Triarylmethyl radicals

There will be a gradual loss of stable radical with these systems as the di- or triarylmethyl radicals produced from the macroinitiator can add monomer, albeit slowly.99 100 This side reaction provides a mechanism for mopping up the excess stable radical formed as a consequence of termination between propagating radicals and may be essential to maintaining polymerization rates. 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

In the absence of 1,4-CHD, the biradical 55e undergoes an intramolecular 1,5-hydrogen shift to form 57, making it possible for an intramolecular radical-radical coupling to occur to produce 58 (Scheme 20.13). The fact that 58 was produced from 54e lends support to the formation of the a,3-didehydrotoluene biradical 55e as a transient reaction intermediate. It is also worth noting that the benzylic radical center in 55 is a stabilized triarylmethyl radical. 

The use of l-iodo-9-fluorenone (59) for cross-coupling with phenylacetylene produced 60, which on treatment with 51 gave the benzannulated enyne-allenes 61 (Scheme 20.14) [43], Thermolysis of 61 in 1,4-CHD at 75 °C promoted the Myers-Saito cyclization reaction, leading to 63 in excellent yields. Again, the benzylic radical center in 62 is a stabilized triarylmethyl radical. 

The diketone 64 was also readily prepared from 59 as outlined in Scheme 20.15. Condensation between 64 and 2 equiv. of 51b gave 65 in excellent yield. Thermolysis of 65 in 1,4-CHD at 75 °C also promoted the Myers-Saito cyclization reaction to generate the biradical 66. The aryl radical center in 66 was then captured by the allenic moiety to form 67, having two stabilized triarylmethyl radical centers. Subsequent hydrogen-atom abstractions from 1,4-CHD then furnished 68. 

Similarly, exposure of 180 to trifluoroacetic acid also promoted an internal SN2 displacement reaction to form 181 (Scheme 20.37) [68], The Myers-Saito cyclization generated the biradical 182 and, subsequently, 183. As in the case of 55, the benzylic radical center in 182 is a stabilized triarylmethyl radical. Several related transformations to produce enyne-allenes have also been reported [69, 70]. 

Triarylmethanes also inhibit oxidation by hydrogen transfer to a peroxy radical. In this case it is the triarylmethyl radical which traps the second peroxy radical. 

The inhibition efficiency of the triarylmethanes decreases as the oxygen partial pressure is increased because of a decrease in the steady-state concentration of the triarylmethyl radicals. 

Figure 4.4. Spectra of radicals. Left ESR (top) and ENDOR (bottom) spectra of a triarylmethyl radical. [Reprinted with permission from B. Kirste, W. Harrer, and H. Kurreck, J. Am. Chem. Soc. 1985, 107, 20-28. Copyright 1985 American Chemical Society.] Right CIDNP effects in the vinyl proton absorbance region in a 60 MHz NMR spectrum from reaction of BuLi and BuBr before mixing (a), 30 s after mixing (b) and after completion of the reaction (c). [Reprinted with permission from H. R. Ward, and R. G. Lawler, J. Am. Chem. Soc. 1967, 89, 5518-5519. Copyright 1967 American Chemical Society.]...

Triarylmethyl compounds also form rather stable triarylmethyl radicals, and indeed the first stable carbon free radical to be reported was the triphenyl-... 

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