Photocatalyst‐Free Wavelength‐Dependant Sequential Ring Transformations of Pyrazolo[1,2‐<i>a</i>]pyrazolones

Abstract We report sequential wavelength‐selective photochemical transformations of 1‐alkenylpyrazolo[1,2‐ a ]pyrazolones to pyrazolo[1,2‐ a ][1,2]diazepines or cyclobuta[ c ]pyrazolo[1,2‐ a ]pyrazolones. Irradiation of 1‐alkenylpyrazolo[1,2‐ a ]pyrazolones with visible‐light (blue LED, 457 nm) induced selective ‘ring switching’ transformation into pyrazolo[1,2‐ a ][1,2]diazepines, which underwent electrocyclisation yielding cyclobuta[ c ]pyrazolo[1,2‐ a ]pyrazolones upon irradiation with UV−A light (black LED, 365 nm). Due to the very narrow irradiation wavelength now available from OLED sources, the selective formation of either 5,7‐bicyclic or 5,5,4‐tricyclic ring systems from the 5,5‐bicyclic starting material is possible by changing the wavelength of the irradiation source. The transformations took place under mild conditions in the absence of additives or photocatalysts. Mechanistic studies indicate that these transformations proceed through the formation of an excited triplet state of the substrate, followed by selective homolytic C(1)−N(8) bond cleavage, intersystem crossing, and cyclization of the zwitterionic intermediate on the ground potential energy surface. The subsequent photoinduced disrotatory stereospecific 4π‐electrocyclization of the 5,7‐bicyclic systems leads to 3D‐rich 5,5,4‐tricyclic products.