Rhodium-mediated C-C bond activation of 2-(2',6'-dialkylarylazo)-4-methylphenols. Elimination and migration of alkyl groups

Abstract

Upon reaction of 2-(2',6'-dimethylphenylazo)-4-methylphenol with [Rh(PPh₃)₃Cl], the azo ligand undergoes an interesting rhodium-assisted C-C bond activation at one ortho position of the 2',6'-dimethylphenyl fragment, leading to the elimination of the methyl group from that ortho position. A similar elimination of an ethyl group takes place when 2-(2',6'-diethylphenylazo)-4-methylphenol is allowed to react with [Rh(PPh₃)₃Cl]. However, when 2-(2',6'-diisopopylphenylazo)-4-methylphenol reacts with [Rh(PPh₃)₃Cl], an interesting migration of an isopropyl group from its original location (say the 2' position) to the corresponding third position (say the 4' position) takes place. In all the three complexes the modified azo ligands bind to rhodium as dianionic C,N,O-donors, and two triphenylphosphines and a chloride are also coordinated to the metal center. Structures of all three complexes have been determined by x-ray crystallography. These complexes show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation on the positive side of SCE and a reduction of the coordinated azo ligand on the negative side.