The perfluoroalkyl effect

Saturated, strained, small ring systems are uniquely stabilised by the introduction of perfluoroalkyl groups, as compared with the corresponding hydrocarbon derivatives, and this has allowed the study, for instance, of many long-lived valence-bond isomers 

The introduction of electron-withdrawing perfluoroalkyl groups into unsaturated systems lowers frontier orbital energies [35], as deduced by theory [36] and photoelectron spectroscopy [37] for a series of fluorinated aUcenes, and manifestations of this effect are seen in much of the chemistry of such systems. 

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D. M. Lemal and L. H. Dunlap, Jr., J. Am. Chem. Soc., 94, 6562 (1972). In this paper, these authors introduce the concept of the perfluoroalkyl effect to explain the relative (kinetic and/or thermodynamic) stabilization of strained species relative to their normal counterparts by CF3 and other Rr groups. 

The triazoline adducts from benzvalene (Scheme 21)162 and diphospha-benzvalene (Scheme 22) photolyze to yield novel tetracyclic aziridine ring systems165 that are valence isomers of azepines,162 whereas that from De-war thiophene (Scheme 20) gives a novel tricyclic aziridine that desulfurizes with triphenylphosphine to yield the trifluoromethylated Dewar pyrrole (Scheme 153).15 9,160 The stabilization of these strained molecules is attributed to the perfluoroalkyl effect.159... 

Fluorochemicals repel both water and oU because they produce an extremely low energy surface (18—26). The effectiveness of the fluorochemicals depends upon uniform surface coverage and orientation of the molecules on the fiber surface so that the perfluoroalkyl chains are directed away from the surface. The result is a GST as low as 5—10 mN /m (dyne/cm). Fluorochemical finishes are often formulated with nonfluorinated resin-based water-repeUent extenders. These water repeUents not only reduce the cost of the finish but may also improve durabUity (27,28). 

Under sonication conditions, the reaction of perfluoroalkyl bromides or iodides with zinc can be used to effect a variety of functionalization reactions [39, 40, 41, 42] (equation 30) Interestingly, the ultrasound promoted asymmetric induction with the perfluoroalkyl group on the asymmetric carbon was achieved by the reaction of perfluoroalkyl halides with optically active enamines in the presence of zinc powder and a catalytic amount of dichlorobisftc-cyclopenta-dienyl)titanium [42] (equation 31)... 

FITS reagents), has undergone considerable development recently [141,142,143, 144, 14S. These compounds, available fromperfluoroalkyhodides (equation 76), are very effective electrophilicperfluoroalkylating agents They react with carban-lons, aromatic compounds, alkenes, alkynes, silyl enol ethers, and other nucleophiles under mild conditions to introduce the perfluoroalkyl moiety mto organic substrates (equation 77) (see the section on alkylation, page 446). 

Perfluocoalkyl groups thermodynamically destabilize double bonds and small rings, but they can kineiically stabilize highly stramed molecules [75]. This remarkable perfluoroalkyl effect has made possible the isolation of stmctures that are uncommon m hydrocarbon chemistry, especially valence-bond isomers of aromatics and heteroaromatics such as 1, 2, and 3 [108],... 

The hydrophobicity index is also suitable for correlating the cM values of various substituted sodium alkane 1-sulfonates [68]. The perfluoroalkyl substituent, e.g., 8 17 has a pronounced hydrophobic effect (/ = 1.66 at 75°C, sodium salt), whereas the hydrophilic disulfonates have values distinctly below 1 (for a-disulfonates, / = 0.75 was derived [70]). Further, it was somewhat surprisingly shown that substituents like 1-hydroxymethyl, 3-hydroxyethoxy, or even the hydroxyethoxyethoxy groups have hydrophobic effects. 

Industrially, the perfluoroalkyl iodides by telomerization are mostly made by a batch system using peroxide initiators. However, the difficulty of mass production, and the production of hydrogen-containing byproducts in the process are disadvantageous [4]. In this study, a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase and the effects of the particle size on the catalytic activities of different the preparation methods and active metals were considered. 

The structure and packing of the complex with n = 8 and m = 3 are shown in Figs. 14 and 15, respectively (d -i = 2.746 A N- T-C = 176.99°). The former shows an expected disposition of the stilbazoles and the perfluoroalkyl entity, while the latter shows an effective segregation of the fluoro alkyl and stilbazole segments. 

Another spacer which was used to insulate the phosphorus atom from the electron-withdrawing effect of the perfluoroalkyl tail is the -0-(CH2) - spacer that contains an electron-donating oxygen atom directly attached to the aryl ring [25]. Fluorous derivatives of triphenylphosphine containing this ether spacer (13a-c) were prepared, though the lower coupling constant of cis-... 

A linear correlation of the oxidation potentials Bp of sulfides with Taft s a values of fluoromethy] groups is obtained as shown in Fig. 1 [42]. This dearly indicates that the polar effect of the fluoroalkyl group plays a significant role in the electron-transfer step from the sulfides to anode. Namely, the oxidation potential increases linearly as the number of fluorine atoms of the fluoroalkyl group increases. However, interestingly the oxidation potential was not appreciably affected by the length of the perfluoroalkyl group (Table 4). 

Because the forces of attraction prevail when molecules are brought into sufficiently dose proximity under normal conditions, release is best effected if both the strength of the interaction and the degree of contact are minimized. Aliphatic hydrocarbons and fluorocarbons achieve the former effect, finely divided solids the latter. Materials such as microcrystalline wax [64742 42-3] and hydrophobic silica [7631-86-9] combine both effects. Some authors refer to this combined effect as the ball bearing mechanism. A perfluoroalkylated fullerene nanosphere would perhaps be the ultimate example of this combined effect (17). These very general mechanistic remarks can be supplemented by publications on the mechanism of specific classes of release agents such as metallic stearates (18), fatty acids and fluorinated compounds (19), and silicone-coated rdease papers (20,21). The mechanism of release of certain problem adherents, eg, polyurethanes, has also been addressed (22,23). 

The reactivity of a terminal 1,13-diene with an interstitial deca(difluoromethylene) chain 1 is lower towards expoxidation with hypofluorous acid/acetonitrile complex21 due to the electron-withdrawing effect of the perfluoroalkyl chain attached directly to the C = C bond. The diepoxide 2 is obtained by repeated reaction with a very large excess of the oxidizing agent.21... 

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

Recently, the electrocarboxylations of benzyl and aryl halides and perfluoroalkyl-halides [39] in supercritical mixture or in supercritical carbon dioxide (scC02) and of aryl and benzyl halides in microemulsion [40], were also investigated in order to exploit the possible effect of the use of these solvents on the selectivity of the... 

The electronic effect of perfluoroalkyl substituents on the absorption spectra of arenes is relatively small (e.g., Xmax 3-perfluorooctylthiophe-ne = 229 nm 3-octylthiophene = 235 nm). Thus, the anomalously low absorption maximum of 504 is the effect of twisting around the backbone. It is apparent that the difference in size of the side chains is sufficiently large to cause twisting of the conjugated backbone of 504 due to steric interactions between the perfluoroalkyl substituents and the adjacent repeat unit. 504 exhibits green fluorescence (Xmax = 512 nm) in solution with a maximum blue shifted by 58 nm relative to POT (570 nm). Accordingly, 504 shows a Stokes shift of ca. 1.4 eV (186 nm) compared to only 0.6 eV (126 nm) for POT. 

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