UV Light is no Longer Required for the Activation 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 non-stabilized 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 diverse reactivities and straightforward synthesis, milder methodologies for the activation of these compounds, that permit the use of UV light-sensitive substrates, are of high significance. Herein, we report the visible-light-induced activation of oxadiazolines by triplet energy transfer catalysis that, in contrast to UV-induced processes, enables generation of carbenes. The formed reactive species react with electron-poor olefins, giving valuable spirocyclopropanes. Mechanistic investigations, both theoretical and experimental, uncover plausible pathways and highlight the importance of the triplet energy-transfer steps.