Perfluoroalkyl reactivity

As already discussed in Section 2.2.1, perfluoroalkyl halides do not act as effective electrophilic perfluoroalkylation reagents, as might be expected by analogy with the reactivity of alkyl halides. Even if some electrophilic perfluoroalkylation reactivity is mimicked with some especially suitable (i. e. easily oxidizable) substrates by an electron-transfer-induced radical mechanism, the practical usefulness of this reaction pathway is limited to very few examples. 

FLUOROTRIAZINES Riag-fluoriaated triaziaes are used ia fiber-reactive dyes. Perfluoroalkyl triaziaes are offered commercially as mass spectral markers and have been iatensively evaluated for elastomer and hydraulic fluid appHcations. Physical properties of representative fluorotriaziaes are listed ia Table 13. Toxicity data are available. For cyanuric fluoride, LD g =3.1 ppm for 4 h (iahalatioa, rat) and 160 mg/kg (skin, rabbit) (127). 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Numerous examples demonstrate that perfluoroalkylated alkynes are quite reactive dipolarophiles. Terminal alkynes, such as 3,3,3-trifIuoropropyne, exhibit... 

In contrast to the relative lack of Diels-Alder reactivity exhibited by fluorinated ethylenes, ethylenes substituted with perfluoroalkyl groups show greatly... 

The dienophilic character of imines parallels that of carbonyl compounds Consequently, electron deficient imtnes are the most reactive dienophiles of this class, particularly those having C perfluoroalkyl [5, 146, 150, 228], /V-acyl [/2i5 127], or A/-sulfonyl groups [148, 229 230]... 

The subjects of this chapter are the exploration of the scope and hmitations of the new Pd-Sn catalyzed hydrogenolysis route for the synthesis of thiols via 2-(perfluoroalkyl)ethane thiocyanate, the characterization of the surprisingly active and robust Pd-Sn catalysts, and the attempted correlation of the characterization of the catalysts with observed onset of hydrogenolysis reactivity and snrprisingly long lifetime in the presence of known catalyst poisons. ... 

Fig. 4 Halogen-bonded adducts are pre-reactive complexes, which, under convenient conditions, can lead to covalent bonds breaking and forming. Perfluoroalkylation occurs when the complexes between iodoperfluoroalkanes and anilines are heated or irradiated in certain solvents...

Not much is known about the reactivity of the phosphinocarbene 2i. Problems arise, at least in part, from the high 1,3-dipolar reactivity of the diazo precursor li, which hides any carbene reactivity. Indeed, although li is stable in a toluene solution at 60°C for hours, the addition of an electron-poor olefin, such as a perfluoroalkyl-monosubstituted alkene, induces the exclusive formation of the thermodynamically more stable anti-isomer of the cyclopropane 14 (see Section V,B,3,a).36 This clearly demonstrates that the cyclopropanation reaction does not involve the carbene 2i, but that an initial [2 + 3]-cycloaddition occurs leading to the pyrazoline 13, which subsequently undergoes a classical N2 elimination.37... 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

Perfluoroalkyl)dibenzothiophenium salts and their selenium and tellurium analogs are novel perfluoroalkylating agents. The synthesis and reactivity of these compounds are covered by T. Umemoto (Ibaraki, Japan). Finally, the first detailed survey of the chemistry of 1,3-oxazinium and 3-azapyrylium salts for over twenty years is provided by S. Lukyanov (Rostov-on-Don, Russia). 

One of the most significant additions to the modern rhodium] ) catalyst ligand family was the development of the hybrid catalysts that combined the carboxamide bridging ligands with the enhanced reactivity of perfluoroalkyl substituents. In this series, rhodium] ) trifluoroacetamidate [Rh2]tfa)4] was the first described [30]. n addi-... 

Despite the lower reactivity of solvated perfluoroalkylzinc reagents, perfluoroalkyl iodides undergo synthetically useful zinc-mediated reactions under Barbier conditions which often employ ultrasound and co-catalysts. Under these conditions, the zinc reagents are not well characterized and radical intermediates and SET mechanisms are proposed in some cases. Representative examples are presented below and include the ultrasound-promoted, zinc-mediated perfluoroalkylation of various substrates as reported by Ishikawa and coworkers (Scheme 10)123 126. Yields of carbinols could be improved by use of Ti(II) co-catalyst. Ultrasound promoted the coupling of perfluoroalkyl iodides with vinyl and allyl halides in the presence of Pd co-catalysts. 

Arguments similar to those stated above can be used to explain the relative chemical inertness of fluoropolymers. Consider the reactivity of alkanes vs. perfluoroalkanes as shown in Table 4.2 (abstracted from Sheppard and Sharts Statistically, FA based materials will have many more types of bonds, in addition to C—F, than fluoropolymers. These bonds will be subject to the same chemical fate during assault by aggressive reagents as bonds in their hydrocarbon counterparts. Similar reasoning can be used to explain the relative thermal stability of FAs compared to fluoropolymers. Thus, incorporation of perfluoroalkyl groups will not make the modified material less stable than the native one. 

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