Fluorinated alkyl radicals

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

Table 2. ESR hyperfine splitting constants for 1°, 2° [4,10-13] and 3° alkyl and fluorinated alkyl radicals ...

Table 3. Experimental ionization potentials, electron affinities and absolute electronegativities of alkyl and fluorinated alkyl radicals [50-55]...

Perfluoroalkyl iodides comprise the most important and commonly-used source of perfluoroalkyl radicals [57-59], and for the most part, the methods which have been developed for such perfluoro systems also work well for generation of partially-fluorinated alkyl radicals from their respective iodides. 

In order to determine whether the enhanced reactivities of perfluoroalkyl radicals could be attributed to some linear combination of the individual contributions of fluorine atoms on the a-carbon (the radical center), the /1-carbon and the y-carbon atom, the absolute rates of addition of a number of parfza/Zy-fluorinated alkyl radicals to a-methylstyrene, styrene, and pentafluorostyrene were determined by LFP [70]. The data in Table 9 clearly indicated that this is not the case, that y- and /1-fluorinated-n-alkyl radicals exhibit little enhancement, while a-difluoroalkyl radicals, although more reactive, remain very much less-reactive than analogous perfluoro species. 

A sample of polytetrafluoroethylene, Aflon G80 (Product of Asahi Glass Company), was y-irradiated up to 40Mrad at room temperature under vacuum of 10 Pa. The irradiated sample was heated at 500 K for 40 min in order to observe only the trapped stable fluorinated alkyl radicals. The sample for the NMR study was irradiated in the same way as the sample for the ESR study. Line widths of ESR spectra of alkyl radicals at various temperatures were measured by a X-band spectrometer, and the line width and second moment of broad-line NMR spectra at various temperatures were obtained at 40 MHz. The temperature dependences of line widths (second moment) of ESR and NMR spectra are shown in Fig. 8.5., from which relaxation spectra I(t) can be obtained both from ESR and NMR data based on Eq. (8.3). values can be estimated by use of Eq. (8.7) both for ESR and NMR observations. 

In this section, evidence will be provided for the formation of HOO , O2, TiOO, and fluorinated alkyl radicals in Nafion/Ti(in)/H202. Separate signals from these radicals species were obtained by variation of sample preparation methods, temperature used for spectra acquisition, and annealing conditions. The formation and stability of these radicals were compared to those of radicals present in the Eenton reagent (TiCl3 + 112 2)... 

Fluorinated alkyl groups (Rp) can be radically attached to Ggp, for example by reaction of mercury-alkyl compounds under irradiation [10, 24] or by reaction with fluorinated alkyl iodides or bromides [25, 26]. Photochemical reaction with (R3Sn)2 or thermal reaction with RjSnH generates the Rp radical from the corresponding iodides or bromides. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

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. 

Fluorine substituents have a dramatic impact upon the structure of alkyl radicals. The methyl radical itself is planar UV, IR, PES and ESR spectroscopy, as well as the highest level of theoretical analysis, all indicate that its conformational properties are best defined as deriving from a single minimum [2], Fluoromethyl radicals, on the other hand, are increasingly pyramidal [3], with the trifluoromethyl radical being essentially tetrahedral [3 - 7], with a significant barrier to inversion [8,9]. 

To summarize the considerable available structural data with respect to fluorine substitution, one can conclude that non-conjugated carbon radicals bearing at least two fluorine substituents will be strongly pyramidal, a-radicals, while //-fluorine substituents appear to have little influence on the geometry of a radical. The strong a-character of CF3, CHF2, and perfluoro-n-alkyl radicals has a considerable influence on their reactivity. 

The influence of fluorine substituents on the stability of alkyl radicals derives from the same complex interplay of inductive and resonance effects that affects their structure. Simple orbital interaction theory predicts that substituents of the -X type (that is, electronegative substituents bearing lone pairs) should destabilize inductively by virtue of their group electronegativities, and stabilize by resonance to the extent of their ability to delocalize the odd electron. 

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. 

Table 9. Absolute rate constants for reactions of alkyl and fluorine-substituted alkyl radicals with three styrenes in Freon 113 at 298 2° K, as measured by LFP [70] ...

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