Perfluorinated ethers, formation

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

The low-temperature chlorination of FC(0)NF2 is also a convenient method for the synthesis of C12NF. Alternatively, FNCI2 can be obtained from CIF and CIN3 in perfluorinated ether. Irradiation of FNCI2 with a high-pressure Hg lamp leads to the formation of m-N2F2. 

Formation of perfluorinated ethers by cobalt trifluoride is generally a low-yielding process, because of fragmentation and partial fluorination. However, incorporation into the substrate of electron-withdrawing polyfluoroalkyl groups moderates the fluorination and allows high yields to be obtained [78]. 

It is worth of mentioning that the cross-coupling of 16 and acid derived from hexafluoropropene oxide trimer leads to clean formation of 18 in relatively high yield (Fig. 9.4). More data on the synthesis of five-membered perfluorinated ethers can be found in Chapter 4. 

The ability of perfluorinated ethers to react with anhydrous AICI3 at elevated temperatures was discovered by Tiers. While the reaction of alicychc perfluorinated ethers results in the cleavage with the formation of acyl chloride and chloro-fluoroalkanes, perfluorinated a-aUcyl tetrahydrofuranes undergo replacement of all three a-fluorines with the formation of cyclic a,a,a-trichloro derivatives. Later, this reaction was extensively studied by Abe (Fig. 9... 

The perfluoroacetamide catalysts, rhodium(II) trifluoroacetamidate [Rh2(tfm)4] and rhodium(II) perfluorobutyramidate [Rh2(pfbm)4], are interesting hybrid molecules that combine the features of the amidate and perfluorinated ligands. In early studies, these catalysts were shown to prefer insertion over cycloaddition [30]. They also demonstrated a preference for oxindole formation via aromatic C-H insertion [31], even over other potential reactions [86]. In still another example, rhodium(II) perfluorobutyramidate showed a preference for aromatic C-H insertion over pyridinium ylide formation, in the synthesis of an indole nucleus [32]. Despite this demonstrated propensity for aromatic insertion, the perfluorobutyramidate was shown to be an efficient catalyst for the generation of isomtinchnones [33]. The chemoselectivity of this catalyst was further demonstrated in the cycloaddition with ethyl vinyl ethers [87] and its application to diversity-oriented synthesis [88]. However, it was demonstrated that while diazo imides do form isomtinchnones under these conditions, the selectivity was completely reversed from that observed with rhodium(II) acetate [89, 90]. 

The aUcoxylamines have a >N—O—C substrucmre and may be considered O-substituted hydroxylamine ethers. The simplest example of these species is MeONH2 with a gas phase enthalpy of formation of —25.1 kJmol which was discussed in a previous section. Consider now the perfluorinated species (CF3)2N0CF2CF20N(CF3)2 and its synthesis shown in equation 9. 

A series of functionalized alkenes has been subjected to selective epoxidation reactions. Oxidation of perfluorinated vinyl polyethers by bubbling oxygen through the liquid that contains a catalytic amount of a Lewis acid, e.g. antimony(V) fluoride, results in the formation of acid fluorides together with a smaller amount of C = C bond cleavage.76 Perfluorinated pentaaIkyI-2.3-dihydrofuran 38, an example of an unsaturated cyclic ether that is quite stable owing to perfluoroalkylation. is epoxidized by hypochlorite to the product 39 at 45 C.62... 

Interestingly, the perfluorinated version of vinyl ether 84 is even less stable to the action of SbF5, and decomposition of this material with formation of 85 and CF4 rapidly proceeds, even at -50°C [155]. A similar reaction was reported for 1-methoxy-F-isobutene, but in this case it produces a mixture of methacroyl fluoride and F-ketene ... 

The formation of 10 is an example of the addition of chloromethyl ethers to perfluorinated alkenes in the presence of potassium fluoride and a catalytic amount of tetrabutylammonium... 

Mercury photosensitization has been used to synthesize 1,3-dioxolane dimers by dehydrodimerization. For 1,3-dioxolane itself or analogues with substitution at C-2 or C-2 and C-4, radical formation occurs at the C-2 position, whereas when substitution is at the C-4 position radical formation occurs at the 4-position. In most cases, a small amount of product from ring opening was also reported <1996TL6853>. For 2,2-dimethyl-l,3-dioxolane, radical formation occurs at the 4-position, and one common way to achieve this is the use of dimethylzinc in air. Addition of various alkenes and related groups can then be performed, such as addition to tosyl imines <2004JOC1531>, and addition to perfluorinated alkenes and alkenyl ethers <1999JFC(94)141>. 

Since the conversion of fluorinated olefins to vinyl ethers is knownl, it should be emphasized that the principal benefit from the silyl ether chemistry is increased selectivity and yield. Base-catalyzed reaction of alcohols and fluoroolefins almost always gives a mixture of substitution and addition products, but clean substitution occurs with silyl ethers. Homogeneity, which persists during bulk polymerization, is an additional attractive feature. Substitution of the first fluorine in a 1, 2-disubstituted perfluorinated olefin proceeds more rapidly than subsequent fluorine substitution in the product vinyl ether. The 2/1 adducts shown in Scheme 4 are therefore conveniently prepared in excellent yield. On a laboratory scale, these difunctional vinyl ethers permit good control of stoichiometry required for polymer formation. 

Alkanes can be fluorinated by means of electrolysis in hydrogen fluoride. This important reaction is reviewed in Reference. Electrochemical fluorination in anhydrous hydrogen fluoride (AHF), the so-called Simons process, involves the electrolysis of organic compounds (aliphatic hydrocarbons, halohydrocarbons, acid halides, esters, ethers, amines) at nickel electrodes. It mostly leads to perfluorinated compounds. It is, however, considerably accompanied by cleavage and rearrangement reactions. As mechanism, the formation of carbocations by an ECE mechanism is assumed through oxidation of the hydrocarbon by higher-valent nickel fluorides. 

In the section on perfluorodimethylvinylamine, above, the conjugation between oxygen or nitrogen lone pairs of electrons and neighbouring 7 bonds was discussed. This positive interaction leads to stabilisation of the sterically unfavourable syn conformation in methyl formate and methyl vinyl ether. However, perfluorination of methyl vinyl ether leads to a major conformational... 

Two other trialkylsilyl trifluoromethanesulphonates, the TBDMS and tri-isopropylsilyl (TIPS) compounds, have been prepared and used in silylation reactions catalysed by 2,6-lutidine " TBDMS triflate allows formation of TBDMS ethers from tertiary and unreactive secondary alcohols, and TIPS triflate reacts effectively with primary and secondary alcohols. Use of the perfluorinated resin sulphonic acid trimethylsilyl ester Nafion-TMS , an immobilized TMS triflate, as a silylating agent for alcohols cf. 5,167) is discussed in a publication on applications of TMS triflate in synthesis. ... 

---