Catalyst Control Over Alkoxyl Radical Reactivity by Electronically Tuned Titanium Photocatalysis

Abstract

Alkoxyl radicals are versatile, high-energy intermediates in organic synthesis, whose reaction fate is typically dictated by intrinsic substrate electronics and geometry, leading to difficulties in reactivity control. Here, we describe the development of a strategy for catalyst-controlled alkoxyl radical reactivity via divergent ligand-to-metal charge-transfer (LMCT) excited state pathways of titanium alkoxide complexes. By electronically tuning features of Ti catalysts, the reaction course can be rationally switched between free alkoxyl radical generation via visible-light-induced homolysis (VLIH) and direct excited-state β-scission from a formally metal-bound alkoxyl radical species. This dual-pathway platform enables tandem β-scission/Giese addition of both scission-amenable and scission-recalcitrant alcohols, with intramolecular competition studies demonstrating a 102–103-fold rate enhancement of β-scission under catalyst-enforced conditions.