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4. Fluorine Aromatic Compounds Synthesis Using Commercial Chladones and Polyfluorolefines The alternative strategy of fluoroaromatic synthesis lies in synthetic production of assigned fluorobenzene-like structures with using reactive fluorine containing fragments (fluorosynthones). The application opportunities for this methods for synthesis of mono-, di-, tri- and polyfluorosubstituted arenas are based on application of commercial chladones (freons) and poly-fluoroolefines as feedstock or fluorine containing structure blocks [94,95]. CHCl3, CHClF2, CHCl2F, CClF3, CF2=CF2, CF3CF=CF2, CF2CFCl, CF2=CFC4F9, CF3CF=CFC3F7 etc. are used as such ones [18]. This approach opens large synthetic perspectives for introducing fluorine into aromatic nucleus. It is based at gas-phase generating and cyclic addition of fluorocarbenes and poly-fluorolefines to unsaturated hydrocarbons followed by thermal isomerization of fluorine containing cyclopropanes and cyclobutanes. Butadiene and its methyl derivatives, cyclopentadiene etc are used as such compounds. Obtaining method of fluorine aromatic compounds using gas-phase reaction of difluorocarbene and coupled dienes was implemented in the form of universal mono- and difluorobenzenes obtaining technology. The technology lies in gas-phase copyrolysis of difluorochlomethane (chladone 22) and cyclopentadiene (fluorobenzene obtaining [96-99]) either by butadiene -1,3 (obtaining of difluorobenzenes [97-100]). The process is carried out in flow system on a continuous mode with water vapour, water-ammonia mixtures [95] or at alkaline packing [87], linking isolating halogen hydrogens, what increases selectivity of synthesis and lowers the resinification [18].
Carbenes method was successfully used to synthesize the fluorine containing bicyclic aromatic compounds: 2-fluoronaphthalene (yield is 68-80 %), starting from indan and CHClF2 at 600-670oC [101], 2,3-difluoronaphthalene (yield is 16-65 %), starting from styrene and CHClF2 at 650 oC [102,103]. Obtaining method of 1,2-difluorobenzene, 3,4-difluorotoluol, 2,3- difluorotoluol and 1-fluoro-2-trifluoromethylbenzene is based on pyrolysis of substituted derivatives of vinylcyclobutane of the type listed below at 600-800 oC in the presence of water vapour [104]. Thus, 2,3-difluoro-2,3-dichlorocylobutane produces ortho-difluorobenzene after water steaming and ammonia treatment, and 1-vinyl-2,3-difluorocyclobutane transforms into 1,2-difluorobenzene with the yield of 61.5 % (conversion of starting one is 80 %) [104].
Fluorolefines can thermally add 1,3-dienes in a mode of [4+2] and [2+2]- cycloaddition [105]. Fluorinated cyclohexene and vinylbutane adducts formed during that processes can be transformed into partly fluorinated aromatic compounds either directly by the following dehydrohalogenation or through the preliminary stage of vinylcyclobutane-cyclohexene rearrangement [106,107]. This approach was used for working out the obtaining method of partly fluorinated benzene derivatives using available fluorolefines. It lies in constructing fluorobenzene-like structures by fluorolefines thermal cyclic addition of 1,3-diens and further aromatization of forming fluorinated carboxylic adducts. Thus, interaction of tetrafluoroethylene and 1,3-butadiene in a flow reactor results either in forming of 1-vinyl-2,2,3,3-tetrafluorocyclobutane (temperature was 450-470 oC, contacting period 4-6 seconds) with the yield within 78-85% or in forming of fluorocyclohexane (temperature 490-520oC) [106]. The reaction has a common nature and trifluorochloroethylene, hexafluoropropylene, perfluoropropylene, perfluorohex-1en, perfluorohex-2en are introduced into it.
Not only derivatives of 1,3-butadiene (2-methyl-1,3-butadiene, piperylene, 2,3-dimethyl-1,3-butadiene, haloisoprene derivatives) but also hem-fluorochlorocyclopropane are introduced into the reaction [106]. For example, the interactrion of trifluorochloroethylene and 1,1-dimethyl-2-fluoro-2-chlorocyclopropane results in forming of isomeric methyltetrafluorocyclohexenes as main products, which not being separated are transformed into 2,4,5-trifluorotolyol by alkali dehydrohalogenation under the conditions of inter-phase catalysis [106].
Thus, new methods of fluoroaromatic compounds synthesis using the reactions of thermal transformation of poly-fluoroolefines allows to offer alternative to traditional methods of solving a problem of obtaining low fluorinatinated benzene derivatives not only for preparative purposes but also for commercial production. The aims of present review are analyzing of new file of information regarding reactivity of fluoroaromatic compounds, which was accumulated during the last decade, describing of fluorine atoms introduction's into benzene ring influence on properties of some benzene derivatives, also describing of development of new methods of fluorine containing aromatic compounds synthesis, latest achievements of this class of compounds. Here we also targeted on presenting the information on practical using of fluorinated aromatic compounds. In our opinion, it can forward a wider attracting of poly-fluorinated organic compounds to solve a number of crucial questions of theoretical organic chemistry and also to purposefully synthesize compounds, possessing useful properties. |
