Photoinduced radical-ionic dihalogen transfer to carbon-carbon multiple bonds using oxime-based surrogates

Functional-group transfer strategies that utilize bench-stable and easy-to-handle surrogates create new synthetic avenues with the prospect to avoid the use of highly toxic and hazardous reagents. We disclose here an operationally simple and mechanistically distinct catalytic transfer protocol for the vicinal dihalogenation of carbon-carbon multiple bonds in alkenes, alkynes and allenes using easily synthesized oxime-based dihalogen surrogates. The reaction is promoted by visible-light photoredox catalysis. The success of this radical-ionic functional-group transfer hinges on the fragmentation of the surrogates into a halogen radical and halide anion with benzonitrile formation and sulfur dioxide evolution as the driving forces. Mechanistic studies suggest that oxidative quenching of the photocatalyst initiates the catalytic process. The released electrophilic halogen radical then engages in a radical addition to the carbon-carbon double or triple bond, followed by a radical-polar crossover and carbocation trapping by the previously formed halide anion to deliver the desired vicinal dihalides in a redox-neutral manner. Functional-group transfer strategies using surrogates to avoid handling hazardous reagents are often limited to monofunctionalization reactions. Now, an operationally simple and mechanistically distinct photocatalytic transfer strategy for vicinal dihalogenation of carbon-carbon multiple bonds, such as in alkenes, alkynes and allenes, using readily synthesized oxime-based dihalogen surrogates is reported.