Ring Transformations of Pyrazolones Via Azo‐Olefins

Abstract

Instead of substitution, 4-halo-pyrazolones undergo ring opening to azo-olefins (2,U,B) on reaction with nucleophiles. Attempted Hofmann degradation of N-Haloamides 4 has to compete with either ring closure, involving NHCI-shift to z, or formation of mesoionic compounds 19, whose pyrazolone ring shows a different pattern of nitrogen insertion. Substitution of cyclic amines for chlorine in azo-olefin a is followed by rearrangement to the bicycle 15. Access to 1,2,3-thiadiazolium salts (I.& 22) is possible by electrophilic attack of sulfur reagents to azo-olefins.4,4-Dihalo-pyrazolones react with nucleophiles by ring opening to unsaturated azo compounds; ester 2, for example, is formed from dibromo-pyrazolone 1 on the action of methoxide. A reverse reaction was also observed : Morpholine converts 2 to the pyrazolone This Hofmann degradation had to compete with a shift of the chloramine group in the course of a pyrazolone formation yielding chlorimino pyrazolones 2. Action of ammonia to 2 gave the triazole 9. Obviously the pyrazolone ring was opened by ammonia to give 8, which cyclized to 9 with chloride ion elimination.A third reaction type was observed in several cases which competed successfully with Hofmann degradation :N-N-bond formation between the chlorine bearing nitrogen and an azo nitrogen in 4 lead to the deep red mesoionic 1p. The structure of j & was confirmed by an X-ray analysis. It is noteworthy that the new pyrazolones 1Q have incorporated only one of the nitrogen atoms of the starting pyrazolone, the other comes from the acid amide group.In an attempt to parallel the reaction of 2 to 3 using pyrrolidine in place of morpholine, azo olefine Q was not the final product. Its isomer L5 was isolated instead.Piperidine reacted analogously.-6 1 -