EFFECTS OF TEMPERATURE ON ATOM TRANSFER CYCLIZATION REACTIONS OF ALLYLIC ALPHA-IODO ESTERS AND AMIDES

Atom-transfer cyclizations of allyl iodoacetates and N-allyl-N-methyliodoacetamides are much more efficient at 80-degrees-C than at 25-degrees-C. At 80-degrees-C, beta-(iodomethyl) lactones and lactams are formed rapidly and in good yield under standard atom-transfer conditions (sunlamp irradiation of iodide and 10% hexabutylditin in benzene for 10-60 min). It is proposed that this temperature effect is responsible for some unusual observations by Jolly and Livinghouse in the cyclization of N-cyclohexenyl-N-methyliodoacetamide. The results suggest that the beneficial effect of temperature arises because an increase in the rate of rotation of the OC-O or OC-N bond in the intermediate radicals begins to convert syn radicals (which cannot cyclize) to anti radicals (which can cyclize). Consistent with this hypothesis, the radical derived from N,N-diallyliodoacetamide (which always has a favorable arrangement for cyclization) closes with excellent efficiency at 25-degrees-C.