Other Sterically Hindered Phenyl Radicals

6-Tri(l -adamantyl)phenyl, [4] 4J, and octamethyloctahydroanthracen-9-yl, [5] 5Jj, have also been generated from their parent bromides and observed by EPR spectroscopy. Both of these radicals decayed with clean first-order kinetics. 

Arrhenius plots of their decay rate constants show pronounced curvature, see Fig. 

By analogy with the 1h 2Jj isomerization it can be concluded that 4 and 5Jj decay by intramolecular H-atom transfers involving 5-center cyclic transition states with QMT playing a dominant role. Unfortunately, product radicals could only be observed in the case of 2Jj, those from 4J, and 5J, could not be detected at any temperature. This is because these two product radicals have a very large number of individual EPR lines which would have made their detection extremely difficult. 

At the same temperature, 4 j and 5Jj are more persistent than 1 h [4, 5] (cf. Fig. 28.1 and 28.3). This was attributed to the fact that the minimum distance that the hydrogen atom must jump is considerably less for IJ, (1.34 A, assuming normal bond lengths and angles) than for the other two hindered phenyl radicals (e. g., 1.84 A for 4Jj) [4, 5]. Attempts to further confirm the importance of QMT in the isomerization of 4Jj and 5Jj by studying these reactions in frozen matrices at really low temperatures were frustrated by poor resolution of the phenyl radicals EPR spectra in the 

The much less sterically hindered phenyl radicals 6J, 7J, and 8Jj were generated 

---

From elementary organic chemistry, we know that the positions and hence reactivities of the electrons in unsaturated molecules are influenced by the nature, number, and spatial arrangement of the substituents on the double bond. As a result of these influences, the double bond reacts well with a free radical for compounds of the types CHj = CHY and CHj = CXY. These compounds constitute the so-called vinyl monomers where X and Y may be halogen, ally l, ester, phenyl, or other groups. It must, however, be noted that not all vinyl monomers produce high polymers. In symmetrically disubstituted double bonds (e.g., 1,2 disubstituted ethylenes) and sterically hindered compounds of the type CHj = CXY, polymerization, if it occurs at all, proceeds slowly. 

The use of Et3B as a radical initiator makes it possible to carry out the addition of other alkyl radicals to nitrone (286) using alkyl iodides. Good yields have been obtained of products (288b-d) when an excess of the appropriate alkyl iodide was used (Scheme 2.110). It has been established that the yield of alkyl by-products (288a) tends to decrease with the increase of the reaction temperature. The stereochemical features of this reaction are explained by the alkyl radical addition taking place predominantly from the less hindered re-face of (286) to avoid steric interaction with the phenyl group (525). 

---