Azo Bridges from Azine 9. [2 + 2 + 1] Cycloadditions of Parallel C=C and N=N Bonds with Dihalocarbenes

Schemes 1 and 2 are examples of type A compounds in which the groups -N=N- and -CH=CH- are held in a close parallel position. They react with dichloro- and fluorochlorocarbene to form a unique pyrazolidine moiety (3–5) in a formal 2 + 2 + 1] cycloaddition which, despite the substitution pattern, is stable to ionization and hydrolysis according to Bredt's rule. The reaction is considered to start with the addition of the carbenes to the azo group. The azomethine imine thus formed subsequently undergoes a rapid 3 + 2] dipolar cycloaddition with the bridging C=C bond. The different isomers from 3–5 are identified from their NMR spectra.