The photochemical mediated ring contraction of 4H-1,2,6-thiadiazines to afford 1,2,5-thiadiazol-3(2H)-one 1-oxides

E.B. and C.G.T. are grateful to Heriot-Watt University and the EPSRC CRITICAT Centre for Doctoral Training (E.B. Ph.D. Studentship: EP/L016419/1, C.G.T Ph.D. Studentship: EP/LO14419/1) for funding and training. C.G.T. is grateful to the Heriot-Watt Annual Fund for financial support. P.A.K. and A.S.K. thank the University of Cyprus for the Internal Grant “Thiadiazine-Based Organic Photovoltaics”, and the Cyprus Research Promotion Foundation (Grant Nos. ΣΤΡΑΤΗΙΙ/0308/06, NEKYP/0308/02 ΥΓΕΙΑ/0506/19, and ΕΝΙΣΧ/0308/83). M.J.P. thanks the EPSRC for funding (Grant Nos. EP/T021675 and EP/V006746), and the Leverhulme Trust (Grant No. PG-2020-208). S.A.M. thanks the EPSRC for funding (Grant No. EP/T019867/1).1,2,6-Thiadiazines treated with visible light and 3O2 under ambient conditions are converted into difficult-to-access 1,2,5-thiadiazole 1-oxides (35 examples, yields of 39–100%). Experimental and theoretical studies reveal that 1,2,6-thiadiazines act as triplet photosensitizers that produce 1O2 and then undergo a chemoselective [3 + 2] cycloaddition to give an endoperoxide that ring contracts with selective carbon atom excision and complete atom economy. The reaction was optimized under both batch and continuous-flow conditions and is also efficient in green solvents.Publisher PDFPeer reviewe