Cyclisation of unsaturated phosphoranyl radicals

Abstract

The reactions of a series of allylic and homoallylic phosphites and related compounds with t-butoxyl, ethoxyl, and methyl radicals have been studied by e.s.r. spectroscopy. Triallyl phosphite reacts to give detectable phosphoranyl radicals which undergo competitive β-scission (giving ally radicals) and cyclisation to give oxaphosphetanylmethyl radicals (four-membered ring). Tris-1-methylallyl and tribut-2-enyl phosphites behave similarly, but tris-2-methylallyl phosphite does not give rise to a cyclised species, probably as a result of steric inhibition of the intramolecular addition. The phosphoranyl radical from tribut-3-enyl phosphite cyclises to give an oxaphospholanylmethyl radical (five-membered ring), but not when the addendum radical is the bulky t-butoxyl. The phosphoranyl radical from tripent-4-enyl phosphite does not give rise to detectable cyclised product. Intramolecular competition experiments show that the four-membered ring is formed more readily than the five in the cyclised radicals. Diallylamino (trialkoxy) phosphoranyl radicals also cyclise to give azaphosphetanylmethyl radicals (four-membered ring).