Thermal and photochemical switching of chiral sugar azoalkenes. A mechanistic interrogation

This manuscript describes a thorough study on the thermal and photochemical evolution of carbohydrate-based azoalkenes, which have been proposed as putative intermediates en route to other heterocyclic derivatives and nucleoside analogues. These substances represent new protagonists in the azo chemical space, under intense study due to reversible photoswitching of this functional group suitable for designing artificial nanomachines and responsive materials. Although structurally simple azadienes derived from monosaccharides have long been known, the dynamics of such species in solution and solid phase remains poorly characterized. Herein, we show that such azoalkenes undergo 3E→3Z thermal isomerization, which is accelerated by the presence of Brönsted acids. On the other hand, they undergo 1E→1Z isomerization when irradiated by sunlight, while the reverse 1Z→1E isomerization occurs thermally in the dark or under acid catalysis. As a result, sugar monoazadienes not only exhibit inherent chirality, but also dual on-off isomerization under external stimuli. Both experiments and DFT-based computational analyses provide a consistent and unifying mechanistic picture of these transformations.