Thermo-analytical determination of intermediates in the copper catalysed rearrangement of o-toluic acid to meta-cresol

Abstract

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.