- Facile halogen exchange reactions: Chloroform with bromoform and carbon tetrachloride with carbon tetrabromide
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Both of the title systems undergo rapid halogen exchange (half-life ca. 1-2 min) in N-methylpyrolidinone with catalytic sodium hydroxide at room temperature. Yet they differ markedly in response to added p-dinitrobenzene. The rate of the haloform exchange is unaffected, whereas the rate of the carbon tetrahalide exchange is severely retarded. The known base-induced halogen exchange reaction between chloroform and bromoform is shown not to proceed through a reversible carbene intermediate as claimed in the literature. It appears to be best described in terms of the so-called RARP mechanism (radical anion-radical pair). The mechanism proposed for the rapid exchange between carbon tetrachloride and carbon tetrabromide is initial electron transfer, halide ion loss, and ensuing radical chain scrambling of halogen atoms. The acronym RARC, standing for radical anion-radical chain, is proposed.
- Orvik, Jon A.
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p. 4933 - 4936
(2007/10/03)
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- Fluoride anion catalyzed halogen dance in polyhalomethanes
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Tetrabutylammonium fluoride catalyzes the exchange of halogens between tetrahalomethanes.The presence of small amounts of haloform is suspected to be a necessary co-catalyst.Key Words: tetrabutyl ammonium fluoride; tetrahalomethanes; halogen exchange in.
- Sasson, Y.,Kitson, F.,Webster, O, W.
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p. 599 - 600
(2007/10/02)
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- Synthesis of fluorinated 1,2,3-butatrienes from α-halovinyl organometallic reagents
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The thermal stability and dimerization reaction of fluorinated α-halovinyl zinc and copper reagents, RR'C=CYM (Y = F, Cl, Br; M = ZnX, Cu), have been explored in detail. Dimerization of these vinyl carbenoids to betatrienes occurred when R was an aromatic (C6H5 or C6F5) and R' was a perfluoroalkyl group (CF3, C2F5, C3F7). The role of the α-halogen was determined; the α-F vinyl copper reagent (R = C6H5, R' = CF3) decomposed by oxidative dimerization to 1,3-dienes while the α-Br and -Cl copper reagents dimerized to butatrienes. The fluorinated butatrienes prepared in this study, (E)- and (Z)-R1R2C==C=C=CR1R2 (R1 = CF3, R2 = C6H5; R1 = C2F5, R2 = C6H5; R1 = n-C3F7, R2 = C6H5; R1 = CF3, R2 = C6F5) are available on a multigram scale and readily obtained with high isomeric purity. The geometry of one member of each isomeric pair of butatrienes was characterized by X-ray crystallography. The mechanism of the dimerization reaction has been determined to be a nucleophilic displacement/β-elimination process. Diels-Alder (1,2-addition), bromination (1,2-addition), and isomerizatioa reactions are described.
- Morken,Bachand, Patrick C.,Swenson, Dale C.,Burton, Donald J.
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p. 5430 - 5439
(2007/10/02)
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- Nucleophilic attack on halogeno(phenyl)acetylenes by halide ions
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Nucleophilic reactions between halogeno(phenyl)acetylenes and halide ions, ArC≡CX + Y-, where Ar = C6H5 or p-ClC 6H4, X = Cl or Br, and Y = Cl or Br, have been examined. Halogen exchange of the Finkelstein type was observed for the first time in acetylene halides in anhydrous dimethyl sulphoxide when X = Br and Y = Cl. This exchange did not occur with other X-Y combinations. In the presence of up to 20% water in dimethyl sulphoxide, or under aqueous-organic phase-transfer catalytic conditions, nucleophilic addition (formally of HY) took place for all the X-Y combinations studied. In the additions, the nucleophile Y- invariably attacked the carbon to which the phenyl group was bound. The mode of HY addition was stereospecifically trans; accordingly, the resulting dihalogenostyrenes always had the (Z)-1,2-dihalogeno-configuration.
- Tanaka, Ryuichi,Zheng, Shi-Qin,Kawaguchi, Kenji,Tanaka, Takehide
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p. 1714 - 1720
(2007/10/02)
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