1574-40-9

The reactions of the ethynyl radical (C2H) with ethene (C 2H4) and propene (C3H6) are studied under low temperature conditions (79 K) in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by 193 nm photolysis of acetylene (C 2H2) and the reactions are studied in nitrogen as a carrier gas. Reaction products are sampled and subsequently photoionized by the tunable vacuum ultraviolet radiation of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. The product ions are detected mass selectively and time-resolved by a quadrupole mass spectrometer. Bimolecular rate coefficients are determined under pseudo-first-order conditions, yielding values in good agreement with previous measurements. Photoionization spectra are measured by scanning the ALS photon energy while detecting the ionized reaction products. Analysis of the photoionization spectra yields-for the first time-low temperature isomer resolved product branching ratios. The reaction between C2H and ethene is found to proceed by H-loss and yields 100% vinylacetylene. The reaction between C2H and propene results in (85 ± 10)% C4H4 (m/z = 52) via CH3-loss and (15 ± 10)% C5H6 (m/z = 66) by H-loss. The C 4H4 channel is found to consist of 100% vinylacetylene. For the C5H6 channel, analysis of the photoionization spectrum reveals that (62 ± 16)% is in the form of 4-penten-1-yne, (27 ± 8)% is in the form of cis-and trans-3-penten-1-yne and (11 ± 10)% is in the form of 2-methyl-1-buten-3-yne.

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The first chemical synthesis of tetravinylallene (3,5-divinylhepta-1,3,4,6-tetraene) is reported. The final, key step of the synthesis involves a palladium-catalyzed, Negishi-type cross-coupling involving 1,5-transposition of a penta-2-en-4-yn-1-ol methanesulfonate. The unprecedented fundamental hydrocarbon is sufficiently stable to be purified by flash chromatography. A similar synthetic pathway grants access to the first substituted tetravinylallenes, which provide insights into the influence of substitution upon stability and reactivity. Tetravinylallenes are shown to break new ground in swift structural complexity creation, with three novel sequences reported.

The reactions of C2H radicals with C4H8 isomers 1-butene, cis-2-butene, trans-2-butene, and isobutene are studied by laser photolysis-vacuum ultraviolet mass spectrometry in a Laval nozzle expansion at 79 K. Bimolecular-reaction rate constants are obtained by measuring the formation rate of the reaction product species as a function of the reactant density under pseudo-first-order conditions. The rate constants are (1.9 ± 0.5) × 10-10, (1.7 ± 0.5) × 10 -10, (2.1 ± 0.7) × 10-10, and (1.8 ± 0.9) × 10-10 cm3 s-1 for the reaction of C2H with 1-butene, cis-2-butene, trans-2-butene, and isobutene, respectively. Bimolecular rate constants for 1-butene and isobutene compare well to values measured previously at 103 K using C2H chemiluminescence. Photoionization spectra of the reaction products are measured and fitted to ionization spectra of the contributing isomers. In conjunction with absolute-ionization cross sections, these fits provide isomer-resolved product branching fractions. The reaction between C2H and 1-butene yields (65 ± 10)% C4H4 in the form of vinylacetylene and (35 ± 10)% C5H6 in the form of 4-penten-1-yne. The cis-2-butene and trans-2-butene reactions yield solely 3-penten-1-yne, and no discrimination is made between cis- and trans-3-penten-1-yne. Last, the isobutene reaction yields (26 ± 15)% 3-penten-1-yne, (35 ± 15)% 2-methyl-1-buten-3-yne, and (39 ± 15)% 4-methyl-3-penten-1-yne. The branching fractions reported for the C2H and butene reactions indicate that these reactions preferentially proceed via CH3 or C2H3 elimination rather than H-atom elimination. Within the experimental uncertainties, no evidence is found for the formation of cyclic species.

