CYCLIZATIONS AND REARRANGEMENTS OF SAMARIUM DIIODIDE-GENERATED VINYL RADICALS

Vinyl bromides 1, 5, 15, 20, 23, 27, 33a,b, 41a-c, 44a-c, 46a,b, 52a,b, 55a, 56a,b, and 60a,b react with samarium diiodide in THF and/or acetonitrile to give the corresponding vinyl radicals. Radicals 3, 7, and 16 afforded products deriving from cyclization on the adjacent triple bond in a 5-(pi-exo)exo, 6-(pi-endo)exo, and 6-(pi-exo)exo mode, respectively. Radical 7 undergoes competitive 1,5-hydrogen translocation. Vinyl radicals 21 and 24 readily cyclize on the proximal double bond, leading to 5- and 6-membered rings, respectively. Thienyl-substituted radical 28 leads to a product deriving from 5-exo cyclization on the thiophene ring and subsequent beta-fission of the C-S bond, whereas thienyl-substituted radicals 34a,b undergo almost exclusively 1,5-hydrogen translocation. Aryl-substituted radicals obtained from vinyl bromides 41, 44, 46, 52, and 55 do not form any products deriving from 5-exo or 6-exo cyclization (or ipso cyclization). Finally, naphthyl-substituted radicals 57a and 61a give only direct reduction products, whereas the radical 61b affords, in addition to the direct reduction product, a rearranged product deriving from a 1,3-radical migration of the naphthylthio group. Evidence is reported for the reduction of EWG-substituted benzene rings and thio-substituted naphthalene rings to radical anions, which can fragment on the side chain with the elimination of allyl radicals. alpha-Oxy(and alpha-thio)-substituted radicals deriving from 1,5-shift rearrange to alcohols (and thiols) through a Wittig rearrangement or a cyclization/fragmentation process.