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.