Perfluoroalkenes, terminal

Perfluoroacetone s. Hydrogen peroxide-perfluoroacetone Perfluoroalkenes, terminal... 

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... 

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. 

Oligomerisation of tetrafluoroethene [55-57] leads to the interesting internal perfluoroalkene (37), a tetramer [58] which, together with the pentamer (66) are the principal products (Scheme 29). Remarkably, the hexamer (67), i.e. with a terminal difluoromethylene group, is also formed rather than an internal isomer and this is probably attributable to the destabilising influence of steric effects on the alternative structures. 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

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). 

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... 

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]. 

The reaction of trialkyl phosphites with perfluoroalkenes in an autoclave at l(X)-140°C results in the formation of dialkyl perfluoroalkenylphosphonates in low to good yields (21-81%, Scheme 3.7). " The mechanism involves the attack of the phosphite nucleophile at the polarized terminal CF2 position to form a perfluoroalkenyltrialkoxyfluorophosphorane, which is stable under the usual conditions. This fluorophosphorane, which may be regarded as an isolable Michaelis-Arbuzov intermediate, decomposes on heating with elimination of alkyl fluoride to give dialkyl perfluoroalkenylphosphonates (Scheme 3.7). ... 

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