Chiral Ti(BOX)–LMCT Catalysis Integrating Alcohol C–C Bond Cleavage with Enantioselective Radical Conjugate Addition

Abstract

Photochemical C–C bond cleavage can convert otherwise inert skeletal linkages into carbon-centered radicals, but its application in asymmetric synthesis requires radical generation to be coordinated with stereocontrolled bond formation. Here we describe chiral Ti(BOX)–LMCT catalysis that enables enantioselective radical conjugate addition through selective fragmentation of native alcohol C–C bonds. Alcohol coordination generates a photoactive Ti(IV)–alkoxide, whose LMCT excitation triggers β-scission to produce a carbon-centered radical and a reduced chiral Ti species in the same photochemical event, with the latter implicated in the subsequent stereocontrolled bond-forming step. Structurally diverse alcohols and electron-deficient alkenes participate with high chemo- and enantioselectivity, including substrates that furnish all-carbon quaternary stereocenters. Single-crystal X-ray diffraction, operando EPR, and kinetic analysis identify substrate-derived Ti(BOX)–alkoxide complexes as catalytically competent photoactive species. Comparative photolysis further shows that alkoxide, halide, and BOX ligands collectively influence the apparent photoreactivity of Ti–alkoxide complexes.