| 2.5.
Isomerization of internal perfluoroolefins.
The isomerization of terminal perfluoroolefins to internal ones takes place under the influence of fluoride-ion [183,184].
For example, the rapid isomerization of perfluoro-1-pentene to thermodynamically more stable isomer occurs in the presence of cesium fluoride [185].
The reactions of allylic displacement occur readily and with a stereo-chemistry steric control [186].
The last product reacts with formation, as a rule, a mixture of isomeric internal perfluoroolefins. For example, the isomerizations of tetrafluoroethylene tetramer under influence of fluoride-ion proceed with the isomers (52) and (53) formation [187].
The nucleophilic reagents react with internal perfluoroolefins and the elimination of fluoride-ion take place. The result of this process is the formation of others isomers. If the rate of reaction with a nucleophile for such isomers is higher then for initial olefin, the result of reaction is the complex mixture. It should be taken into account at realization of these processes. Both type of isomerization are possible at the action of fluoride-ion on ramified internal perfluoroolefins (with double bond participation) and rearrangement with migration of CF3-. The authors of [17] found the intermediate trifluoromethyl anion in the reaction with perfluoropyrimidine. The new perfluoroolefins with unexpected structure may be produced [12].
The generation of carbanions under the influence of fluoride-anion can to give the transformation of initial structure. So, at the reaction of CsF with trimer of a perfluorocyclobutene a complex cycle is formed [18,19]:
Isomerization also can be carried out by transformation of C-carbcation, which generate, for example, by the action of SbF5 on perfluoroolefin. Thus, terminal perfluoroolefins (perfluoropent-1-ene, perfluoro-hex-1-ene etc.) at the action of SbF5 (catalytic amounts) smoothly isomerizate to corresponding trans-perfluoroolefins with double bond in position 2 (yield 80-85 %) [16, 188-190].
Other catalytic agents (for example AlClxFy) are effective in these processes too [191].
The rearrangement of erfluoroisopropylethylene to tris(trifluoromethyl)-ethylene at heating up to 30-40 oC was observed [188-190]. If molecule had two terminal double bond, they isomerizate simultaneously with formation of 80-90 % E, E-isomers and 10-20 % E, Z-isomer [192,193].
When boiling perfluorohex-2-ene (perfluorohept-2-ene) reacts with SbF5 and equilibrium mixture containing 75-80 % of olefin (54) and 20-25 % of (55) is observed [194].
The shift of double bond is also observed in the reactions of some perfluorinatered dienes with SbF5 [195]. So, perfluoro-2-methylpent-2,4-diene in the reaction with SbF5 at temperature 0-5 oC isomerizate to perfluoro-2-methylpent-1,3-diene. Last quantitatively isomerizate to tris(trifluoromethyl)fluoroallene at 20-25 oC .
German with co-workers was shown [191-196], that the realization of an intramolecular cyclization with formation of perfluoroalkylcyclopentene is the important moment.
Perfluorinated dienes at presence of SbF5 also yield the derivative of cyclopentene in soft conditions. However, if diene contains only CF3 of group, the cyclic product is not formed at room temperature, whereas perfluoro-1,2-dimethylcyclobutene was produced at temperature 100oC [196].
The presence only one CF3-group results the formation of cyclobutane system [196]. Probably, the rearrangement of perfluoropent-1,3-diene to perfluoro-1,3-dimethyl-allene with further cyclodimerization is take place.
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