Leveraging Divergent Ligand-to-Metal Charge-Transfer Excited State Pathways for Catalyst Control over Alkoxyl Radical Reactivity

Ligand-to-metal charge-transfer (LMCT) excitation has emerged in recent years as a powerful modality in organic synthesis, namely for the generation of heteroatom-centered radicals through formal metal-ligand bond homolysis from the LMCT excited state. However, the exploitation of alternative LMCT excited state processes has been extremely limited. Here, we describe a general strategy for tuning the reaction course from LMCT excited states of titanium alkoxides. This reactivity paradigm has been exploited for tandem β-scission/Giese addition reactions of both scission-amenable and scission-recalcitrant alcohols under divergent reaction pathways of metal-ligand bond homolysis and excited state β-scission through judicious choice of electronically tuned Ti catalysts. Through intramolecular competition studies, catalyst-controlled scission is shown to facilitate a rate enhancement of up to 103-fold over the intrinsic scission of free alkoxyl radicals, highlighting the impact of accessing the excited state scission paradigm. Computations support the relevance of a scission-promoting LMCT excited state with stereoelectronically aligned alkoxyl radical cation character to enable direct, selective β-scission.