Megawatt KrF laser gas-phase photolysis of benzene and acetonitrile-benzene mixture was studied by using mass spectroscopy-gas-chromatography and Fourier transform infrared spectroscopy for analyses of volatile products, and by Fourier transform infrared, Raman and X-ray photoelectron spectroscopy, electron microscopy and magnetization measurements for analyses of solid products deposited from the gas-phase. The results are consistent with carbonization of benzene and decomposition of non-absorbing acetonitrile in carbonizing benzene through collisions with excited benzene and/or its fragments. The solid products from benzene and acetonitrile-benzene mixture have large surface area and are characterized as nanomagnetic amorphous carbonaceous soot containing unsaturated C centers prone to oxidation. The nanosoot from acetonitrile-benzene mixture incorporates CN groups, confirms reactions of benzene fragments with CN radical and has a potential for modification by reactions at the CN bonds.

Thermal extrusion of Ph3PO from β-oxoalkylidenetriphenylphosphoranes 4 to give the alkynes 5, which under conventional pyrolysis conditions is restricted to cases in which R1 is an electron withdrawing group, has been successfully achieved for R1=H or alkyl by using FVP.The method allows convenient construction of multigram quantities of the alkynes 5 from alkyl halides 1 and allows convenient construction of multigram quantities of the alkynes 5 from alkyl halides 1 and acid chlorides 3 in three steps with good overall yields.Under the conditions used the ylides with R2 = cyclobutyl also undergo less of ethene to provide convenient access to the vinylalkynes 6.

For the alkynyl-substituted olefines 1 - 14 activation parameters for the geometrical isomerisation have been determined in the gasphase by the single-pulse shoke-tube technique.By comparison of these barriers with the corresponding one of the isolated double bonds, each corrected by the steric energy contribution of the ground and transition state, a value of 7.8 +/- 1.3 kcal * mol-1 for the propargyl stabilisation energy (PrSE) has been derived. - Key Words: Resonance energy / Stabilisation energy / Propargyl resonance / Force field calculation / Intrinsic rotational barrier / Single pulse shock tube / Gasphase kinetics / Heats of hydrogenation

The enynes HCCCH=CHCH2R (R = H, Me, OMe, NMe2, SMe), are converted into dimers by treatment with slightly more than two equivalents of butyllithium.The Z-configuration predominates in the dimers obtained after aqueous work-up.The lithiated enynes (R = H, Me, SMe) and acetylenic compounds LiCCCH2R (R = SMe, C6H5) add in an analogous way to the double bond of LiCCCH=CH2.

The gas phase pyrolysis of cyclopropylacetylene (3) has been investigated in a flow system between 600 and 900 deg C.At the lower temperature limit 3 isomerizes to give 1,2,4-pentatriene (9), cis- (10) and trans-3-penten-1-yne (11), as well as 1-penten-4-yne (12), all isomers being formed in roughly equal amounts.At 850 deg C none of these C5H6-hydrocarbons is formed; rather, 1,3-cyclopentadiene (13), benzene (14), toluene (15), indene (16), naphthalene (17), and acenaphthylene (18) are the major constituents of a complex pyrolysate.The mechanisms of formation of the major pyrolysis products are discussed.

Treatment of the bromobicyclobutanes 4a - c with LDA led to the formation of the 1,2,3-butatrienes 6 which were isomerized by excess base to the alkynes 8.Reaction of 4c with LDA afforded 8d, indicating that bicyclobut-1(3)-ene 5 was not an intermediate.

