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The gradient extremals can be taken as a representation of reaction paths. We prove that this type of curve possesses a variational nature. We report the conditions such that these curves have the character of a minimal curve. Finally we discuss the relations between the points of these curves being turning points with respect to other special points of the potential energy surface, like the valley-ridge inflection points.

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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 TiCl4. However, when the nucleophilic path is quenched by appropriate ligand modification, the initial equilibria between the possible neutral complexes of TiCl4 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.