Electron spin resonance evidence for rapid cyclization of alkenyl radicals formed by readily occurring 1,5- and 1,6-hydrogen shifts in vinyl radicals

Abstract

Vinyl radicals have been generated (for in situ study by e.s.r. spectroscopy) by the addition of first-formed alkyl radicals to the triple bond in butynedioic acid. It is shown that rearrangements occur readily via 1,5- and 1,6-hydrogen shifts (with k > 10⁴ s^–1), even from unactivated C–H bonds. Subsequent rapid cyclization of the resulting alkenyl radicals (with k 10³ s^–1) can evidently be assisted by both electronic and steric effects: regioselectivity of ring closure leads to the formation of five-membered rings by reaction in the exo mode of hex-5-enyl radicals (formed via 1,6-shifts) and, in an unusually rapid reaction, by endo closure of the pent-4-enyl radicals (formed via 1,5-shifts).