Unsaturated Radicals Bearing a Heteroatom in the Chain

The origin of the rate enhancement of the cyclization process caused by introduction of an allylic heteroatom such as oxygen in the chain is not clear. Furthermore, with other groups such as carboxylic esters, cyclization processes are greatly slowed down. Thus, even in a Cy5/Cy6 case, Walling was unable to observe cyclization products from CH2=CHCH2-0-C-CH2, and low yields 

At present there is no explanation for the rate enhancement or retardation of the cyclization process afforded by an allytic heteroatom in the chain. With regard to the possible interesting but more complicated effect of a vinylic heteroatom more studies are clearly needed. 

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