In Green Chem.
We report a novel, CO2-free hydrazine-free and reductant-free photocatalytic access to azines from oxime esters. Oxime esters normally react by EnT photocatalysis, extruding CO2 and radical (cross-coupling) recombining. We divert this to SET reactivity by a preassembling triarylamine photocatalyst, releasing unreactive acetate as a valuable, recoverable by-product. The resulting N centered radicals have no choice but to dimerize, affording azines in surprisingly high yields!
Classical azine access with hydrazine (toxic, explosive = rocket fuel!) needs high temperature for forcing aryl ketone condensations, well above the ignition point of hydrazine. Though we're limited to homodimers so far, our reaction is safe, high in atom economy & scalable in flow.
Intriguingly, we discovered that the triarylamine photocatalyst photodecomposes to a carbazole in the substrate's presence, yet fully in its absence. The resulting 65:35 mixture of triarylamine : carbazole gives a better reaction yield and rate vs either catalyst alone, as a unique example of co-operative photocatalysis!
A hot debate in the photocatalysis field is that the catalyst 'X' you put in the reaction may decompose to 'Y' as the active catalyst. Yet, everyone's foci is on 'is it X, or is it Y'? When actually, X+Y may BOTH play roles! Here, we reckon the carbazole up-shuttles electrons from the liberated acetate to the generated triarylamine radical cation that would otherwise be too endergonic.
A big congratulations to Jonathan for his first, 1st author paper and to the whole team for valuable contributions! We're excited to share even more examples of preassembling photocatalysis in the near future, that also strongly benefit sustainability aspects of reactions - stay tuned!
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