Clear cut example of preference for a 6-membered ring radical cyclisation over the homologous 5-membered ring radical cyclisation. The reaction of 5-[bromoalkyl]-5H-dibenzo[a,d]cycloheptenes with Bu3SnH

Article abstract of DOI:10.1016/0040-4039(96)00695-8

The radical cyclisation reaction of the 5-[2-bromo-ethyl]-5H-dibenzo[a,d]cycloheptene 4 with Bu₃SnH and AIBN led to the cyclised hydrocarbon 5 as major product, whereas the bromo-derivative 8 with Bu₃SnH and AIBN gave only the straight chain product 9. An explanation based on the conformational rigidity of dibenzo[a,d]cycloheptene system calculated by AMI method is proposed. Copyright