Uncovering outer-sphere mechanisms governing chemoselectivity in copper-photocatalyzed ATRA reactions of CF₃SO₂Cl with alkenes

This work presents a detailed DFT-based mechanistic investigation of copper-photocatalyzed atom transfer radical addition (ATRA) reactions between CF₃SO₂Cl and alkenes. Depending on the electronic nature of the alkene substrate, these reactions yield either RCl or RSO₂Cl products. The unusual divergence in product selectivity has led to the proposal of multiple mechanistic pathways. In this study, we show that all productive pathways proceed exclusively via outer-sphere single-electron transfer and identify two previously unrecognized mechanisms: an S(vi)/S(iv) redox cycling mechanism responsible for RSO₂Cl formation, and a 2c–3e Cl-coordination-induced SET mechanism accounting for RCl formation. These two pathways represent the first models to explicitly demonstrate the bifunctional role of the [SO₂Cl]⁻ anion in governing divergent product formation. Additionally, we identify a third, cationic mechanism, in which the carbon-centred radical is oxidized to a carbocation by Cu(ii), competing with the other pathways and likewise leading to RCl. Taken together, these results provide a useful framework for understanding chemoselectivity in this class of photocatalytic transformations and may help guide the design of future ATRA protocols.