Regiochemistry in aryl radical cyclisations (5-exo versus 6-endo) of N-vinylic 2-iodobenzamides

Bu3SnH-mediated aryl radical cyclisations of a range of N-vinylic 2-iodobenzamides 10 were examined. The enamides 10a–d gave exclusively the 5-exo cyclisation products 11a–d, whereas the enamides 10e,f having a phenyl substituent on the vinylic carbon atom α to the nitrogen atom gave predominantly the 6-endo cyclisation products 14 and 18, respectively. The experiments on the effect of concentration of Bu3SnH or temperature on the products distribution showed that the formation of the 6-endo cyclisation products 14 and 18 was a result of a 5-exo cyclisation of the aryl radicals formed from 10e,f followed by neophyl rearrangement of the intermediate radicals.