2.4. Fluoride ion as a catalyst in generating C-nucleophile from perfluoroolefins and its interaction with electrophiles. The significant part of internal perfluoroolefins reactions are the processes with intermediate generation of a C - carbanion. There are different ways of such carbanions generation. First way is based on the reaction between internal perfluoroolefins and fluoride-ion. The structure of carbanion after interaction can be both changed and unchanged. The key step is the opportunity of isomerization to more thermodynamically stable olefin. Second way is based on a generation of C - carbanion during decomposition of intermediate compound which receive in the reaction of some nucleophiles (phosphines, for example) with internal perfluoroolefins. The formation of carbanions in the reaction of fluoride ion with fluorinated alkenes [127] is a very simple way to the introduction of perfluorinated groups into organic molecules and especially into aromatic systems [128-130]. It is known that the fluoride-ion in aprotonic bipolar solvents has high nucleophility and therefore it is capable to attack the double bond of internal perfluoroolefin with formation of primary C-carbanion [124,131]. Perfluoroisobutylene reacts with CsF and (CF3)3CCs is formed [132]. Perfluoro-2-methylpent-2-ene reacts with CsF in diglyme and (CF3)2(C3F7)CCs is formed [133]. 1-Chloroacetylenes containing a strong electron-acceptor substituent react with (CF3)3CCs and the formation of acetylenes derivatives take place [132]. (CF3)3CCs reacts with arylsulfenylchlorides in acetonitrile with perfluoroalkylarylsulfide formation [134]. The reaction with fluoroolefins gave 50-84 % of (CF3)3CCF=CFR. Analogous reaction with perfluorocyclobutene and -cyclopentene gave 75-89 % of cycloalkene derivatives [135].
The various forms of alkaline metals fluorides [136], including their complexes with crown-ethers [137,138], (Me2N)3S +. Me3SiF2- ("TASF") [139,140] (Table 2), salt of a perfluorocyclobutene with a triethylamine [143,144] and others are used as sources of a fluoride-ion. Perfluoroallen and perfluorinated
The salt [(CF3)2CCFC(CF3)2]-Cs+ reacts with compounds having mobile halogen atom at elevated temperature [145]:
The nature of a source of ion and nature of a solvent are extremely important for stability of these C-anions and cesium and the "TAS" cation (i.e. {Me2N}3S+) are the most effective. The "TAS" cation has especial value, because the starting matherial TAS+ Me2SiF2- is soluble in various organic solvents. Examples of stable anions are shown in Table 2.
Table 2. Salts obtained as the result of the reaction (Me2N)3S+ Me3SiF2- or CsF with perfluoroolefins [141,142].
Perfluoro-4-ethyl-3,4-dimethylhexan-3-yl tetramethylammonium carbanion was prepared as a stable salt from the pentamer tetrafluoroethylene and Me4NF in acetonitrile. The reaction is fast and quantitative [146].
Reaction of perfluoro-2-methylbut-2-ene with cesium fluoride in tetraglyme gave the stable carbanion [29]. This ion is analogous to the polyfluoro-t-butyl anion that was described previously in [122,129].
Reaction of 1(pentafluoropropyl-2-enyl)-3H-decafluoro-2,2-dimethylcyclopentene with cesium fluoride gave a stable allyl anion [29]. This is the first example of observable fluorinated carbanion where a fluorine atom is formally directly attached to a charged site, although more charge probably resides on the (CF3)2C- site. Thus, when these ion was treat by bromine, 1-(2-bromohexafluoro-2-propyl)-3-hydrofluoro-2,2-dimethyl-cyclopentene was formed, i.e. bromine attack of (CF3)2C- site took place.
13C and 19F NMR spectra were measure in solution down to -95 oC. These results with combination of semi-empirical quantum chemical calculations show that the decision could be made between the occurrence of a planar sp2 or a tetrahedral inverting carbon at the carbanion site. It is seems from DNMR-data a planar existing rather than a tetrahedral carbanion, where the lone pair is not stereo active. Perfluoroalkyl groups stabilize this carbanion and the following reactions are possible: 1) isomerization of double bond; 2) reaction with electrophile; 3) reaction with initial when a high nucleophilic mobil fluorine atom at double bond take place. It should be note, that it is not necessary to generate steady tertiary perfluoro-carbanion during the reaction of perfluoroolefine with a fluoride of metal. Reaction generates enough intermediate C-carbanion in situ. Tertiary fluorinated carbanions, generated from corresponding unsaturated precursors, react with variety electrophiles. The examples of reactions of C-carbanion generated from perfluoro-2-methylpent-2-ene are given in Scheme [124,141,147,148].
