Photochemical Reactions of Diazo Compounds

Diazo compounds, characterised by a diazo functional group (–N₂), serve as versatile precursors to carbenes upon exposure to light. Photochemical activation of these species enables clean N₂ extrusion under mild conditions, affording highly reactive singlet or triplet carbenes that engage in a broad spectrum of transformations. Key processes include O–H and C–H insertions, cyclopropanation of alkenes, Wolff rearrangement of diazoketones to ketenes and subsequent multicomponent couplings. The use of visible light, often through photocatalysts or direct absorption by suitably coloured diazo substrates, enhances selectivity and sustainability by avoiding hazardous reagents and harsh conditions. These methods have found application in the synthesis of complex heterocycles, late-stage functionalisation of bioactive molecules and the construction of diverse building blocks for materials science and drug discovery.

Research from Nature Portfolio

Metal-free photochemical activation of diazo compounds via iodine catalysis has unlocked radical carbene pathways. Under light irradiation, the iodine catalyst promotes N₂ extrusion to generate carbene radicals that undergo efficient cyclopropanation of olefins, epoxidation and heterocycle formation without reliance on transition-metal carbenoids. This strategy extends the scope of diazo chemistry, offering excellent functional-group tolerance and a complementary route to traditional metal-catalysed processes.

Photochemical Reactions of Diazo Compounds publication trend

The graph below shows the total number of articles in photochemical reactions of diazo compounds across all publications each year (not limited to Nature Index journals).

Technical terms

Diazo compound: An organic molecule containing the –N₂ group, which can extrude dinitrogen upon activation to generate reactive carbenes.

Carbene: A highly reactive divalent carbon species with two nonbonded electrons, existing in singlet or triplet states.

Wolff rearrangement: A photochemical transformation of diazoketones to ketenes via N₂ loss and 1,2-shift of an alkyl or aryl group.

Electron donor–acceptor (EDA) complex: A ground-state assembly between an electron-rich and electron-poor species that absorbs light to initiate charge transfer and subsequent reaction.

Photochemical activation: The process of using photons, typically UV or visible light, to excite a molecule and induce chemical bond cleavage or rearrangement.

References

1. Iodine-catalyzed diazo activation to access radical reactivity. Nature Communications (2018).
2. Into the Blue: Ketene Multicomponent Reactions under Visible Light. The Journal of Organic Chemistry (2021).
3. Aryl versus Alkyl Redox-Active Diazoacetates  Light-Induced C–H Insertion or 1,2-Rearrangement. Organic Letters (2023).
4. Unique Reactivity of Triazolyl Diazoacetates under Photochemical Conditions. ACS Organic & Inorganic Au (2024).