The dual role of thiourea in the thiotrifluoromethylation of alkenes† †Electronic supplementary information (ESI) available. CCDC 1474239–1474241. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc02790c Click here for additional data file. Click here for additional data file

We report the stereoselective and metal-free trifluoromethylation of alkenes followed by S -cyclization using thiourea as the S-source and SET initiator. Alkenes substituted with a thiourea undergo C–CF 3 followed by intramolecular C–S bond formation with the Togni reagent and trifluoroacetic acid (TFA) at room temperature; thiols and thioamides are not suitable S-sources for this reaction. This anti-addition process involves a CF 3 radical, and affords CF 3 -substituted thiazolines and thiazines for medicinal applications. A metal or photoredox catalyst is not required as the thiourea acts as a reductant, as well as serving as an S-source capable of adding to a C-centered radical. Mechanistic work comparing the reactivity of thiourea, urea, thioamide and thiol in the context of alkene trifluoromethylation demonstrates that in this series, the thiourea is unique for its ability to release CF 3 radical from the Togni reagent, and to orchestrate trifluoromethylation followed by S -cyclization with both activated and unactivated alkenes.