Alkenes perfluoroalkenes

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluonnated alkenes become better dipolarophiles when vinylic fluonnes are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that IS observed in this reaction... 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

In the presence of freeze-dried potassium fluoride, perfluoro-2-methylpent-2-ene reacts with activated methylene compounds to yield pyrans (81MI22400). The fluoride ion abstracts a proton from the methylene group and subsequent condensation of the carbanion with the perfluoroalkene affords a dienone (19) which ring closes to the pyran (20 Scheme 3). In the case of pentane-2,4-dione a divinyl ether (21) is also formed. This product is considered to arise from reaction of the alkene at the oxygen of the enolate ion. 

Due to their industrial applications, tetrafluorooxirane and perfluoro(2-methyloxirane) are the most frequently mentioned epoxides in the literature, and a number of methods have been developed for their synthesis. Although the epoxidation of perfluoroalkenes with hydrogen peroxide in alkaline media appears to be the most general method for the synthesis of per-fluorinated epoxides, it cannot be used in the preparation of tetrafluorooxirane due to the hydrolytic decomposition of the alkene. 

Alkaline hydrogen peroxide oxidation52 has been extended to higher perfluorinated alk-1-enes, perfluorinated cycloalkenes and certain alkenes with internal C = C bonds.52 57 A convenient reagent for the preparation of perfluoroalkene epoxides is sodium hypochlorite in a mixture with aqueous acetonitrile or another aprotic solvent. Several cis- and traw.s-perfluoroalkenes are oxidized to 32 with retention of configuration (Table 4).58-63... 

Finally, intense ultraviolet irradiation of perfluoro(2.3-dimethylbut-2-ene) (14) yields terminal alkene 15 quantitatively.35 Unfortunately, photolysis of more complex perfluoroalkenes gives mixtures of products resulting from migration of both allylic fluorine and allylic per-fluoroalkyl groups. Perfluorobut-2-ene only undergoes geometric isomerization. Pyrolysis of 14 at temperatures up to 300 C in the dark does not lead to detectable rearrangement. 

The rates and orientation of free radical additions to fluoroalkenes depend upon the nature of the attacking radical and the alkene, but polar effects again are important For instance, methyl radical adds 9 5 times faster to tetrafluoroethylene than to ethylene at 164 °C, but the tnfluoromethyl radical adds 10 times taster to ethylene [7551 The more favorable polar transition states combine the nucleophilic radical with the electron deficient olefin 17 and vice versa (18) These polar effects account for the tendency of perfluoroalkenes and alkenes to produce highly regular, alternating copolymers (see Chapter starting on page 1101)... 

Preparation of nitronitroso dimers from a variety of straight chain alkenes has been patented. " The reactions of nitric oxide with alkenes are extremely complex (e.g. isobutylene) and are rarely usefiil. Perfluoroalkenes add nitric oxide at room temperature in the da tetraflucMoethylene gives ONCF2CF2NO (68%) the reaction with perfluoropropene is more complex. ... 

Internal perfluoroalkenes react with trimethyl(perfluorophenyl).silanc under more forcing conditions than terminal alkenes. The reactions can also give biphenyl derivatives. An interesting dependence of the reaction results on the structure in the case of perfluorophenylation of isomeric perfluoro(methylpentenes) has been identified. Perfluoro(2-methylpent-2-ene) is transformed into pcrfluoro(2-methyl-3-phenylpent-2-cnc) (4) perfluoro(4-methylpent-2-cnc) forms pcrfluoro( 1.1,3-triinethy lindan) (6) as well as perfluoro(4-inethyl-2-phciiylpen t-2-ene) (5). 

It should be noted that under some conditions reaction of internal perfluoroalkenes and -cycloalkenes with tributylphosphane and triphenylphosphane results in dimerization, preceded by defluorination and formation of a 1,3-diene and an appropriate difluoro-A -phosphane. For example, reaction of perfluoroalkenes containing three perfluoroalkyl groups at the double bond w ith a phosphane[tributylphosphane, triphenylphosphane, tris(diethylamino)phosphane] results in reductive dimerization of these alkenes, to give 1,3-dienes and difluoro-A -phosphanes. 

