Radical cyclization of 6-bromohex-1-ene

With tributyltin hydride and AIBN, 6-bromohex-1-ene forms the hex-5-enyl radical. This radical can take a hydrogen from the stannane and give hex-1-ene, or attack its own double bond.

5-exo cyclization, which forms a five-membered ring and a primary radical, is much faster than 6-endo for stereoelectronic reasons, and the cyclopentylmethyl radical gives methylcyclopentane. As the rate constant of the cyclization is well known, this reaction is used as a “radical clock”: the ratio of cyclic to open-chain product measures the rate of hydrogen transfer, and at low stannane concentrations the cyclic product predominates.