Simple benzoselenadiazolium salts for counteranion-controlled chalcogen bond catalysis

Herein, we demonstrate that chloride, triflate, and tetrafluoroborate salts of an N-methyl benzoselenadiazolium cation catalyze bromination of activated aromatic substrates with N-bromosuccinimide (NBS). Departing from the common strategy of employing the heaviest chalcogen, tellurium, with weakly coordinating counteranions in catalyst design, our chloride-bearing catalyst instead affords better catalytic performance than the other investigated salts. At ambient temperature, a quantitative yield of 4-bromoanisole was obtained within 3 h and >90% conversion of 9,10-dibromoanthracene was achieved within minutes. Crystal structures of the chloride salt obtained from mixing with NBS reveal bromide and polybromide anions, and DFT analysis of the optimized catalyst•NBS adduct suggest that chalcogen bonds mediate NBS carbonyl activation while chloride stabilizes the electrophilic bromine derived from NBS via a halogen-bond Br … Cl − motif. Our results highlight that in chalcogen bond catalysis, the nature of the counteranion can override simple σ-hole strength considerations and must be regarded as an integral contributor to NBS activation rather than a passive spectator.