Fluorine substituent stabilising radicals

Do fluorine substituents have an effect on thermodynamic stabilisation, or not We might expect fluorine to have a similar stabilising influence to that of oxygen on the formation of radicals (Figure 4.44). However, we have already noted the schizophrenic nature of fluorine in carbanions, where inductive electron withdrawal wrestles with I, electron repulsion, and it is a similar situation with radicals. 

Even less obvious in this particular case is evidence that the less-substituted radical is actually more stabilised, presumably by negative hyperconjugation with the f3 C—F bonds, than the more-substituted radical is by the attached F atoms.997 Furthermore, in the other striking reversal of expectations, the tert-butoxy radical, which is certainly more electrophilic than methyl, adds to 1,1-difluoroethylene 7.45 at the substituted carbon with a selectivity of 80 20,998 whereas the methyl radical is normal in this case. Thus the coefficients in the LUMO are hardly likely to be the explanation, and one suggestion in this particular case is that there is a growing anomeric effect between the oxygen atom and the two fluorine substituents in the transition structure for the formation of the major intermediate 7.46.988... 

For every radical the activation energy is greater for addition to CHF-end of vinyl fluoride than for addition to the CH2" end, and in every case the activation energy for addition to the CH2" end of vinyl fluoride is greater than that for addition to ethylene. In other words radicals add preferentially to the least substituted end of vinyl fluoride because attack at this site is less deactivated than attack at the substituted end. Not because attack at the CH2- end is activated. A single fluorine substituent is deactivating to all radicals, including methyl. This conclusion shows that simple resonance pictures in which a substituent fluorine atom is supposed to stabilise an adduct radical is not an adequate picture. 

Stepwise reactions by way of diradical intermediates are also possible such reactions often require rather high temperatures, but radicals are probably involved in the formation of cyclobutanes like that from the halogenated alkene 6.138 and butadiene giving the cyclobutane 6.140.717 As we saw in Chapter 2 (Section 2.1.5), any group, C, Z or X, can stabilise a radical. Both radical centres in the intermediate 6.139 are stabilised, the one on the left by the a-chlorines and the /3-fluorines, and the one on the right because it is allylic. There are a number of reactions like this—all that is required is enough radical-stabilising substituents. 

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