Perfluoroalkenes react readily with tertiary phosphanes to form various products depending on the position of the multiple bond in the alkene. Internal alkenes (perfluorobut-2-ene and perfluorocyclobutene) undergo addition of phosphane at the multiple bond with subsequent... 

The A -phosphane 1, formed in the reaction of triphenylphosphane with hcxafluoro-propene, reacts further with hcxafluoroacetone in the presence of the fluoride ion. affording triphenylphosphane oxide and perfluoro(2-methylpcnt-2-ene) (2). Formation of fluoro-A -phos-phancs underlies the method for the reduction of perfluoroalkenes which involves the reaction of an alkene with tributylphosphane and subsequent hydrolysis of the A -phosphanc formed. ... 

It was noted that for the perfluorobutanoic acid salts of the alkaline earth metals the yield of perfluoroalkene increased with increasing weight of the metal cation (vide supra). Decomposition of the barium salt gave hexafluoropropene in 78% yield while the reaction with the magnesium salt gave > 5 % of the same product. An exception was ammonium per-fluorobutanoate (4) which formed no alkene, but instead gave an almost quantitative yield of... 

Problems of orientation of attack and reactivities of fluorinated alkenes arise in a way that is analogous but entirely complementary to the classical problems of electrophilic attack on alkenes. For example, typical of the results that we must be able to account for is the reaction of methoxide in methanol which occurs specifically at the Cp2= site in perfluoropropene (Figure 7.15). Also, there is a very wide range of reactivity with perfluoroalkenes for example, reactions of tetrafluoroethene usually require base catalysis, whereas perfluoroisobutene reacts with neutral methanol. 

Several perfluoroalkene derivatives have been made and used successfully in synthesis. Trifluorovinylmagnesium bromide and the lithium derivative may be obtained [39 1] from bromotrifluoroethene but preparation from HFC 134a, which involves metallation of trifluoroethene generated in situ, is now the more accessible route (see Section lA). However, direct metallation of fluorinated alkenes and fluorinated cycloalkenes has also been reported [26, 28] (Figure 10.9). 

Perfluoroalkenes react readily with tertiary phosphanes to form various products depending on the position of the multiple bond in the alkene. Internal alkenes (perfluorobut-2-ene and perfluorocyclobutene) undergo addition of phosphane at the multiple bond with subsequent 1,2-migration of the fluorine atom to form stable phosphanes. Terminal fluoroalkenes under similar conditions form the corresponding fluoro(perfluorovinyl)-/5-phosphanes. The exception is perfluoro(2-methylpropene) which reacts with triphenylphosphane to form the triene... 

The acylation of alkenes can also be similarly effected with SbFs [32], For example, a,/3-unsaturated carboxylic acid fluorides react with perfluoroalkenes to form a,/3-unsaturated ketones (Eq. 16) [32b]. 

Isomerization of perfluoroalkenes can be realized by use of SbFs catalysis [37]. The terminal carbon-carbon bonds of these alkenes are usually moved to the 2-position under the influence of this catalyst (Eq. 19). A further inward shift generally occurs only if H or Cl atoms are present at the 4-position of the alkenes. As a rule, isomerization leads to the predominant formation of trans isomers. Terminal fluorodienes also isomerize exothermally into dienes containing internal double bonds in the presence of SbFj. With a catalytic amount of SbFs, perfluoro-l,4-cyclohexadiene disproportion-ates to hexafluorobenzene and perfluorocyclohexene. SbFs promotes the rearrangement of perfluoroepoxides to carbonyl compounds (Eq. 20) [38]. 

Perfluoroalkenes and perfluorinated chloroalkenes add secondary amines to give a,a-difluo-roamines (47 equation 28). The A -halogenated amine (48) reacts with perhalogenated alkenes in a similar fashion, (equation 29). ... 

Arsines are expected to show greater reactivity than phosphines, and H3AS, (CHsiHaAs, and (CHsiaHAs react spontaneously with perfluoroalkenes under mild conditions CgHsAsHa reacts similarly with CH2=CHCN . These reactions are summarized in Table 2 . No reactions of stibines and bismuthines with alkenes have been reported. 

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