Methoxy-Substituted α,<i>n</i>‑Didehydrotoluenes. Photochemical Generation and Polar vs Diradical Reactivity

Abstract

The photoreactivity of differently substituted (chloromethoxybenzyl)­trimethylsilanes in alcohols and alcohol/water mixtures has been investigated by means of a combined computational and experimental approach. Subsequent elimination of the chloride anion and the trimethylsilyl cation gives the corresponding methoxy-substituted α,<i>n</i>-didehydrotoluenes (α,<i>n</i>-MeO-DHTs). The rate of desilylation is evaluated through the competition with arylation via phenyl cation (ca. 10⁸ s^–1). α,2-MeO- and α,4-MeO-DHTs show a purely radical behavior (H abstraction from the solvent, methanol), while α,3-MeO-DHT shows mainly a ionic chemistry, as when the parent α,3-DHT is thermally generated. This is likely due to triplet–singlet surfaces crossing occurring during desilylation