On the Mechanism of Phenolic Formylation Mediated by TiCl₄Complexes: Existence of Diradical Intermediates Induced by Valence Tautomerism

Abstract The conventional electrophilic intramolecular aromatic substitution pathway proposed by Cresp et al. [ J. Chem. Soc., Perkin Trans. 1 1973 , 340–345] is confirmed by the observed products of phenolic formylation mediated by TiCl 4 . However, when the nucleophilic path is quenched by appropriate ligand modification, the initial equilibria between the possible neutral complexes of TiCl 4 with 3,5‐dimethoxyphenol and/or diethyl ether lead to different stable diradical intermediates induced by valence tautomerism that provide valuable activated reagents. Some of these species have been detected by EPR, characterized theoretically and captured by TEMPO, thus providing a consistent mechanism for the reaction with one or more equivalents of TEMPO per phenol.