NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition state phase.
Click the structures and reaction black arrows in sequence to view the 3D models and animations respectively
Even though the six membered ring gives a stabilised radical as an intermediate, the formation of the five membered exo product is more favoured. The reaction proceeds via an abstraction of iodine by a trimethyltin radical. The chain can then twist into a position to cyclise. Not shown here is the abstraction of hydrogen from trimethyltinhydride by the methylcyclopentane radical to form the endo product.
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H. Fischer and L. Radom, Angew. Chemie Int. Ed., 2001, 40, 1340–1371.