Recent methodology [149] has revealed, that solvent employment may be avoided, when use the organic base, such as trimethylamine. Such compounds are uses in situ as very active fluoride-ion sources. This methodology simplifies the procedure of product isolation. This C - carbanion reacts with diazonium salt of aniline, giving diazonium compounds, which are used for producing of a primary amine with spatially complicated perfluoroalkyl substituent [119].
It is reported that perfluoro-2-methylpent-2-ene react with fluoride ion with perfluoro-t-carbanion formation. Under appropriate conditions, this carbanion reacts with specific electrophile with addition products formation. The probable reaction pathway is shown in Scheme.
These carbanions are the active nucleophiles and also very active alkylation reagents. So, various benzylbromides (RC6H4CH2Br (R = H, p-, m-, o-CN, p-, m-, o-NO2, p-, m-CH2Br)) easily react with perfluoro-2-methylpent-2-ene in bipolar aprotic solvents at KF presence or with iodides of alkaline metals. The yield of products of bromine replacement is more than 70 %. This method is used for production industrial oil for hydraulic liquids in electronic branch and for surface-active substances production [150-152].
Eters of para-(bromomethyl)phenylbenzoate reacts in DMF (50-60 oC; 20 hours, 85 % yield) giving the product of replacement, which further is hydrolyzed by KOH treatment in ethyl alcohol- water mixture to oxy-derivative (42) (yield 30 %) [153].
42 Benzylchlorides react too. So, 3,5-dinitrobenzyl chloride in DMF at 60 oC reacts with a dimer of a hexafluoropropylene, giving a product of replacement of atom of chlorine (43) [154]. This compound is used as an intermediate product in synthesis of aromatic diamines.
The reaction with benzylbromide results the formation of a product of bromine replacement, which react with dimethylchlorosilane with adducts formation. It contains active bond Si-Cl. Such compounds (for example, 1,1-H-, 2,2-H-) are used for creation of chromatography phases for 3,3-H- tridecafluoro (4,4-dimethylheptyl) silicon (Fluotix 120 N (A)) [155].
Halogen alkyl also is effective in the given processes, that allow to produce the partially fluorinated paraffins for various purpose [147].
Chloromethyl ethers react with dimers of a hexafluoropropylene with partially fluorinated ethers formation [142].
R = C8H17 (90%), Ph (81%), PhCH2 (82%), CH2CH=CH2 (50%), CH2=CH(CH2)8 (86%), CH2=CHCH2OCH2CH2 (85%) At the same time, the reaction with trimer of a hexafluoropropylene is not observed even when temperature was increased up to 150 oC. It shows that the essential influence of shield of anionic center by the spatial substituents take place.
At the same time the reactions of C-carbanion with perfluoroalkyl iodides yield the fluoride-bridged products 44 [156].
RFI = C6F5I (86%), I(CF2)4I (86%), C6F13I (76%), I(CF2)2I , I(CF2)8I, I(CF2)10I The high mobility of chlorine in arylsulphenylchloride and arylselenylchloride allows to develop the method of producing of partially fluorinated arylperfluoroalkyl-sulphides and - selenides [157,158], which can be employed as fungicides and insecticides. When dimers of perfluoropropylene is stirred with KF in DMF at 50-60 oC the perfluoro(1,1-dimethylbutanide) anion is generated. It smoothly reacts with arenesulfenyl chloride and the corresponding arylperfluoro(1,1-dimethylbutyl)sulfines with 58-86 % yields are produced [157]. It is known that perfluoro-4-methylpent-2-ene undergo isomerization into thermodynamically more stable perfluoro-2-methylpent-2-ene in the presence of fluoride ion [159]. |
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