Neighboring group free radical

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

Allylstannanes are also available from allyl sulfoncs24 and sulfides25 by treatment with tri-alkyltin hydrides under free-radical conditions. Of some interest is the stereocontrol exercised by a neighboring hydroxy group, possibly because of interaction with the tin26. 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

See Glaser, R. Horan, C.J. Nelson, E.D. Hall, M.K. J. Org. Chem., 1992,57, 215 for the influence of neighboring group interactions on the electronic stmcture of diazonium ions. Aryl iodonium salts Ar2l also undergo substitutions by this mechanism (and by a free-radical mechanism). 

Neighboring-Group Assistance in Free-Radical Reactions... 

The emphasis here is on the word forced . As amply discussed in Section 2.1, the mutual repulsion between adjacent CN groups in the same chain is large, and tends to hold them apart in a helix-like structure. The dipole moment is certainly still quite high, if a free radical attack has transformed a CN group into a R—C=N radical. Neighboring CN groups from different macromolecules, on the other hand, attract themselves. This kind of arguments appears to favor the intermolecular reaction (Formula 5) over the intramolecular reaction (Formula 3). 

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