Mechanism of lithium-halogen exchange of primary alkyl iodide

On Clayden's page 190 this reaction is given for preparation of organometallics: 


[![enter image description here](https://i.stack.imgur.com/DvQHW.png)](https://i.stack.imgur.com/DvQHW.png)


What is the mechanism of this reaction? I expected the "almost carboanion" part of the $\ce{t-BuLi}$ to attack as a nucleophile and form a longer 7 membered chain? But instead, that almost anionic $\ce {t-Bu^-} $ has attached to iodine.


I realised that $\ce{t-Bu-}$ is too unstable because of electron releasing alkyls, but what is the exact mechanism of this reaction? 




With primary alkyl iodides the (*simplified*) mechanism is likely similar to a $\mathrm{S\_N2}$ process, but with attack of $\ce{t-Bu}$ at iodine instead of carbon.


[![Simplified mechanism](https://i.stack.imgur.com/BBSDM.png)](https://i.stack.imgur.com/BBSDM.png)


Note that this is not necessarily the case for other substrates, e.g. aryl iodides, or aryl/alkyl bromides. In general lithium-halogen exchange can occur either via a polar mechanism (such as the one shown above) or via a radical mechanism (by two successive electron transfers from RLi to RX). There is also a solvent and temperature dependence. Why does the solvent matter? We know that ethereal solvents such as Et2O or THF are capable of acting as Lewis bases - or ligands for lithium - and possibly assist in deaggregation of the alkyllithium. This sort of thing can't be depicted easily with curly arrows, which is why I add the disclaimer that the mechanism above is (heavily) simplified.


In any case, this particular substrate ([*Tetrahedron Lett.* **1986,** *27* (17), 1861–1864](https://doi.org/10.1016/S0040-4039(00)84395-6)) is fairly interesting because if the Li/I exchange were to occur via a radical pathway, the radical formed would cyclise onto the alkene via a 5-*exo*-trig pathway (even at −78 °C):


[![Radical cyclisation](https://i.stack.imgur.com/RbPS2.png)](https://i.stack.imgur.com/RbPS2.png)


However, if you quench the reaction at −78 °C with methanol, no methylcyclopentane is observed, which rules out the intermediacy of a radical. [If you warm up the reaction before quenching then the *organolithium* can cyclise by nucleophilic attack onto the alkene.]


This particular case (and general mechanisms for Li/Hal exchange) are described quite thoroughly in [Clayden's *Organolithiums: Selectivity for Synthesis*](https://www.elsevier.com/books/organolithiums-selectivity-for-synthesis/clayden/978-0-08-043261-8), Chapter 3.


There is also an [excellent set of slides](https://www.princeton.edu/chemistry/macmillan/group-meetings/SL-LiExchange.pdf) on Li/Hal exchange reactions, courtesy of the MacMillan group at Princeton; and also [another set](http://www.scs.illinois.edu/denmark/wp-content/uploads/gp/2005/gm-2005-03_08.pdf), courtesy of the Denmark group at the University of Illinois.


 5