Trifluoromethyl radicals polarity

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

Competition studies from Szwarc s group provided excellent quantitative insights into the relative affinities of methyl and trifluoromethyl radicals for a host of alkenes [86-88], and from this work came the first general recognition that substituted alkyl radicals could exhibit polar characteristics ranging from nucleophilic to electrophilic. On the basis of such early work, methyl and trifluoromethyl were taken to be the prototypical nucleophilic and electrophilic radicals, respectively, characterizations which it turns out are somewhat exaggerated in both cases. 

As concluded by Tedder, it would appear that a combination of polar and steric effects on the part of both the olefin and the trifluoromethyl radical are sufficient to determine the observed regioselectivities [93]. 

Tedder (reference 181) also discussed the importance of polar effects, for example, in the case of attachment of methyl or trifluoromethyl radicals to vinyl fluoride. 

This kind of divergent behaviour is often interpreted in terms of relative electronegativity, according to which picture the methyl radical is "nucleophilic" when compared to the "electrophilic" trifluoromethyl radical. We shall find a lot of evidence to support the idea that "polarity" plays an important part in determining the relative reactivity of fluoroalkyl radicals. 

Polar effects also appear to exist in the addition reactions of certain radicals to substituted ethylenes. It is clear from Table II that for electronegative radicals like difluoroamino or hydroxy and to a lesser extent trifluoromethyl radical and hydrogen atom, substitution with donor groups like methyl increases the rate of addition. Substitution with electronegative atoms like fluorine or chlorine has the effect of slowing the rate of addition by hydroxy or trifluoromethyl radicals but the rate of addition of methyl radicals is increased. 

Similarly, electrophilic CHj radicals add faster to ethylene than to tetrafluo-rethylene, in which the inductive effects of the fluorine substituents make the double bond somewhat electron deficient. The reverse selectivity is shown by CH3 radicals, which are nucleophilic in character [24]. This parallels the observations in organic chemistry thatcarbonium ion formation is facilitated by replacement of hydrogens by alkyl groups and hindered by CF3 groups. There is a polarization of a electrons towards the trifluoromethyl group, whereas these electrons can be more effectively released by alkyl substituents. 

The behavior of methyl and halomethyl radicals in their reactions with the fluoro-olefins (Table 1.2), can thus be rationalized hi terms of a more dominant role of polar factors and the nucleophilic or electrophilic character of the radicals involved. Methyl radicals are usually considered to be slightly nucleophilic, trifluoromethyl and trichloromcthyl radicals arc electrophilic (Tabic 1.4). 

Sigma electron withdrawing groups like trifluoromethyl have no effect at C4 which is consistent with partial radical and not polar character at that site. Trifluoromethyl at C2 does provide some rate acceleration. However, the origin of this effect is not clear since the SDKIEs are similar to those of the parent molecule. 

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