- Removal of water - a factor influencing the synthesis of alkynes in a phase-transfer catalyzed β-elimination reaction
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Acetylene derivatives 4 were synthesized from the corresponding vicinal bromo compounds 2 in the phase-transfer catalyzed hydrogen bromide β-elimination reaction using solid potassium hydroxide as a base, xylene as a solvent, and a phase-transfer catalyst. The yields of the synthesized acetylene derivatives 4 were substantially improved when water formed in the process had been removed.
- Zakrzewski,Huras,Sas,Zelechowski,Bombinska
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p. 1051 - 1057
(2008/09/21)
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- Polylithiumorganic compounds. Part 28. The reaction of allene and alkyl substituted allenes with lithium metal
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The reaction of allene (3a) and alkyl substituted allenes 1,2-hexadiene (3b), cyclopropylallene (3c), and vinylidene cyclopropane (3d) with lithium metal was investigated in order to access 2,3-dilithioalkenes 4a-d. These dilithioalkenes 4a-d are very reactive in polar solvents like THF and act as strong bases, either metalation of the starting allene 3a-d, the solvent, or sufficiently acidic intermediates like 8 a-d is observed. The metalation products 5-7 show follow-up reactions like 1,3-H shift to the corresponding 1-lithio-1-alkynes 8 and subsequent metalation to the dilithioalkynes 9. Additionally, lithium hydride elimination and ring-chain rearrangement (for 5c) are observed. 1,2-Hexadiene (3b) can be brought to reaction with lithium metal in the apolar solvent pentane, here the follow-up reactions are much slower due to the insolubility of 4b. In all cases the elucidation of the reaction pathways is hampered by the formation of complex mixtures of, amongst others, regio- and stereoisomeric products upon quenching with simple electrophiles.
- Maercker, Adalbert,Tatai, Andrea,Grebe, Burkhard,Girreser, Ulrich
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- Gas-phase kinetic and mechanistic studies of some interconverting alkylcyclopropene pairs: Involvement of dialkylvinylidene intermediates and their quantitative behaviour
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The pyrolyses of two isomeric pairs of alkylcyclopropenes, namely 1,3-dimethyl- (15) and 1-ethyl-cyclopropene (16), and 1,3,3-trimethyl- (5) and 1-isopropyl-cyclopropene (17), have been studied in the gas phase. Complete product analyses at various conversions up to 95% were obtained for the decomposition of each compound at five temperatures over a 40°C range. The time-evolution data showed that the isomerisation reactions 15?16 and 5?17 were occurring. Kinetic modelling of each system allowed the determination of rate constants for these and all other decomposition processes. Tests confirmed that all reactions were unimolecular and homogeneous. Arrhenius parameters are reported for overall reactions and individual product pathways. Further kinetic analysis allowed us to extract the propensities (at 500 K) for 1,3-C-H insertion of the dialkylvinylidene intermediates involved in the rearrangements as follows: kprim:ksec: ktert = 1:16.5:46.4. Additional experiments with 13C-labelled cyclopropenes yielded alkyl group migration aptitudes for the dialkylvinylidenes (from the pattern of 13C in the alkyne products) as follows: Me:Et:iPr=1:3.1:1.5. Explanations for these trends are given. Another important finding is that of the dramatic rate enhancements for 1,3-diene product formation from the 1-alkylcyclopropenes; this can be explained by either hyperconjugative stabilisation of the vinylcarbene intermediates involved in this pathway, or their differing propensities to 1,2 H-shift. The observed large variations in product distribution amongst these four cyclopropenes is interpreted in terms of these specific effects on individual pathways.
