Single Electron Transfer in Metal-Halogen Exchange. The Reaction of Organolithium Compounds with Alkyl Halides

Article abstract of DOI:10.1021/jo00383a023

Halogen-metal exchange has been studied by allowing tert-butyllithium to react with a series of alkyl halides containing a cyclizable radical probe in order to evaluate the occurrence of a radical intermediate in the reaction.It was found that radical intermediates, formed via a single-electron-transfer pathway, are involved in the reactions of t-BuLi with the radical probe, endo-5-(2-bromoethyl)-2-norbornene in pentane/Et2O at -78 deg C, since cyclized hydrocarbons were formed during the reaction.However, there was no evidence to support an electron-transfer pathway in reactions of the corresponding iodide and chloride with t-BuLi under the same conditions since only the straight-chain organolithium compound was formed.However, evidence indicative of a radical intermediate was obtained from reactions of the iodo compound with t-BuLi in pentane/Et2O at higher temperatures (-45 deg C and -23 deg C) and in pure pentane at -78 deg C and -23 deg C in which stable cyclized hydrocarbon product was obtained.Lithium complexing agents such as TMEDA, HMPA, and 18-crown-6 were employed in these reactions in order to increase the carbanionic nature of the organolithium products.It was found that cyclization of the straight-chain organolithium compound to the corresponding cyclized organolithium compound in the presence of such complexing agents is relatively slow.On the other hand, the effectiveness of the complexing agents to increase carbanion character of the straight-chain organolithium compound was demonstrated by a significant lowering of the deuterium content of the straight-chain product due to ether cleavage by the straight-chain organolithium compound.Reactions of the 6-halo-1-heptenes with t-BuLi were also examined.It was shown that 6-bromo-1-heptene reacted with t-BuLi via SET pathway in both pentane/Et2O and pentane at -78 deg C since cyclized product with a high cis/trans ratio was the major product in these reactions.On the other hand, the reaction of the corresponding iodide with t-BuLi in pentane/Et2O at -78 deg C produced a high yield of cyclized product with a low cis/trans ratio (1.7).The reaction of the alkyl iodide with t-BuLi in pure pentane produced the cyclized product with a cis/trans ratio (4.0) indicative of a radical intermediate.Lithium complexing agents were also added to these reaction mixtures, in which the product ratio remained constant although the deuterium concentration decreased.Hence, the presence of the cyclized products with a high cis/trans ratio in the presence of cation complexing agents is indicative of the formation of radical intermediates both in the presence and absence of such intermediates.