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Pure Appl. Chem., 2010, Vol. 82, No. 4, pp. 1055-1063

Published online 2010-03-19

Novel anodic concepts for the selective phenol coupling reaction

Siegfried R. Waldvogel

Rheinische Friedrich-Wilhelms-Universität Bonn; Kekulé Institute for Organic Chemistry and Biochemistry, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany

Abstract: The oxidative phenol coupling reaction of phenols with simple methyl substituents can be difficult due to several by-products. Such a challenging substrate is 2,4-dimethyl-phenol. We studied the electrochemical access to the ortho-coupled dehydrodimer. Anodic treatment in a basic electrolyte supports the formation of a molecular tricyclic architecture called Pummerer’s ketone. Employing a two-step sequence involving anodic conversion of 2,4-dimethylphenol to a preliminary substrate and different workup protocols yield exclusively and diastereoselectively polycyclic architectures. The selective ortho-coupling reaction was achieved by two strategies: A tetraphenoxy borate can be used as template substrate which can be easily made in large scale. Due to the ionic nature of the borate, no supporting electrolyte is required. This methodology can be applied to several related phenolic substrates. The second route is described by the direct conversion of 2,4-dimethylphenol on boron-doped diamond anodes. Surprisingly, the electrochemical transformation does not destruct or mineralize the substrate. Fluorinated additives allow the conversion of a broad scope of substrates.