In their study, the researchers selected sulfinamides as substrates and identified a chiral indium(III) bromide salen complex as a suitable photocatalyst for the enantioselective construction of sulfur stereocentres by light-driven deracemization (pictured). Screening experiments revealed that the catalyst components indium, bromine and the salen ligand were all important for enantioselectivity. The reaction was temperature-dependent, leading to the highest enantiomeric excess at −55 °C under irradiation with a violet light-emitting diode. Moreover, a broad substrate scope with high functional group tolerance was demonstrated. Beyond the initial transformation, the products can be converted into valuable chiral sulfur(VI) derivatives, including sulfonimidamides, sulfoximines, and ligands for asymmetric catalysis, underscoring the method’s synthetic utility.
Mechanistic investigations indicate that photoexcitation of the catalyst–substrate complex triggers homolytic cleavage of the sulfur–nitrogen bond, generating sulfinyl and aminyl radicals. Crucially, these radicals remain close to each other by confinement within a solvent cage forming a geminate pair, which enables recombination under chiral induction rather than random coupling. The researchers were also able to obtain X-ray structures of the catalyst–substrate adduct, revealing an oxygen-indium bond between the indium(III) bromide salen complex and the sulfinyl group as well as stabilizing interactions of the sulfur atom with the salen ligand. Computational studies provided further insights into the control of the enantioselectivity exerted by the photocatalyst by highlighting noncovalent interactions that favour one transition state over the other.
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