Photoreduction on Irradiation of 3,3,5,5-Tetramethyl-4-methylene-1-pyrazoline. - An Overlooked Example of a Reaction Competing with the Extrusion of Molecular Nitrogen from Reluctant Azoalkanes

The photolysis of the 4‐methylene‐1‐pyrazoline 1 is reinvestigated under a variety of experimental conditions. Direct irradiation (350‐nm light) affords the alkylidenecyclopropanes 4 and 5 as predominant products, besides molecular nitrogen, and in a constant ratio of (55 ± 1): (45 ∓ 1), which is not altered in the presence of 1,4‐diazabicyclo[2.2.2]octane (DABCO) or piperylene nor in [D6]acetone solution. When oxygen is excluded, an additional compound is formed whose proportion depends on the capability of the solvent to act as hydrogen donor. In the presence of three equivalents of DABCO, it eventually becomes the major product. The 4‐methylenepyrazolidine structure 13 of the novel photoproduct is established by NMR spectroscopy, a comparison with an authentic sample, obtained in quantitative yield by zinc reduction of 1, and reoxidation to 1 as well. An extensive investigation to detect any traces of the 4‐isopropylidene‐2‐pyrazoline 6 in the irradiated solutions has failed. This result is at variance with recent disclosures reporting evidence for a turnaround photoisomerization of 1 via diazenylallyl diradicals. The contribution of photoreduction to the decay of the first excited singlet state of reluctant azo compounds is emphasized.