3.2. Other methods of stable radicals generation and problems of stability and reactivity of perfluoroalkyl radicals. Really, the reactions of ramified perfluoroolefins (dimers and trimer
of a hexafluoropropylene, perfluoro-4,4-dimethylpent-2-ene) with
peroxydisulphurildifluorides resulted the stable
It was shown that perfluoro-4-methyl-2-pentene reacted with peroxydisulfuryldifluoride regiospecifically with formation of radical 59, which was stable in the oxygen absence [257,258].
When peroxidisulfuryldifluoride (FSO3) 2 reacts with perfluoro-4-methylpent-2-ene, the radical attacks perfluoro-4-methylpent-2-ene in position 2 with formation of a-fluorosulphatotetrafluoroethylperfluoroiso-propylmethyl radical. Last ones is capable to addition either a second radical FSO3 or halogen atoms. But it is not able to undergo the dimerization [259]. In the case of more branched perfluoro-4-methyl-3-isopropylpentene-2 the addition of peroxydi-sulfyryldifluoride led to a stable radical A, which was isolated as individual material. The use of more violent conditions results the formation of bis-fluoro-sulfate B [257].
The presence of functional FSO3 group carries out the chemical transformations of radical A with retention of radical center. The reactions of unsaturated carbonyl compounds iso-C3F7CF=CORF (RF = F, C2F5, i-C3F7) with (FSO3)2 yield the considerable amounts (30-35 %) of fluorosulphatodimers [260].
Obviously, dimerization is associated with changing of the radical attack site, which result the thermodynamically more stable radical B. Remains can react with formation of the less hindered, and therefore, more reactive O-centered radical. This fact probably determines the dimerization of unsaturated carbonyl compounds. The kinetic of hydrogen absorption from hydrocarbons by air-stable perfluoroacetyldi-iso-propylmethyl radical was studied by the method of ESR (Table 6) [261].
Table 6. The kinetic and thermodynamic data of
hydrogen atom detachment from paraffins by
It is interesting, that the radical center does not complicate the reactions ways with other functional groups in all molecules. So, CsF reacts with formation of ketoradical with fluorosulphuryl elimination. If this reaction carries out in acetonitrile, the stable radical is formed as replacement of OSO2F-group by fluorine [249-252]. The radical (58) react with SbF5 at 45-60 oC without of solvents and a product of replacement of FSO3 group by fluorine atom (that is perfluorodi-iso-propylmethyl radical (60)) is obtained [259,261].
The interaction of intermediate formed from methylvinyl-dimethoxysilane or methylvinyl-diethoxysilane radicals at UV-irradiation and -10 --20 oC with perfluoroolefins having ramified alkyl groups results the formation of stable radicals [259].
R = [CH2=CHSi(Me)(OCH2X)(OCH2X)]. (X = Me, Et) The radical reaction of vinylsilane with ramified perfluoroolefins can be the ways of introduction of fluorinated substituents. The stable radicals are received from fluorinated imidoylchlorides too [263].
These radicals are stable at room temperature. Reactions proceed as photolysis in isopropyl alcohol with Et3SiH [264].
The ultraviolet influence on solutions of hydrosilanes in perfluoroolefins results the formation of steady adduct 61 (time of life 5-10 h) [265]. When di-tert-butyl-peroxyde was added the signals intensity increased in 10-30 times. The reactionary ability of intermediates obtained from free radical 61 determines by steric shielding of the radical centers.
RF = CF(CF3)2, C(CF3)2CF(CF3)2 Y = SiCl3, SiMeCl2, SiMe3, SiEt3 RF = C(CF3)3 Y = SiCl3, SiMeCl2, SiMe3, SiEt3 RF = CF(CF3)2, C(CF3)3 Y = HP(O)(OR) (R = Me, Et, Pr)
The similar results are received for irradiation of dialkylphosphites too. Allylic radical of a similar structure 62 is formed when initial olefin reacts with silicon-containing radical.
62 However, photolysis of this olefin without the donors of hydrogen also results the formation of allylic radical with similar structure 63.
The authors of this work have established, that the lifetime of such allylic radicals is approximately 1 hour. This fact disagrees with the data of such type of radicals described in [266]. The irradiation of perfluoroolefins (even not very spatially complicated) in alcoholic solutions results the formation of steady radicals 64 [267]. Thus, the addition of peroxyde tert-butyl (5 % volumetric) increases the signal strength of ESR in 10-40 times..
