Reactions of tert-Butyllithium with α,ω-Dihaloalkanes. Evidence for Single-Electron-Transfer-Mediated Metal-Halogen Interchange Involving Alkyl Radical-Halide Ion Adducts

Article abstract of DOI:10.1021/jo00186a004

The reactions of tert-butyllithium (t-BuLi) with primary α-iodo-ω-haloalkanes and α,ω-dibromoalkanes have been investigated in experiments conducted at -23 deg C in n-pentane-diethyl ether (3:2 by volume) solution.It has been found that production of an α-lithio-ω-haloalkane (1) by metal-halogen interchange at one end of a 1,3-, 1,4-, or 1,5-dihalide results in intramolecular coupling to give cycloalkanes in high yield.As the chain length of the dihalide is increased beyond five carbon atoms the production of an α,ω-dilithio species by interchange at bothends of the dihalide ceases to be excluded by cyclization of 1 and a precipitous drop in the yield of cycloalkane was found to occur in reactions of 1,6-diiodoalkanes with t-BuLi.Metal-halogen interchange was the exclusive process observed in reactions of α,ω-dihalides with t-BuLi provided at least one halogen of the substrate was an iodine.By contrast, similar treatment of α,ω-dibromoalkane provided only minor amounts of product attributable to metal-halogen interchange.Under conditions that provide an essentially quantitative yield of carboxylic product from reductive cyclization of 1 generated from 1,5-diiodo-3,3-dimethylpentane, the corresponding 1,5-dibromide was converted to a mixture composed of cycloalkane, products from Wurtz-type coupling with t-BuLi, and quantities of parent alkane from formal reduction of the dihalide.The mechanism of the metal-halogen interchange was further probed by using 6-halo-1-hexene substrates.Observation of cyclized product from the reaction of t-BuLi with 6-bromo- and 6-iodo-1-hexene demonstrated that the interchange between t-BuLi and primary alkyl bromides and iodides occurs predominantly via single-electron-tranfer (SET) process.Incorporation of alkyl radical-halide ion adducts with halogen dependent lifetimes as intermediates following SET from alkyllithium to alkyl halide serves to explain the disparate behavior of alkyl bromides and iodides when treated with t-BuLi.A unifield SET-mediated mechanism for metal-halogen interchange and Wurtz coupling is presented.