Radical fluorination

Several different methods of sidewall functionalisation, such as fluorination, radical addition, nucleophilic addition, electrophilic addition and cycloaddition, have been developed (Tasis et al., 2006). The sidewalls of vertically aligned CNTs have been functionalised with DNA using azide units as photoactive components. The azi-dothymidine reacted photochemically with sidewalls of CNTs utilising [2+1] cycloaddition. The oligonucleotides were grown in situ on the sidewalls of CNTs and the DNA-modified CNTs were obtained after the deprotection of the nucleic acid (Moghaddam et al., 2004). 

The cyclization of a range of fluorinated radicals has attracted interest. - The rate constants for the 5-exo cyclization of a range of fluorinated 5-hexenyl radicals have been studied as a function of the position and number of fluorine substiments. For fluorine substituents at or close to the alkene there is little effect on the rate, whereas... 

The )S-fluorinated radicals, such as the a-trifluoromethyl ones, are also destabilized due to the inductive effect. The dissociation energy grows from CH3CH2—H (97.7kcal/mol) to CF3CH2—H (102.0kcaFmol). ... 

In accordance with these early findings, a recent detailed study of the perfluorination of neopentane by Adcock [36] found the order of hydrogen reactivity to be CH3 > CH2F > CHF2 by a comparison of statistical and actual yields of the hydrofluorocarbon products obtained upon polyfluorination. Thus, the hydrogen abstraction step 2 a (Table 1) becomes less favourable as the C-H bond becomes increasingly electron poor and, consequently, less reactive towards highly electrophilic fluorine radicals. 

The Exfluor-Lagow method [43] involves the slow addition of both the hydrocarbon substrate and an excess of fluorine into a vigorously stirred chlorofluorocarbon or perfluorinated inert solvent. If required, reactions are completed by adding a small quantity of a highly reactive hydrocarbon, such as benzene, which reacts spontaneously with fluorine to produce a very high concentration of fluorine radicals ensuring perfluorination of the substrate. 

In the Liquid-Phase Photofluorination [39,44] process the reactant is injected at a very slow constant rate into an inert fluorocarbon solvent which is saturated by fluorine and under U.V. irradiation. Conditions are chosen to ensure that the concentration of fluorine and fluorine radicals is always much higher than the concentration of the substrate. This method is only suitable for the perfluorination of substrates, such as partially fluorinated ethers (see Section 2.5) and amines (see Section 2.7), that are both soluble in perfluorocarbon solvents and can withstand such vigorous reaction conditions. 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

Unlike the short-lived and highly reactive fluorinated radicals, there is a class of persistent radical species, stable in solution at room temperature also in the presence of air. One of these,40 Scherer s radical, perfluoro(l,l-diisopropylpropyl) [(CF3)2CF]2CF3CF2C is obtained by elemental fluorination of the parent hexafluoropropene-trimer alkene. Clearly, the unprece-... 

Recently, there have been a number of reports on stable fluorinated radicals, which include functionalized radicals and different fluorination methods, for example, electrochemical fluo-rination.41... 

Fluorination has little effect on the geometry of fluorinated radicals.96 The dramatic effect of fluorination on structure is clearly evident in the ESR spectroscopy of fluorinated radicals. 

All aspects of the structure, reactivity and chemistry of fluorine-containing, carbon-based free radicals in solution are presented. The influence of fluorine substituents on the structure, the stability and the electronegativity of free radicals is discussed. The methods of generation of fluorinated radicals are summarized. A critical analysis of the reactivities of perfluoro-n-alkyl, branched chain perfluoroalkyl and partially-fluorinated free radicals towards alkene addition, H-atom abstraction, and towards intramolecular rearrangement reactions is presented. Lastly, a summary of the synthetically-useful chemistry of fluorinated radicals is presented. 

Keywords Radicals, fluorinated radicals, radical structure, radical reactivity, radical rearrangements... 

Fluorinated radicals have played a significant role in the history and development of the field of free radical chemistry, and it was recognized quite early that they have natures which are quite different from those of their hydrocarbon counterparts. As a result, there has been much effort directed towards defining and understanding these differences with respect to their structure, reactivity and chemistry. 

Within our discussion of the stabilities of fluorinated radicals we have, of course, been referring to thermodynamic stabilities. In fact, most fluorinated radicals will be seen to have enhanced kinetic reactivity in reactions with closed shell molecules. However, appropriate fluorine substitution can also give rise to long-lived, or persistent radicals, the most dramatic example being Scherer s radical, 3, which persists at room temperature, even in the presence of molecular oxygen [39] ... 

Unfortunately, few experimental ionization potentials or electron affinities of fluorinated radicals have been reported, and the calculation of such molecular properties is fraught with difficulties, although reasonable trends can be predicted [48-49]. Table 3 provides what numbers are presently available [50-55],... 

In this section, each method of generating carbon-based fluorinated radicals will be introduced and discussed in terms of mechanism, and it will be seen that virtually all of the useful methods for generating perfluoro and partially fluorinated radicals in a practical manner involve well defined and controlled free radical chain reactions. Some representative examples which demonstrate the preparative use of these methodologies will be presented in the final section of this review. 

Fluorinated radicals, in contrast, would be expected to derive their electro-philicities virtually entirely from fluorine s inductive effect. One would expect the reactivity of perfluoro-n-alkyl radicals to differ significantly from that of their hydrocarbon counterparts, since the latter are electron-rich, planar n-radicals, whereas the former are electron poor, nonplanar cr-radicals. 

The data in Table 9 also allows one to reach conclusions regarding the elec-trophilicity vs nucleophilicity of the partially-fluorinated radicals since the three styrene substrates have a considerable range in IP values. /1-Fluorine, and to a lesser extent y-fluorine, substitution would appear to have a small impact on electrophilicity, whereas a single a-fluorine substituent seems to impart slightly nucleophilic properties. In a recent study, Takeuchi et al. have examined both computationally and experimentally radicals which bear both an a-fluorine substituent and an electron withdrawing ester function. They found that the a-fluorine substituent makes such radicals more electrophilic, but that they still add more readily to styrene than they do to acrylonitrile [127],... 

The trend of reactivities which is observed for hydrogen atom abstraction by partially fluorinated radicals is qualitatively similar to that for their addition to styrene. However, the absolute rates and the range of reactivities for each type of process can be seen to differ significantly. Thus, absolute rate constants for... 

Table 14. Rate constants for hydrogen abstraction from n by various fluorinated radicals at 25 3°C [31] -Bu3SnH and for addition to styrene ...

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