Vinyl fluoride reduction

Lithium, sodium, potassium, calcium or aluminum can also be used in addition to zinc dust (see Table 9). The influence of the reduction agent in the formation of vinylic fluorides has been investigated, e.g. formation of l.152... 

Table 12 there is a marked increase in the orientation ratio as the attacking radical becomes more bulky. With vinylidene fluoride the orientation ratio for perfluoro-t-butyl radicals is too large to measure. Even with perfluoro-isopropyl the orientation ratio (kjk is 103. In the other tables the orientation ratios for addition to vinyl fluoride and trifluoroethylene vary only slightly, but in Table 12 there is a marked increase as the radical becomes more branched. Perhaps the most striking feature of the table is that the increase in orientation ratio is almost entirely due to a reduction in the rate of addition to the most substituted end of the olefin. The relative rates of attack at the unsubstituted ends of vinyl fluoride and vinylidene fluoride vary only slightly across the table. In other words, the high orientation ratio is almost entirely due to the difficulty of addition at the substituted end of the olefin, a conclusion supported by the great variation in the relative rate of addition to tetrafluoroethylene (i.e. where both ends are fully substituted). 

Hydrogenolysis of vinylic fluorides is hardly ever done with the aim of removing fluorine finom a molecule. Rather, it occurs as a usually undesirable side reaction, accompanying reductions of other functions in fluorinated compounds. " It frequently takes place during catalytic hydrogenation of fluorinated alkenes. A clear-cut saturation of the double bonds in polyfluorinated alkenes is rare and usually requires mild reaction conditions,and hydrogenolysis of fluorine without reduction of the double bond in catalytic hydrogenation is unknown. Numerous examples are quoted in the review literature, " and a few samples are shown in Table 1. 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

Figure 25 shows such plots for vinyl chloride and fluoride reduction on... 

Fig. 25. Specificity-potential relationship for vinyl fluoride (a) and chloride (b) reduction on Pd 61). (Data obtained from Ref. 31. Reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

Electrocatalytic cathodes, Pt, Rd, Ru, and Ag, do not follow the behavior of medium and high overpotential metals. Thus, electrogenerative reduction of vinyl fluorides and chlorides gives saturated halides oj nonhalogenated olefins and alkanes, depending on catalyst, potential, and electrolyte or reactant concentration 31). In contrast to this, conventional electroreduction is pH independent, while fluorides are not reduced 371). Other differences between electrogenerative, catalytic, and conventional electrochemical reduction of organic halides have been outlined recently 31). 

Tsai, H.-J., and Burton, D.J., Synthesis of phenyl and ester substimted vinyl fluorides via reduction and olefination of esters, Phosphorus, Sulfur Silicon Relat. Elem., 140, 135, 1998. 

Bromofluoro compounds are themselves useful starting materials for the preparation of monofluorinated compounds. For example, reduction of 1-bromo-2-fluorocyclododecanes with tributyltin hydride gives fluorocyclododecane, while other methods to produce this compound have been unsuccessful.15 The elimination of hydrogen bromide from vicinal bromofluorides also gives vinyl fluorides in good yields,10 16 as is illustrated by the recent synthesis of several substituted a-fluorostyrenes for studies related to [4+2]-cycloadditions.17... 

Copolymers of vinyl fluoride with such monomers as vinylidene fluoride or l-chloro-2-fluoroethylene were prepared in the presence of trichloroacetyl peroxide at 0 C in sealed tubes. The chlorine-containing copolymers were then reductively dechlorinated at 60 C in tetrahydrofuran with tri- -butyltin hydride in the presence of 2,2 -azobis(isobutyronitrile) for up to 40 hr. This general procedure led to the formation of polymers with a reasonable control of the level of head-to-head, tail-to-tail linkages in the product [27]. 

The assignment was confirmed by examination of poly (vinyl fluoride) free from inverted units. This material was made by reductive dechlorination of poly(l-fluoro-l-chloroethylene) by means of tri-n-butyltin hydride the bulky chlorine atom in the monomer effectively prevents head-to-head growth. The F spectrum of poly (vinyl fluoride) prepared by this indirect route contained only the signals in the range 178-185 ppm. 

The polymerization temperature has a pronounced effect on the degree of polymerization and on the structure of the products. Polymers prepared at high temperatures are reported to be branched and of lower molar mass. Increasing pressure increases the rate of polymerization and favors the formation of high-molar-mass polymer. If organic solvents, especially alcohols, are used as a polymerization medium instead of water, a high tendency of telomerization of vinyl fluoride accompanied by a drastic reduction of molar mass is observed [457]. 

A nucleophilic mechanism can be applied in reductions with complex hydrides of highly fluori-nated aliphatic and alicyclic fluoroalkenes with electron-deficient C = C bonds the hydride anion adds as a strong nucleophilic agent to the more electrophilic carbon atom the intermediate anion can then lose a fluoride ion either from the original C = C bond, or from the allylic position finishing an SN2 displacement of the fluorine. Thus, the reductions of vinylic C-F bonds with hydrides proceed by a nucleophilic addition-elimination mechanism. Displacement of fluorine in highly fluorinated aromatic compounds proceeds by the same mechanism ... 

Nickel at 100 C is a much more efficient catalyst than palladium in the catalytic reduction of vinylic C-F bonds in perfluoro(2-methylpropene) (21). It is thought that the reduction takes place via the following reaction sequence addition of hydrogen to the C = C bond, elimination of hydrogen fluoride, and then addition of hydrogen to the new C = C bond.26... 

In analogy to the preparation of the cyclopropanone hemiacetal 3 (vide supra, Eq. (1)7), reductive cyclization of the piperidide of 3-chloropropionic acid 193 with sodium in ether in the presence of ClSiMe3, gave the 1-piperidino-l-trimethylsilyloxy-cyclopropane 194 a which was converted to the cyclopropanol 194 b upon treatment with methanolic tetrabutylammonium fluoride. Both 194 a and 194 b can be used for the ready generation of cyclopropane derivatives thus the silyl ether 194a could be reacted directly with the vinylic Grignard reagents 195 to provide the vinyl cyclopropane derivative 196, (Eq. (61)128). 

For PEMFCs, the solid electrolytes are polymer membranes polymers modified to include ions, usually sulfonic groups. One of the most widely used membranes today is the polymer Nafion , created by the DuPont company. These membranes have aliphatic perfluorinated backbones with ether-linked side chains ending in sulfonate cation exchange groups [6, 7], Nafion is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether [8] and has a semi-crystalline structure [9], This structure (which resembles Teflon ) gives Nafion long-term stability in oxidative or reductive conditions. The sulfonic groups of the polymers facilitate the transport of protons. The polymers consist of hydrophilic and hydrophobic domains that allow the transport of protons from the anode to the cathode [10, 11],... 

Oxygenation of vinyls Hanes ally He alcohols. Singlet oxygen undergoes the ene reaction with vinylsilanes to give, after reduction with sodium borohydride, -silylated allylic alcohols. These products are desilylated by tetra-n-butylam-monium fluoride with preservation of the geometry of the double bond. 

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