Redox-Neutral 1,3-Dipolar Cycloaddition of 2-Azirines with 2,4,6-Triarylpyrylium Salts under Visible Light Irradiation.

A novel visible light mediated redox-neutral 1,3-dipolar cycloaddition of 2-azirines with 2,4,6-triarylpyrylium tetrafluoroborate salts providing tetrasubstituted pyrroles has been developed. The 2,4,6-triarylpyrylium salt acts as dipolarophile as well as photosensitizer in the reaction, under blue light irradiation. The control experiments indicated single electron oxidation of 2-azirines by photoexcited pyrylium salts, followed by coupling between an azaallenyl radical cation and triarylpyranyl radical as the key mechanistic feature. The mild conditions, wide substrate scope, and complete regioselectivity are the noticeable attributes of the reaction.