Ethers, reactions with fluorinated alkenes

Barton and co-workers have studied30 the reactions of trifluoromethyl hypofluorite with various alkenes and found that (Z)- and (T -stilbene give vicinal fluoro trifluoromclhoxy products and vicinal difluorides in chlorotrifluoromethane or diethyl ether, while vicinal fluoro methoxy adducts predominate when the reaction is carried out in methanol at —78 C. svn Addition predominates in all cases on the basis of the fluorination regioselectivity of many alkenes, the formation of a /1-fluoro carbcnium ion is proposed.30... 

Model compound studies reveal factors which influence free—radical additions of ethers, polyethers, and crown—polyethers, to fluorinated alkenes. Stereo-electronic effects significantly affect relative reactivities of cyclic amines. Novel dienes are obtained by reactions of some oligomers of fluorinated alkenes with sodium amalgam and some reactions of these systems with nucleophiles are described. 

Cyclodimerization of fluorinated alkenes was observed as early as 1947 during the pyrolysis study of polytetrafluoroethylene (PTFE) [54]. The ability of fluorinated alkenes to dimerize with themselves has been attributed to the energy relief of fluorinated double bond strain [4]. In the same fashion, the aryl trifluorovinyl ether groups (Ar—O—CF=Cp2) underwent thermally activated [2-1-2] cycloaddition to form bisaryloxy-substituted PFCB rings. Scheme 14.9 depicts the model reaction of cycloaddition of aryl trifluorovinyl ethers. The thermal cyclodimerization of methyl... 

Radical cyclization continues to be one of the most popular radical reaction classes used in synthesis. By far the majority of published cyclization reactions are 5-exo in nature and this year is no exception. The rates of 5-exo cyclization reactions continue to be measured. For example, the rate constants for the 5-exo cyclizations of the 6,6-diphenylhex-5-enyl, 1-methyl-6,6-diphenylhex-5-enyl and the 1,1-dimethyl-6,6-diphenylhex-5-enyl radicals have been measured by laser flash photolysis studies and Arrhenius parameters determined. The relative rate constants for cyclization, and the reaction with PhSeH, were determined at 20 °C. At 20 °C the rates of the three primary, secondary and tertiary radicals with PhSeH were approximately (1.2 0.1) X 10 lmol s The rate constants for alkyl radicals calibrated by competition reactions with PhSeH and PhSH were found to be 30-40% smaller than previously reported and thus it was concluded that derived rate constants for some fast radical reactions may have to be adjusted accordingly. The rate constants for the 5-exo cyclization of secondary radicals on to hydrazones and oxime ethers have been determined. The fastest rate constants were observed for the IV-benzoylhydrazone acceptor. The rate constants were found to be approximately 800 times faster than for the corresponding 5-exo cyclization on to alkenes." Other work has measured the rate constants of cyclization of a range of fluorinated hex-5-enyl, hept-6-enyl, oct-7-enyl and non-8-enyl radicals. Not only were the rate constants measured but the regioselectivity of cyclization was compared with that for the aU-carbon homologues." The 6-exo cyclization of 1,1,2,2-tetrafluoro- and l,l,2,2,3,3,4,4-octafluorohept-6-enyl radicals were found to be approximately 10 times faster than those for the parent... 

The same group of Japanese authors studied the photochemical reaction of fluorinated quinoxalines. Photochemical cycloadditions with quinoxaline derivative 1294 occurred and C=N double of the diazine ring, leading to the formation of azetidine derivatives (Scheme 299). The presence of trifluoromethyl group in the molecule of 1294 activated the substrate towards cycloaddion, so that even electron-deficient methyl methacrylate was introdnced in the reaction [797], In the case of ethyl vinyl ether as the alkene, the addnct 1296 also as in a case with 1289 was not stable and underwent azetidine ring-opening npon action of the solvent. Ketene was also successfully introduced in [2+2] cycloaddition with trifiuoromethyl-substituted quinoxaline derivatives [797]... 

Bromine monofluoride [13863-59-7], BrF, can be prepared by the direct reaction of Br2 and F2, but because it readily disproportionates it has never been prepared in pure form (57). However, BrF can be prepared in situ by the reaction of Br2 with AgF in benzene (58) or by the reaction of A/-bromoacetamide and HF in ether (59). BrF adds to simple alkenes at room temperature to give products of trans-addition. Bromine trifluoride [7787-71-5], BrF, can be formed from gaseous fluorine and Hquid bromine (60). Bromine pentafluoride [7789-30-2], BrF, is formed from the reaction of BrF vapor with gaseous fluorine at 200°C (60). The tri- and pentafluorides are commercially available. As strong fluorinating agents they are useful in... 

Replacement of fluorine occurs when l,l,l-trifluoro-2,2-dihaloethanes 1 are treated with alkaline alkoxides.59 The products, alkyl 1,1 -difluoro-2,2-dihaloethyl ethers, are probably formed by two separate reactions these are elimination of hydrogen fluoride to give an alkene, followed by addition of the alcohol.59... 

Nonfluonnated a- and p diketones give the corresponding tetrafluoroalkanes as the major products together with considerable amounts of alkenes, ethers, and sulfites Formation of side products is reduced, and yields of tetrafluoroalkanes are greatly improved by conducting the fluorination reactions in an excess of anhydrous hydrogen fluoride [176, 177]... 

In the reaction of / -hydroxycarboxylic acids 20 and 21 with sulfur tetrafluoride, the formation of alkenes by elimination of hydrogen fluoride and of fluorinated ethers and esters by inter-molecular reaction is observed." Overall yields are low because of extensive resinification. 

By reaction of a 1,1-difluoroethene with tributylphosphane in the presence of boron trifluoride-diethyl ether complex, the vinylphosphonium salt is obtained. On subsequent hydrolysis the selectivity of formation of the Z- and F-isomer is 100%, but it diminishes after the introduction into the alkene. together with the perfluoroalkyl group, of substituents such as phenyl and 4-methoxyphcnyl. In accordance with this methodology, a method of substitution of fluorine by hydrogen at the fluoroolefinic double bond has been developed.194,206 The first stage of this reaction includes the formation of a fluoro-/5-phosphane from the fluoroalkene and tributylphosphane. The decomposition of this /l5-phosphane in the aqueous solvent leads to a fluoroalkene, e.g. formation of l.206... 

Applications of Crown Ethers.—Crown ethers have been found to form molecular complexes with bromine that can be used as reagents in bromination reactions, e.g. of alkenes. The crown-halogen interaction seems not to be encapsulation but rather to be the formation of a polymeric structure. The complexes of nitro-nium tetrafluoroborate (NO2BF4) with various crown ethers, for example the 1 1 complex with 18-crown-6, have been reported to be selective nitrating agents. A new rapid synthesis of alkylseleno- and alkyltelluro-arenes from arenediazon-ium tetrafluoroborates and dialkyl diselenides or ditellurides employs KOAc and 18-crown-6 as a phase-transfer catalyst a radical mechanism has been suggested. Fluorine-19 n.m.r. evidence has been presented to show that KF forms... 

---