- Graf Von Der Schulenburg, Wilhelm,Hopf, Henning,Walsh, Robin
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p. 1963 - 1979
(2007/10/03)
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- Kinetic Investigation of the Reactions of Methyl Radicals with But-2-yne
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The reactions of methyl radicals with but-2-yne have been studied in a greaseless static vessel in the temperature range 543...583 K.The methyl radicals were generated by thermolysis of azomethane (initial pressures 2.66 kPa and 3.19 kPa) in mixtures with the alkyne (initial pressures 0...7.8 kPa).The primary steps are H-abstractions producing 3-methylpropargyl radicals and additions to the triple bond forming trimethylvinyl radicals.Both radicals were stabilized by hydrogen abstraction from the parent compounds and by combination processes. The temperature dependence of the rate constant k2 for the addition reactio n can be expressed by the equation k2 = 108.65+/-0.20exp(-40300 +/- 1300)Jmol-1/RT M-1s-1 For the rate constant of hydrogen abstraction a value of k1 = 1.65 . 105 M-1s-1 at 573 K was estimated.Therefore the H-abstraction proceeds about two times faster than the addition reaction.
- Claus, P.,Marquardt, M.,Trung, Tran Kim,Scherzer, K.
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p. 533 - 539
(2007/10/02)
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- TRANSITION-STATE GEOMETRIES AND STEREOSELECTIVITY OF ALKYLIDENECARBENE ADDITION TO OLEFINS. AN EXPERIMENTAL AND THEORETICAL INVESTIGATION.
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A careful investigation of the addition of unsymmetrical alkylidenecarbenes R(CH//3)C equals C:, where R equals Et, i-Pr, and t-Bu, to two unsymmetrical olefins, isobutylene and tert-butylethylene, was carried out. In all cases, the E adduct predominated over the Z adduct, with increasing stereoselectivity being observed upon going from R equals Et to R equals t-Bu in the carbene. Greater stereoselectivity was also observed with tert-butylethylene as substrate compared with isobutylene. Detailed theoretical calculations were carried out on the reaction pathways and transition-state geometries of unsaturated carbene-olefin interactions. Model studies on the H//2C equals C: plus H//2C equals CH//2 system were done both by MNDO and by standard ab initio methods at the STO-3G level, whereas the more substituted systems were evaluated by the MNDO method. These calculations indicate that attack of R(CH//3)C equals C: on (CH//3)//2C equals CH//2 leads to favored 'C//2-inward' transition states, which yield the observed E adducts.
- Apeloig,Karni,Stang,Fox
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p. 4781 - 4792
(2007/10/23)
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- Alkyne Formation in the Reaction of α-Bromo Ketones with Arylsulfonylhydrazines
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Alkynes can be produced by treating some α-bromo ketones with arylsulfonylhydrazines.The reaction is acid-catalysed and mesitylsulfonylhydrazine was the most efficient hydrazine reagent examined.The mechanism of the reaction has not been elucidated although it is shown not to proceed via an α-bromo mesitylsulfonylhydrazone.
- Bryant, Michael W.,Smith, Robin A. J.,Wong, Lisa
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p. 2529 - 2540
(2007/10/02)
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- KETENE-ANTHRACENE ADDUCT, A PRECURSOR OF SUBSTITUTED ACETYLENES
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The ketene-anhracene adduct 1 serves as a good precursor in the synthesis of substituted acetylenes, in wich the key step is the retro Diels-Alder reaction.
- Tarnchompoo, Bongkoch,Thebtaranonth, Yodhathai,Utamapanya, Suchada,Kasemsri, Prapani
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p. 1241 - 1242
(2007/10/02)
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- Application of Phase Transfer Catalysis, 14.- Preparation of Alkynes from Halides with Solid Potassium tert-Butoxide and Crown Ether
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Preparatively very simple and mild HX eliminations with solid potassium tert-butoxide in petroleum ether in the presence of catalytic amounts of crown-6 are described. 1,2-Dihalides (from alkenes) and 1,1-dihalides (from aldehydes) yield 1-alkynes; internal geminal dihalides (from symmetric ketones) give internal alkynes in excellent yields. 2,2-Dihalides (from methyl ketones) yield homogeneous 1-alkynes only if the 3-position is blocked. (E)-Haloalkenes lead also to alkynes in a syn-elimination process.
- Dehmlow, Eckehard V.,Lissel, Manfred
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