Continuous-flow photogeneration of nitrosobenzaldehydes enables indazolone formation

1,2-Dihydro-3H-indazol-3-ones (indazolones) are nitrogen-containing heterocycles with broad biological relevance, yet their synthesis often relies on harsh conditions or expensive metal catalysts. Herein, we report a metal-free continuous-flow photochemical protocol that enables rapid access to indazolone cores under mild and sustainable conditions. Direct irradiation of o-nitrobenzyl alcohols in ethanol at 365 nm generates o-nitrosobenzaldehydes in situ, which subsequently undergo condensation-cyclization with primary amines to afford indazolones. UV-visible spectroscopy combined with conceptual DFT analysis suggests that reaction efficiency is governed by wavelength-dependent substrate absorption: weak absorbers generate nitroso intermediates inefficiently, whereas strongly absorbing substrates undergo competitive photodegradation. By matching the irradiation wavelength to the absorption properties of individual substrates, the accessible chemical space can be extended beyond that obtained under fixed-wavelength conditions. Conceptual DFT further rationalizes substituent-dependent electronic effects, clarifying how photophysical and polar reactivity factors jointly control productive indazolone formation. The reaction scalability is assessed with commercial flow reactors. Indazolones are nitrogen-containing heterocycles with broad biological relevance, yet their synthesis often relies on harsh conditions or expensive metal catalysts. Here, the authors report a metal-free continuous-flow photochemical protocol that enables rapid access to indazolone cores under mild and sustainable conditions.