Reactivity of Carbenes and Related Compounds towards Molecular Nitrogen

The thermal N2 exchange of a number of 15N‐labelled diazo compounds was studied in solution. The compounds involved were 3‐diazo‐1‐methylindolin‐2‐one (1), 9‐diazofluorene (2), 5‐diazo‐1,3‐cyclopentadiene‐1,2,3,4‐tetracarbonitrile (3), 2‐diazo‐2H‐imidazole‐4,5‐dicarbonitrile (4), 4‐diazocyclohexa‐2,5‐dienone (5), and the conjugate acids of 4 and 5, namely 4,5‐dicyano‐1H‐imidazole‐2‐diazonium ion (6) and 4‐hydroxybenzenediazonium ion (7). Only 1, 4, 6, and 7 exchange their diazo group with ‘external’ molecular N2. The results are explained on the hypothesis that only organic species which have an empty σ orbital and which are effective in π electron back‐donation are able to react with N2. Thus, reaction with carbenes is likely to occur only if the carbene is in the 1A2 singlet state and if its electrophilicity is high.