Fluoroalkyl radicals

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

Minato ct a/.1(12 proposed that the transition state for disproportionation has polar character while that for combination is neutral. The finding that polar solvents enhance kJkK for ethyl170 and /-butyl radicals (Section 2.5.3.5), the very high kjktc seen for alkoxy radicals with a-hydrogens,171 and the trend in kJkK observed for reactions of a scries of fluoroalkyl radicals (Scheme 1.13, Table 1.7) have been explained in these terms.141102... 

Aryl radicals are much more reactive than alkyl radicals (k ca. 2 x 106M-1 s 1), and rather more reactive than cyclopropyl radicals (ca. 8 x 107) or vinyl radicals (ca. 3 x 10s). Fluoroalkyl radicals are also about 100 times more reactive than alkyl radicals, because of contributions from structures Rf 6 - H -4+SnR3 to the transition state. Singlet-excited acetone is twice as reactive as triplet-excited acetone, which shows a similar reactivity to that of the /-butoxyl radical (k ca. 2 x 108M-1 s-1). 

Table 1.19 Absolute rate constants of addition of fluoroalkyl radical to styrene and methylstyrene ...

However, substitution of sterically shielded hydrogen atoms appears to be possible, in 7 (Fig. 2) for example, but rearrangement of fluoroalkyl radicals (Fig. 2) [38], rather than direct substitution, has been suggested as the fluorination mechanism. 

The reactions of ethylenes substituted with fluorine and chlorine were determined by Anicich, Bowers and coworkers211-216. Su and collaborators217 tested several reactions of fluoroalkyl radical anions with C2F4 ... 

Recently, a quantitative study of the regiochemistry of addition of a number of different fluoroalkyl radicals to CHF = CF2, summarized in Table 6, indicated that the observed selectivity could be correlated with the postulated relative electrophilicity of the radicals, with the conclusion being reached that the secondary H-C5Fn(CF3)CF radical was the most electrophilic [94]. 

Table 6. Regioselectivities of addition of some fluoroalkyl radicals to trifluoroethylene [94]...

As would be expected for reactions with polar transition states, additions of per-fluoroalkyl radicals to alkenes are faster in CH3CN than in Freon 113 with the observed solvent effects being greater for additions to alkenes which are more electron-rich [70,117]. Table 10 provides comparisons of rates in the two solvents. 

Because perfluoroalkyl radicals are easily generated by a variety of means, and because they exhibit high reactivity towards diverse types of organic substrates, the use of fluoroalkyl radicals has become a preferred method for the incorporation of fluoroalkyl groups into organic compounds. 

A difficult process to execute successfully is addition of an electrophilic per-fluoroalkyl radical to an ethene which is itself substituted with a perfluoroalkyl group. However, Hu and Hu have reported a smooth conversion of this type using NiCl2 6H20/Al/alkene/RFI in a ratio of 0.2 2 1.5 1 in CH3CN at RT [258],... 

An acid of roughly similar strength as acetic acid is HE. In fact, elimination of F from ot-hydroxy-y9-fluoroalkyl radicals has been observed [69], e.g. ... 

Mason has reported on the photolysis of perfluoronitromethane and hepta-fluoronitropropane. Primary dissociation into NO2 and fluoroalkyl radicals is presumed to be important. 

Effect of fluorine substituents on the structure> stability, and reactivity of fluoroalkyl radicals... 

Taking into account the marked pyramidalization of carbon radicals by a-fluorine substituents, it is interesting to see whether benzylic or allylic conjugation would make the fluoroalkyl radicals planar or not. The observed a(19F) hfs constants (42.6 and 39.7 G) for 1,1,3,3-tetrafluoroallyl radical [18] are smaller than that for CH3CF2 radical (94.0 G)... 

Generation and stereospecific reaction of fluorocyclopropyl radicals provided experimental evidence for the nonplanarity of the fluoroalkyl radical [23]. Hunsdiecker reaction... 

The ionization potentials (IP), electron affinities (EA), and absolute electronegativities of fluoroalkyl radicals are useful in order to elucidate the nucleophilic and electrophilic reactivities of the fluoroalkyl radicals. Table 1.36 summarizes available IP and EA data, indicating that (1) all of the a-fluoromethyl radicals have lower IPs than methyl radicals in spite of the strong inductive effect of the fluorine atom, and (2) trifluoromethyl and pentafluoroethyl radicals are, of course, more electrophilic than methyl and ethyl radicals because of their higher values of EA [30]. The former result may arise from the electron-donating conjugation of lone-pair electrons on the fluorine atom, and the latter is due to the strong inductive effect of the fluorine atom. 

