N-Azinylpyridinium N-Aminides: An Approach to Pyrazolopyridines via an Intramolecular Radical Pathway

Abstract

Intramolecular radical arylation, under thermal condi- tions, to a -deficient pyridinium, linked to a -excessive 2-azi- nyliminopyridine moiety is described. The method allows a new entry to pyrazolo(1,5-a)pyridine nucleus. The generation and subsequent reactions of aryl radicals, derived from aryl halides using tri-n-butyltin hydride (Bu3SnH) and azobisisobutyronitrile (AIBN) is now well documented, and several syntheses, based on aryl radical cyclisations have been reported. 1 Few examples of het- eroaryl radicals are known, and presumably they would behave similarly to aryl radicals, since the lone electron would be in an orbital orthogonal to the aromatic -system and hence, its nature ( -excessive or -deficient) should have little or no effect on the reactivity of such radicals. 2 Particular attention has been devoted to pyridyl radicals; both Snieckus 3 and Nadin 4 have reported pyridyl radical cyclisations. Harrowven 5 has published some papers, which include pyridyl radical cyclisations and aryl radical cyclisations onto pyridines. Jones 6 has disclosed the use of radicals derived from 3-bromopyridine and the exten- sion of this chemistry to pyridinium radicals. 7 However, to the best of our knowledge, pyrazinyl radical cyclisa- tions have not been reported before.