Fluorinated diene preparation

Reductive coupling reaction of fluonnated vinyl iodides or bromides has been used as a route to fluorinated dienes [246, 247, 248, 249, 250. Generally, the vinyl iodide is heated with copper metal in DMSO or DMF no 1 ntermediate perfluorovmy I-copper reagent is detected. Typical examples are shown m equations 163-165 [246, 247, 249. The X-ray crystal structure of perfluorotetracyclobutacyclooctatetraene, prepared via coupling of tetrafluoro-l,2-diiodocyclobutene with copper, is planar... 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

The preparation of fluorinated dienes is particularly advantageous via the palladium catalyzed coupling protocol. By proper selection of the appropriate vinylzinc reagent and vinyl iodide, a wide variety of fluorinated dienes can be stereospecifically prepared, as illustrated in equation 22. 

As noted, these zinc reagents find extensive application in the preparation of fluorinated styrenes [113,114], aryl-substituted fluorinated propenes [114], fluorinated dienes [115,116], and trifluorovinyl ketones [117], as illustrated in equations 83-88... 

Dienes.—Poly(perfluorobuta-l,3-diene), prepared by free-radical polymerisation of the monomer initiated with bistrifluoromethyl peroxide, di-iso-propyl peroxydicarbonate, or benzoyl peroxide, has been cross-linked by treatment with fluorine and with hexamethylenediamine. ... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

Certain trifluoromethyl-substituted 1,2,4,5 tetrazines [260, 26i] and 1,2,4 triazines [i06] can be used as cyclic hetero-1,3-dienes and provide efficient preparative routes to partially fluorinated heterocycles (equations 55 and 56)... 

Aromatic fluoro-compounds have been prepared by thermal cycloaddition of fluorinated 1,3-butadienes 10-12 (Figure 2.1) with several dienophiles. Fluorophenols were obtained by cycloaddition of diene 10 with quinones [11]... 

Synthetic Rubbers. Synthetic rubbers are polymers with rubberlike characteristics that are prepared from dienes or olefins. Rubbers with special properties can also be prepared from other polymers, such as polyacrylates, fluorinated hydrocarbons, and polyurethanes. 

The DMS method has not been employed yet for the generation of 117 and 123, since the dibromocarbene adducts of norbomadiene and norbornene rearrange under the usual conditions for the preparation [89]. However, they could be synthesized at -60 °C by taking advantage of tetrabromomethane and methyllithium as a source of the carbene [90] and could prove stable enough to serve as precursors of 117 and 123. On the other hand, the adducts of bromofluorocarbene to norborna-diene and norbornene having the fluorine atom in a cis-orientation should be isol-able at room temperature and hence be usable as stable precursors of 117 and 123. These variations ofthe DMS method were published on the occasion ofthe preparation of cycloadducts of l-oxa-2,3-cyclohexadiene (351) (Section 6.3.6) [35, 91], 1,2,4-cyclohexatriene (162) and 3d2-lJ-f-naphthalene (221) (Section 6.3.4) [35, 92],... 

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

This effect cannot be explained by simply considering differences in frontier orbital energies. A useful monofluorinated dienophile has been prepared [364] using metallated difluoroenol carbamate chemistry (Eq. 143) cycloaddition occurred smoothly with a range of dienes, and desulfination could be achieved under mild conditions without loss of the fluorine atom. Wakselman and co-workers [365] synthesised a rare competent difluorinated dienophile. Lewis acid catalysed Diels-Alder reaction with furan afforded an acceptable yield of (unfortunately) unstable cycloadduct which decomposed to a phenolic product via a dehydrofluorination reaction, circumscribing its utility (Eq. 144). 

These novel a, co-diiodides underwent functionalisations, especially for the preparation of original fluorinated nonconjugated dienes [388] utilised in the preparation of hybrid fluorosilicones [389] which exhibit excellent properties at low and high temperatures. 

The [4+1] cycloadducts formed from 4,4-bis(trifluoromethyl)-l-oxa-buta-1,3-dienes (92JPR219) and tin(II )-chloride are transformed on heating into 4,4-difluoro-3-trifluoromethylbut-3-en-l-ones, which on treatment with sodium hydride yield 2-fluoro-3-trifluoromethylfurans (92CC348). When heated with phosphorus pentasulfide, l-aryl-4,4-difluoro-3-trifluor-omethylbut-3-en-l-ones give 2-fluoro-3-trifluoromethylthiophenes. The fluorine atom at C—2 of the furans and thiophenes can readily be substituted by a wide variety of nucleophiles (92JPR3II). This reaction sequence represents a preparatively useful method for the selective introduction of biologically relevant substituents into the C—2 position of 3-trifluoro-methyl-substituted furans and thiophenes (Scheme 87). 

For reasons of symmetry the resulting acids are identical when a symmetrically substituted alkcne is oxidized. For the preparation of short-chain fluorinated acids, such as trifluoroacetic acid. this method is more economical than the oxidation of an alkene with a terminal C = C bond (see Table 1). Oxidation of polyfluorinated dienes and polyenes gives fluorinated dicar-boxylic acids. 

Flash-vacuum pyrolysis (FVP) of isomeric fluorinated cyclohcxadicnoncs is a route for the preparation of perfluorocyclopentadiene (7)/ a versatile diene in the Diels Alder reaction. 

Trifluoropyrazine has been prepared from tetrafluoropyrazine through 2,3,5-trifluoro-6-hydrazinopyrazine by removal of the hydrazino substituent with aqueous copper sulfate (885). Fluorination of tetrafluoropyrazine in nitrogen in the presence of cobalt(lll) fluoride and calcium fluoride at 80° gave perfluoro-1,4-diazocyclohexa-13-diene (IS) (886). 

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