Ring Transformations of Pyrazolones Via Azo-Olefins

Instead of substitution, 4-halo-pyrazolones undergo ring opening to azo-olefins (2, 12, 16) on reaction with nucleophiles. Attempted Hofmann degradation of N-Haloamides 4 has to compete with either ring closure, involving NHCl-shift to 7, or formation of mesoionic compounds 10, whose pyrazolone ring shows a different pattern of nitrogen insertion. Substitution of cyclic amines for chlorine in azo-olefin 12 is followed by rearrangement to the bicycle 15. Access to 1,2,3-thiadiazolium salts (18, 22) is possible by electrophilic attack of sulfur reagents to azo-olefins.