Photosensitizer-Free Photoinduced Ground-State Triplet Carbene-Assisted Aryloxy Radical Generation via HAT

Carbene is the pivotal reactive intermediate and its reactivity potential has been extensively explored in a variety of synthetic transformations ranging from cycloaddition to site-selective C–H functionalizations. However, controlling the reactivity of carbene is a key parameter for achieving the selective carbene transfer reactions. Usually, strategies rely on the combined effect of metal catalysts and ligands to accomplish the selective transformations via the generation of singlet carbene intermediate. Contrarily, the triplet carbene has gained a very little attention from the researchers as its generatation and reactivity relies on the use of specific metal complexes thus limiting its wider utility. Neverthless, the organic transformations involving a triplet spin state carbene have been explored with diazo compounds under visible light utilizing photosensitizer. However, the development of the visible light induced sensitizer-free strategy for the generation of triplet carbene is highly challenging and is seldom reported. Herein, we have demonstrated vinyl diazosuccinimide as a new class of diazo compound which upon exposure to blue LEDs (456 nm) generates ground-state triplet carbene (without ISC) in the absence of any photosensitizer. We have successfully explored this strategy and utilzed to construct the biologically relavant scaffold such as 2H-chromenes. A series of control experiments, EPR studies as well as DFT calculations have supported the existence of ground-state triplet carbene followed by the aryloxy radical during the course of the reaction.