On the Mechanism of Phenolic Formylation Mediated by TiCl₄ Complexes: 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 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.

Room Temperature Magnetic Switchability Assisted by Hysteretic Valence Tautomerism in a Layered Two-Dimensional Manganese-Radical Coordination Framework

“…In this redox process, the Co­(III) ion is reduced to Co­(II) and the CAT ligand is oxidized to its semiquinonato (SQ) form. VT transitions involving SQ-CAT ligand are by far the most reported in the literature, mainly in Co, − and Mn isolated complexes, − but also in various d block elements such as Fe, Ni, Rh, and V. − VT transitions were also observed in four unidimensional (1D) polymers and one Co-based bidimensional (2D) coordination polymer. ,− Other ligands than SQ-CAT are less common in VT systems, with a dozen examples reported so far, involving porphyrin derivatives, cyclopentadienyl, or bis­(diisopropyl-phenylimino)­acenaphthene. − In the latter case, VT was reported for Yb …”

Formylation of Electron-Rich Aromatic Rings Mediated by Dichloromethyl Methyl Ether and TiCl4: Scope and Limitations

“…We have reported the successful ortho -formylation of electron-rich phenols mediated by dichloromethyl methyl ether and titanium (IV) tetrachloride [ 27 ], as well as a description of the reaction mechanisms in phenolic compounds [ 28 ]. This methodology was based on the outstanding procedure pioneered by Gross [ 29 ] and Cresp [ 30 ] that affords aromatic aldehydes ( Scheme 1 ).…”

Experimental and Computational Evidence of the Biradical Structure and Reactivity of Titanium(IV) Enolates

“…These spectroscopic data are in close agreement with the computational calculations and all together the results lend a strong support to the coexistence of a biradical intermediate and a closed shell titanium complex that can interconvert through a low energy transition state . This result is in line with previous studies in similar Ti­(IV) complexes in which the EPR signals have been unambiguously assigned based in the observed fine structure and temperature dependence of the spectra. , …”

“…requires much less energy than the direct oxidation of 5S, which makes necessary to take into account the formation of the non-chelated complex 6 and the subsequent ligand exchange. 13 Scheme 5. Third step: oxidation of 5S.…”

“…Within this framework, some years ago we reported the unexpected biradical character of titanium(IV) enolates derived from α-benzyloxy ketones 12 and TiCl 4 -phenoxy complexes 13 that were assigned to coexisting Ti(III) species. Rather than being a simple expansion of the resonance model, this biradical character arises from a valence tautomery in which there is a nuclear configuration with two utterly unlike but almost degenerate electronic configurations.…”