反应生成3-甲基苯基-2-甲基苯甲酸酯4,该产物可水解为邻甲基苯酸2和目标化合物中甲基酚1。