Reactivity of Carbenes and Related Compounds towards Molecular Nitrogen

The thermal N 2 exchange of a number of 15 N‐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‐2 H ‐imidazole‐4,5‐dicarbonitrile ( 4 ), 4‐diazocyclohexa‐2,5‐dienone ( 5 ), and the conjugate acids of 4 and 5 , namely 4,5‐dicyano‐1 H ‐imidazole‐2‐diazonium ion ( 6 ) and 4‐hydroxybenzenediazonium ion ( 7 ). Only 1 , 4 , 6 , and 7 exchange their diazo group with ‘external’ molecular N 2 . 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 N 2 . Thus, reaction with carbenes is likely to occur only if the carbene is in the 1 A 2 singlet state and if its electrophilicity is high.