Synthesis of Heterocycles Via Free Radical Cyclization

Abstract

<p> Aryl radical ring closures onto the azo functional group were investigated. A series of ortho-substituted aryl radicals (83a-f) have been generated by bromine abstraction from the corresponding 1-(orthobromophenyl)-1-methoxy-azoethanes (82a-f) by tributyltin radicals. The radicals generated underwent cyclization in the 5-endo sense, to ultimately afford the substituted indazoles (86a-f). There was also some evidence for cyclization to the other azo nitrogen (closure in the 4-exo sense) to form a 4-membered ring. The aryl radical also underwent hydrogen atom abstraction from tributyltin hydride in competition with cyclization. Since the rate constant for hydrogen atom abstraction from tributyltin hydride by aryl radicals is known, this makes it possible to estimate the rate constants for cyclization throughout the series.</p>