A stereochemical test was conceived to determine if the photochemical extrusion of carbon monoxide from 3,5-cycloheptadienones to give 1,3,5-hexatrienes is a concerted process, and thus an example of a photochemical cheletropic fragmentation.The cis- and trans-2,7-dimethyl derivatives of this ring system were synthesized.Spectroscopic data could not distinguish unambiguously between the two materials, and configurations were therefore assigned to the two dienones based on the NMR spectra of the adducts of the dienones and the reactive dienophile PTAD and an X-ray crystal structure of the cis adduct.The several isomeric 2,4,6-octatrienes were prepared and their photoisomerization was studies preliminary to study of the photolysis of the dienones.Since no conditions could be found which afforded fragmentation of the dienones without secondary isomerization of the trienes, the composition of the product mixture was deterrmined as a function of time on excitation of the dienones at 313 nm.It was found that the isomeric dienones indeed gave different triene photoproducts, the cis-dienone affording the (E,Z,Z)-octatriene and the trans-dienone affording the (Z,Z,Z,Z)-octatriene.In both cases, these products were the predominant if not exclusive initial products, with other trienes appearing soon afterward.It is concluded that these photochenmical reactions proceed by a concerted reaction path, involving conrotatory opening of the cycloheptadienone ring system.The same course of reaction was observed some time ago for thermal cleavage of the analogous cyclic sulfones.Various theoretical rationales for this behavior are discussed.The photochemical reactions clearly do not follow the predictions for a linear cheletropic reaction, and a nonlinear path is a distinct possibility.Howeverr, the formation of the relatively strained (Z,Z,Z)-triene from the trans-2,7-dimethylcycloheptadienone is difficult to rationalize on the basis of any concerted mechanism except by invoking the principle of least nuclear motion.Thus, reaction of the trans-dienone appears to proceed selectively from the conformation in which both methyls are pseudoaxial, which according to MM2 calculations is less stable than the diequatorial conformation in the electronic ground state by 2.7 kcal/mol.It is suggested that orbital symmetry may not play a major role in controlling the course of this type of photochemical cheletropic reaction.

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By using flash vacuum conditions the thermal elimination of Ph3PO from β-oxoalkylidenetriphenylphosphoranes, previously confined to cases with an α-electron withdrawing group, has been extended to provide a general, high yielding synthesis of aliphatic and terminal alkynes.

Pent-2-en-4-ynyl radicals were generated from pent-1-en-4-yne and cis- and trans-1-bromopent-2-en-4-yne and observed in the trans (4) and cis (5) conformations by e.s.r. spectroscopy.The rate constant for the isomerization (5)->(4) was determined by kinetic e.s.r. to be given by equation (i) logk1=(13.2+/-1.3)-(47.5+/-8.0 kJ mol-1/2.3 RT)s-1 and for the isomerization (4)->(5) the rate constant was found to be given by equation (ii) logk-1=(13.0+/-1.3)-(49.2+/-8.0 kJ mol-1/2.3RT)s-1.From the observed barriers to rotation the methane based stabilization energies of (4) and (5) were estimated to be 112 and 110 kJ mol%-1.E.s.r. spectra were also recorded for the 1-trimethylsiloxy-derivatives of (4) and (5), pent-2-en-4-ynyl radicals, 1-trimethylsiloxypent-2-en-4-ynyl radicals, and penta-1,4-diynyl radicals.The enthalpies of formation, structures, and stabilization energies of the hydrocarbon radicals were computed using the MINDO/3 method.The conformational interconversion of radicals (4) and (5) was also examined by MINDO/3.

On irradiation in the presence of triplet sensitizers having a triplet energy >250 KJ/mol, vinylacetylene (1a) dimerizes to cis- and trans-1,2-diethynylcyclobutane (cis- and trans-2) as well as minor amounts of 4-ethynyl-1-vinylcyclobutene (3).The effect of substituents on the course of the reaction is investigated: whereas alkyl, vinyl, and phenyl substituents, respectively, in the 4-position of 1a do not influence the photoaddition, 2-substituted enynes yield the corresponding cyclobutanes in poor yields only.Finally, 1-substituted vinylacetylenes (besides the substituents mentioned above the influence of ethynyl, chloro, and methoxy groups has been investigated) do not provide photodimers; they are cis-trans-isomerized instead.The mechanism of the photoaddition is discussed.