NXS/TBHP as a Reagent System for Expedient Generation of Halogen Radicals: Site-Selective Halogenation of Quinoxalin-2(1H)-ones and Conversion of Terminal Alkynes to gem-Dihaloketones.

We report an efficient and practical protocol for the generation of halogen (Br and Cl) radicals using inexpensive and readily available N-bromo/N-chlorosuccinimide (NXS) and t-butylhydroperoxide (TBHP) at rt. The NXS/TBHP reagent systems are demonstrated for site-selective bromination and chlorination of the benzene-core of quinoxalin-2(1H)-ones. It is shown that the reagent system permits regioselective halogenation at the C7 position of the benzene core over the readily-functionalized C3 position of quinoxalin-2(1H)-ones under mild reaction conditions. We further show that terminal alkynes can be converted to gem-dihaloketones in a facile manner by NXS/TBHP. Mechanistically, single electron transfer between TBHP and NXS leads to instantaneous formation of electrophilic halogen radicals, which attack arenes and alkynes.