Lithium-iodine exchange mediated atom transfer cyclization: Catalytic cycloisomerization of 6-iodo-1-hexenes

Treatment of 6-iodo-1-hexene (1) in a variety of solvent systems with either MeLi or PhLi at room temperature or above effects clean cycloisomerization of 1 to (iodomethyl)cyclopentane (4). This novel transformation, which is mediated by rapid and reversible lithium-iodine exchange processes, most likely involves the following discrete steps: (i) generation of 5-hexenyllithium (2) from 1 by exchange with MeLi or PhLi, (ii) irreversible cyclization of 2 to give (cyclopentylmethyl)lithium (3) and (iii) virtually complete conversion of 3 to (iodomethyl)cyclopentane (4) via a highly favorable lithium-iodine exchange equilibrium. The isomerization sequence 1 --> 4 has been found to be effectively catalytic in PhLi when conducted in hydrocarbon-ether solvent mixtures at room temperature.