- Rate coefficients for the gas-phase reactions of chlorine atoms with cyclic ethers at 298 K
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Rate coefficients of reactions of Cl atoms with cyclic ethers, tetrahydropyran (THP), tetrahydrofuran (THF), and dihydrofurans (2,5-DHF and 2,3-DHF) have been measured at 298 K using a relative rate method. The relative rate ratios for THP and THF are 0.80 ± 0.05 and 0.80 ± 0.08, respectively, with n-hexane as the reference molecule. The relative rate ratios for THF and 2,5-DHF with n-pentane as the reference molecule are 0.95 ± 0.07 and 1.73 ± 0.06, respectively, and for 2,5-DHF with 1-butene as reference is 1.38 ± 0.05. The average values of the rate coefficients are (2.52 ± 0.36), (2.50 ± 0.39), and (4.48 ± 0.59) × 10-10 cm3 molecule-1 s-1 for THP, THF, and 2,5-DHF, respectively. The errors quoted here for relative rate ratios are 2σ of the statistical variation in different sets of experiments. These errors, combined with the reported errors of the reference rate coefficients using the statistical error propagation equation, are the quoted errors for the rate coefficients. In the case of 2,3-DHF, after correcting for the dark reaction with CH3COCl and assuming no interference from other radical reactions, a relative rate ratio of 0.85 ± 0.16 is obtained with respect to cycloheptene, corresponding to a rate coefficient of (4.52 ± 0.99) × 10-10 cm3 molecule-1 s-1. Unlike cyclic hydrocarbons, there is no increase with increasing number of CH2 groups in these cyclic ethers whereas there is an increase in the rate coefficient with unsaturation in the ring. An attempt is also made to correlate the rate coefficients of cyclic hydrocarbons and ethers with the molecular size as well as HOMO energy.
- Alwe,Walawalkar,Sharma,Pushpa,Dhanya,Naik
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p. 295 - 305
(2013/08/25)
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- Stereoselective Synthesis of Alcohols containing (Z)- and (E)-Olefins, Dienes, Enynes and Styrenes: Cyclic β-Halogeno Scissions using Samarium Diiodide as the Electron-transfer Agent
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In contrast to the sodium-mediated ring scission of 2-substituted 3-chloro ethers of the tetrahydrofuran series, samarium diiodide gives olefins of high (E)-stereoselectivity and provides (E)-conjugated and unconjugated dienes, styrenes and enynes in good yield without appreciable over-reduction.Whilst the SmI2 scission of 3-chloro-2-alkyltetrahydropyrans gives (Z)-rich (Z)/(E) olefin mixtures, the 2-(alk-1'-ynyl) members give (Z)-enyne alkohols with high stereoselectivity, providing a valuable complement to the (E)-enyne synthesis employing the tetrahydrofuran series.In electron-transfer scissions using sodium, the stereochemistry of the product alcohols is related to the ground-state conformation of the cis- and trans-pyrans and -furans.The slow SmI2-mediated reactions appear to involve samarium-complexed intermediates having structures independent of the original conformation, or of the cis- or trans-geometry of the furan or pyran, and it is the transition states from these intermediates that determine the stereochemical outcome.Scissions in the tetrahydrofuran series can be accelerated by addition of HMPA or DMPU with only a little deterioration in stereoselectivity, but in the tetrahydropyran series there are drastic changes in product stereochemistry when DMPU is added.Brief comment is made on the synthesis of tetrahydro-furan and pyran precursors.
- Crombie, Leslie,Rainbow, Linda J.
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p. 673 - 688
(2007/10/02)
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- CATALYSED QUANTITATIVE SYNTHESIS OF 2,3,3-TRICHLOROTETRAHYDROFURAN
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Tetrahydrofuran was chlorinated in the presence of N,N-dimethyloctylamine (1 mmol per mol optimal) with an excess of sulfuryl chloride at 55-80 deg C.After a first stage producing trans 2,3-trichloro THF (1) the reaction proceeded to complete conversion to 2,3,3-trichloro THF (2) in less than 22 h. The mechanism is discussed in terms of an inhibitory effect of hydrogen chloride and a catalytic role for anions unintentionally derived from the sulfuryl chloride. Equilibration of 2 with the isomer 2,2,4-trichlorobutanal (3) was observed at 130-170 deg C with hydrogen chloride catalysis, at 70 deg C with titanium tetrachloride or zinc chloride.
- Buyck, Laurent de,Lepeleire, Carine de
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p. 553 - 558
(2007/10/02)
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- β-Halogeno Ether Synthesis of Olefinic Alcohols: Stereochemistry and Conformation of 2-Substituted 3-Halogenotetrahydro-pyran and furan Precursors
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Ring-scission of cis- or trans-2-alkyl- (or aryl-) 3-chlorotetrahydropyrans proceeds regioselectively and highly stereoselectively to give (E)-alk-4-en-1-ols, but in the parallel tetrahydrofuran series (Z)-/(E)-mixtures, dependent on percursor geometry, are formed.In this paper the stereochemistry and conformation of the tetrahydro-pyran and - furan precursors are considered.The cis/trans-composition of 2,3-dihalogenotetrahydro-pyrans and-furans made by various routes is reported.Reaction with Grignard reagents gives separable cis-/trans-mixtures the stereoisomeric composition of which, in the cases examined, does not depend on the stereoisomeric composition of the dihalide, but does vary with the halogen and the composition of the Grignard or dialkylmagnesium; possible reasons are discussed.The stereochemistry and conformation of the 2-alkyl- (or aryl-)3-chlorotetrahydropyrans is analysed by n.m.r. methods (J2a,3acis ca. 1.5 Hz; J2a,3atrans ca. 9.8 Hz) but assignments for the two tetrahydrofuran series with J2,3 2.6-3.6 and 4.3-5.9 are made uncertain by pseudorotation.The stereochemical identity of the two series is rigorously proved by isolation of cis- and trans-2-allyl-3-chlorotetrahydrofuran.On the one hand the former is hydrogenated to the cis-2-propyl compound, correlated with other members of the alkyl series, but on the other it is oxidised and the acid is converted into the cis-p-bromophenyl ester.The stereochemistry and conformation of the latter is rigorously demonstrated by an X-ray structure.The stereochemistries and conformations of the 2-deuterio- and 2-methoxy-3-chlorotetrahydropyrans are discussed, and consideration is then extended to the 2-alkyl-3-chloro-2-methyltetrahydro-pyran and -furan series.
- Crombie, Leslie,Wyvill, Robert D.
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p. 1971 - 1982
(2007/10/02)
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