The perfluoroalkyl radicals (65 and 66) with cyclic substituents were received by photolysis of corresponding bromides at presence of Hg (C2B10H11)2 [268]. They were stable up to 220 oC.
Perfluorinated alkenyl radical 67 was received by the reaction of terminal perfluoroolefin and methyl radical which was formed from photolysis of di-tert-butyl-peroxyde. Radical 67 contain the double bond in position 4 or 5 to the radical center and further turn into the rather stable radical 68 [269,270].
In the reaction of perfluoro-2-methylpent-2-ene with di-tert-butyl-peroxyde at 140 oC or at 20 oC (UV light) (CF3)2CMeCHFC2F5 and (CF3)2CHCFMeC2F5 (ratio 1:4) was obtained (30 % yield) [271]. The intermediate radical (CF3)2CMeCFC2F5 was identified by ESR spectrum. These radicals are the models for study of a structure and reactionary ability of cyclic tertiary perfluorinated radicals. It is established, that the view of received kinetic curves (range of temperatures +20 - 120 oC), and dependence of signal of stationary amplitude from intensity of light support the idea of radicals dimerization with following kinetic parameters: k20 =102 L/mols, Eact = 3.4 kkal/mols. It is assumed that the reduction of dimerization speed constant of these radicals over against the acyclic fluorinated radicals [246, 271] was provided not only by steric factors, but also by the stereo chemical rigidity of radical canter. The stable perfluoroalkyl radicals were received also by other methods, which include interaction of perfluoroolefins with various radicals. So, at electrochemical fluorinations of perfluoroolefins and their alkoxy- and alkylamino-derivatives the radicals 69 - 71 with high stability are received [272].
Radicals 69-71 was identified by ESR spectrums. During the electrochemical fluorination of steric ramified polyfluoropyrrolizidine the stable radical (with concentration not less 5 % and time live > 1 month) was recognized in products of reaction by the method of ESR [273, 274]. The radicals of this type rather stable and can be received when perfluoroolefins reacted with either element fluorine, or other radicals, for example, CH3., [P(O)(OMe)2]..
Radicals were fixed during electrochemical fluorination of following compounds:
Electrolysis of perfluoroolefins in environment of fluorosulphonic acid produce the stable fluorosulphonyl-oxyperfluoroalkyl radical [249-252, 275]. The characteristic features of this reaction are confirmed by the formation of the
perfluoroalkyl radicals from the
SbF5 replaces FSO3 group on atom of fluorine, giving a new stable radical 73 (anode - glass - carbon SU 2000, cathode - titanium, 1=0.6 A, t = 0.5 h) (current yield 75 %)
RF = F, CF3 [267], i-C3F7, t-C4F9 [245] Due to the essential differences in boiling points of radicals, received by electrochemical fluorosulphatation of hexafluoropropylene trimer and the initial olefin as well as a product of the further transformation, radical was isolated. The method of fluoroolefins fluorosulphatation is attractive becouse allows to receive the fluoroalkyl radicals with functional FSO3 group which permit their further chemical transformations. So, under the action of CsF this radical was transformed to the stable perfluoroacetyldi-iso-propylmethyl radical (last also was recognize during photolysis of ketofluorosulphate).
The electrochemical fluorination of hexafluoropropylene trimer at presence of NaF produce the stable perfluoro-3-ethyl-2,4-dimethyl-3-pentenyl radical [276]. The method of stable perfluoroalkylic radicals producing by radiolysis of perfluoroparaffins, having tertiary atom of carbon, is most advanced [228,232,239,277-279]. Method is simple and universal. The formation of radicals proceeds with fluorine atom elimination from a cycle [269]. It is possible to use ultraviolet irradiation of perfluorinatered compounds. However photolysis is less effective method then radiolysis. So, during the photolysis of hexafluoropropylene the concentration of radicals was three times less than during radiolysis [232].
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-fluoro-sulphonyloxy-tetrafluoroethylperfluoro-di(isopropyl)methyl
radical (58) [249-254], which was isolated with yield more than 80 % [255,256].
Because of the ramified structure the radical 58 has high stability [235]. Radical 58
can produce the perfluoroisoninyl radical as a product of FSO3 group
substitution under the action of either SbF5 [248,254] or CsF in acetonitrile
[255]. Radical 58 is the first example of new type of fluoroalkylating agents.
Their alkylation properties are determined by the stabilizing influence of unpaired
electron on the adjacent carbcation center.
.
-fluorosulfatoperfluoroalkyl radicals under the action of SbF5
[249].
