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

Abstract

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 UVinduced 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.

Photogenerated donor–donor diazo compounds enable facile access to spirocyclopropanes

“…To tackle some of these challenges, activation of either oxadiazolines or tosylhydrazones by energy transfer photocatalysis has been reported very recently. 27,28 Nevertheless, these methods require the addition of elaborate photocatalysts and can only access the corresponding carbene species directly without harnessing the unique reactivity of diazo compounds.…”

Visible-Light-Mediated Energy Transfer Enables Cyclopropanes Bearing Contiguous All-Carbon Quaternary Centers

“…During the preparation of our manuscript, the Carreira group displayed the photoredox cyclopropanation of olefins with α-bromo-β-ketoesters and five cyclopropanes bearing contiguous quaternary carbon centers were afforded. Later, the Gryko group exploited the energy-transfer-induced activation of 1,3,4-oxadiazolines, a well-investigated donor/donor diazo surrogate, resulting in the generation of donor/donor carbenes; these carbenes then reacted with electron-poor olefins to afford cyclopropanes. Despite tremendous advances to avoid using UV light to activate 1,3,4-oxadiazolines, only 6 substrates bearing contiguous quaternary carbon centers have been reported with compulsory electron-withdrawing groups.…”

Catalytic Olefin Cyclopropanation with In Situ-Generated Dialkyl Diazomethanes via Co(II)-Based Metalloradical Catalysis

“…14,15 In this context, the Gryko group recently disclosed a study on the photocatalytic reaction of spirocyclic oxadiazolines that�upon triplet sensitization�gives a cyclic triplet carbene that reacts in cyclopropanation reactions with electron-deficient olefins under visible light conditions. 15 In a related context, the Zhang and Bruin groups have reported a series of studies on cobalt-catalyzed carbene transfer reactions of diazoacetates where a metalloradical species is involved to achieve cyclopropanation reactions without participation of a conventional singlet carbene intermediate (Scheme 2b). 16 Moreover, the Uyeda group has recently uncovered that gemdihalogenated alkanes can be employed as precursors of unstabilized carbenes in cyclopropanation reactions using stoichiometric amounts of reducing agents.…”

“…The vast majority of these methods are, however, restricted on the reactivity of metal-bound or free-singlet carbene intermediates, which are commonly accessed from aryl diazoacetates, diazoacetates, and aryl aryl diazoalkanes (Scheme a). Applications of dialkyl diazoalkanes in carbene transfer reactions remain scarce, and existing approaches employ, for example, the UV light photolysis of oxadiazolines under continuous-flow conditions, yet with limitations in general applicability because of the nature of the strong light source. , In this context, the Gryko group recently disclosed a study on the photocatalytic reaction of spirocyclic oxadiazolines thatupon triplet sensitizationgives a cyclic triplet carbene that reacts in cyclopropanation reactions with electron-deficient olefins under visible light conditions . In a related context, the Zhang and Bruin groups have reported a series of studies on cobalt-catalyzed carbene transfer reactions of diazoacetates where a metalloradical species is involved to achieve cyclopropanation reactions without participation of a conventional singlet carbene intermediate (Scheme b) .…”