Diphenylmethyl radical stability

Just as several alkyl substituents increasingly stabilize a radical center (Table 1.2), so do two phenyl substituents. The diphenylmethyl radical ( benzhydryl radical ) is therefore more stable than the benzyl radical. The triphenylmethyl radical ( trityl radical ) is even more stable because of the three phenyl substituents. They actually stabilize the trityl radical to such an extent that it forms by homolysis from the so-called Gomberg hydrocarbon even at room temperature (Figure 1.8). Although this reaction is reversible, the trityl radical is present in equilibrium quantities of about 2 mol%. 

The enantioselective reaction mechanism can be explained by considering the photochemical aspects and also the molecular arrangements (Scheme 36 and Fig. 7). Hydroacridine radical species 155 and 156, which satisfy both such conditions, should be preferably produced with their higher stability. Next, decarboxylation of 154 gives the diphenylmethyl radical 150. In the chiral crystal lattice of M-150 DPA, the molecular pairs of acridine and diphenyl acetic acid stack with... 

It was reasoned that, as with diphenylmethyl radicals, perchlorination might stabilize dephenylaminyl radicals. The synthesis of perchlorodiphenylaminyl [PDA ] has been effected (i) by oxidation of A -H-decachlorodiphenylamine (PDA—H) with either Fe(CN)6 "/HO" (in H O/CgHg) or Ag"0 (in CgHg) (ii) by dechlorination of the quinonoid perchloro compound [134] with molecular silver (in CCI4) (192) (Ballester et al., 1974a). 

The biradical catalysts described previously for double-base propints (Ref 80) are also effective for hydrocarbon propints. Table 34 shows how p,p,-biphenylene-bis(diphenylmethyl) compares to n butyl ferrocene as a catalyst in a carboxy-terminated polybutadiene. These catalysts are claimed to overcome all of the processing difficulties, chemical stability and volatility disadvantages attributed to catalysts based on ferrocene and carborane derivatives. Another somewhat similar functioning catalyst, the free radical compd, 2,2-diphenylpicrylhydrazyl,... 

In fact, the stability of the triphenylmethyl radical we know to be due mainly to steric, rather than electronic, factors. X-ray crystallography shows that the three phenyl rings in this compound are not coplanar but are twisted out of a plane by about 30°, like a propeller. This means that the delocalization in this radical is less than ideal (we know that there is some delocalization from the ESR spectrum) and, in fact, it is little more delocalized than the diphenylmethyl or even the benzyl radical. 

The bond from H to sp2 carbon is stronger than that to sp3 carbon so the vinyl (ethenyl), ethynyl and phenyl radicals are all less stable than the methyl radical. On the other hand, delocalized radicals such as allyl and benzyl are significantly more stable than methyl. The delocalization is demonstrated by ESR, so that delocalization and stabilization go hand-in-hand. Delocalization is increased if the radical can be spread over more than one ring, as in diphenylmethyl and triphenylmethyl. 

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