UV Light Is No Longer Required for the Photoactivation of 1,3,4-Oxadiazolines

Carbenes play a key role in a plethora of organic transformations. Although stabilized diazo carbonyl compounds predominate as a source of electrophilic carbenes, the hazardous nature of nonstabilized analogues calls for their in situ generation from stable precursors. Among these, 1,3,4-oxadiazolines serve as diazoalkane surrogates under UV light irradiation. In view of their high stability, diverse reactivities, and straightforward synthesis, milder methodologies for the activation of these compounds that permit the use of UV-light-sensitive substrates are highly valued. Herein, we report the visible-light-induced activation of oxadiazolines by triplet energy transfer catalysis that, in contrast to UV-induced processes, alters their reactivity and enables the generation of carbenes. The formed reactive species react with electron-poor olefins, thereby giving valuable spirocyclopropanes. Mechanistic investigations, both theoretical and experimental, uncover plausible pathways and highlight the importance of the triplet energy transfer steps.