Perfluoroalkyl effect

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],... 

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... 

Aromatic perfluoroaLkylation can be effected by fluorinated aUphatics via different techniques. One category features copper-assisted coupling of aryl hahdes with perfluoroalkyl iodides (eg, CF I) (111,112) or difluoromethane derivatives such as CF2Br2 (Burton s reagent) (113,114), as well as electrochemical trifluoromethylation using CF Br with a sacrificial copper anode (115). Extmsion of spacer groups attached to the fluoroalkyl moiety, eg,... 

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). 

The importance of solvent effects in the preparation of perfluoroalkyzinc reagents is further illustrated in the reaction of perfluoroalkyl iodides with zinc-copper couple. In DMSO, DMF, and HMPA, the main products are the fluo-roolefins The formation of the fluoroolefin is facilitated when the reaction is carried out in the presence of potassium thiocyanate [30] (equation 21)... 

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). 

Azaaromatics can be perfluoroalkylated selectively in the 2-position using RLi-BF3 in a Ziegler-Zeisser-type reaction. The BF3 was essential as a promoter and other Lewis acids were found to be less effective. 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

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. 

Perfluoroalkyl)-5 -deoxy-5 -fluoro- and 5-(perfluoroalkyl)-2, 5 -di-deoxy-5 -fluoro-uridines were prepared " from 840 and 834, respectively, using perfluoroalkyl-copper complexes. Among them, 5 -deoxy-5 -fluoro-(846) and 2, 5 -dideoxy-5 -fluoro-5-(perfluoroethyl)uridine (839) were particularly effective against Ehrlich ascites carcinoma. 5-Hydroxyl (847) and 5-amino or 5-alkylamino (5-NHMe, -NHBu, -NHCH2Ph, -morpholino, -piperidino, and -pyrrolo) analogs (848) of 840 were prepared. The a anomer of 5 -deoxy-5 -fluorouridine (840) was also synthesized. "... 

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. 

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