Table 1.36 Experimental ionization potentials, electron affinities, and absolute electronegativities of some alkyl and fluoroalkyl radicals (eV)...

S02,403 on the photoreduction of quinones,404 on acetyl and acetyl peroxide radicals in gas-phase reactions,405 on fluoroalkyl radicals in solution,408 on the photochemistry of 2,2-dimethoxy-2-phenylacetophenone,407 on pyridine-3,5-dicarboxylic acid,408 on photolytically reduced pyrazine,409 410 on the dimerization... 

D.M. Guldi, M. Kumar, P. Neta, and P. Hambright, Reactions of Alkyl and Fluoroalkyl Radicals with Nickel, Iron, and Manganese Porphyrins, J. Phys. Chem., 96 (1992) 9576. 

The propagation step in this free radical polymerization has been demonstrated to involve radical attack on episulfide sulfur (as depicted above) by telomerization studies with cyclohexane. From the structures of the two major products, 20 and 21, cyclohexyl radical attacks at the sulfur atom by displacing fluoroalkyl radical. This new radical, a prototype of the growing polymer chain, then abstracts hydrogen from cyclohexane to form 20 or attacks another molecule of 6 (n = 1) at sulfur to give 21. 

For the reaction of OH radicals with vinyl chloride and vinyl bromide the halogen elimination reactions are thermo-chemlcally favorable, the overall reactions being <11 kcal mole and <24 kcal mole exothermic for X = Cl and Br, respectively (203). The elimination of Br atoms from activated chloro-bromoalkyl radicals (206, 207), and of H, Cl, or Br atoms from activated fluoroalkyl radicals (208-213), have been studied using molecular beam techniques, these Intermediate radicals being produced by the reaction of Cl atoms with bromlnated alkenes (206, 207) or of F atoms with alkenes and halogen-substituted alkenes (208-213). For the elimination of Br atoms In the reactions... 

Fluorinated diacylperoxides were also used as the source of fluoroalkyl radicals. The electron-transfer mechanism was proposed for the reaction of compound 21 with furan. ... 

Lachinov and co-workers [52] have performed a more detailed study of the kinetics of block radical polymerisation of perfluoroalkylmethacrylates (FMA) in the solid state. The main method of investigation of the kinetics of FMA polymerisation was isothermic calorimetry [58]. Due to the absence of data on the heat effects of their polymerisation in the reference literature, these values were measured [52]. The values of AQ and glass transition point (T ) of polymers formed are shown in Table 8.3. Obviously heat of polymerisation of monomers of the fluoroacrylate sequence is quite close to heat of polymerisation of non-substituted monomers of the AMA sequence [59], and a significant influence of the length of the fluoroalkyl radical on this parameter is absent [52]. In accordance with the Polyani-Semenov rule, the present result makes it possible to consider that chain propagation constant of FMA with the accuracy of the pre-exponential multiplicand being equal to each other [57]. 

Therefore, initial polymerisation rates of these monomers (as in the case of AMA [60]) grows with length of fluoroalkyl radical, and degree of the gel effect display decreases. 

The similarity in the behaviour of the FMA sequence of monomers with that of the AMA sequence was observed that not only on the initial stages of the polymerisation, but also of the ones that followed. The onset of the gel-effect in the FMA sequence is shifted to the higher side of the degree of transformation (Table 8.5) with growth of fluoroalkyl radical length of the monomer, all other factors being the same. 

It was also shown [52] that at radical homopolymerisation of monomers from the fluoroalkyl methacrylic sequence in mixture conversion of the autoacceleration onset increases, and intensity of the gel-effect display decreases as the length of fluoroalkyl radical of the monomer increases. 

Structure, length of hydrocarbon chain, location [26] and amount of fluorine atoms in fluoroalkyl radical significantly affect properties of appropriate methacrylic polymer (see Table 8.9). Increasing the content of fluorine atoms in the polymer, increases the chemoresistance, thermo- and heat resistance and water absorption decreases. It should be noted that the fluorine atom in the ro-position in fluoroalkyl radical leads to a decrease of the surface tension, which is also the the reason for poor adhesion of the fluorine-containing polymer to the core and water absorption. In the a-position, the heat resistance and other properties of polymers obtained on this basis are presented in Table 8.10 [25]. 

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