Alkynes In Presence Research Articles

The Peculiar Kinetics of the Reaction between Acetylene and the Cyclopentadienyl Radical

The cyclopentadienyl radical (cC(5)H(5)) is a fascinating molecule characterized by several peculiar properties, such as its high internal symmetry and resonance enhanced stability. This makes cC(5)H(5) one of the most abundant radicals present in high temperature gaseous environments, such as flames. Therefore it is generally considered an interesting candidate as the starting point of reaction pathways leading to the formation of polycyclic aromatic hydrocarbons (PAH) and soot in combustion processes. However, known reaction pathways are not able to explain some recent experimental findings concerning the rapid conversion of cC(5)H(5) into C(7)H(7) and C(9)H(8) in the presence of acetylene. In this work, we used ab initio calculations and quantum Rice-Ramsperger-Kassel (QRRK) theory to investigate the cC(5)H(5) + C(2)H(2) reaction kinetics. We found that cC(5)H(5) can add acetylene to form, through a fast and not previously known reaction, the heptatrienyl radical (cC(7)H(7)), which, in many ways, can be considered the superior homologue of cC(5)H(5). The calculated reaction kinetic constant is (2.2 x 10(11))exp(-6440/T(K)) cm(3) mol(-1) s(-1) and is in good agreement with experimental data, while that of the inverse process is (4.2 x 10(16))T(-1) exp(-30 850/T(K)) s(-1). In a successive reaction, cC(7)H(7) can add a second acetylene molecule to form indene, cC(9)H(8), and H. The forward and backward kinetic constants are (6.6 x 10(11))exp(-10 080/T(K)) and (4.2 x 10(14))exp(-27 300/T(K)) cm(3) mol(-1) s(-1), respectively. These two successive reactions, leading from a single C5 cycle to a bicyclic C5-C6 species, represent a new PAH growth mechanism, characterized by a C5-C7 ring enlargement reaction.

Irradiation of Imine-Group VI Carbene Complexes in the Presence of Alkynes. 2. Control of Product Distribution

The photoreactivity of iminecarbene complexes in the presence of alkynes has been explored. Up to four different reaction paths are available depending on the alkyne and carbene complex substituents, although in each case only one type of product is isolated. 2H-Pyrrole derivatives are formed mainly from aryl alkynes. When alkyl alkynes are used, the method affords a new type of aza-dendralene product in good yields. Isoquinoline derivatives can also be formed in a two-step one-pot photochemical process when the appropriate substituents are present. Finally, indene derivatives are also available through a benzannulation reaction. To explore the underlying mechanism, we carried out computations using DFT methods. Experimental and theoretical results compare well, which allows control over the reaction and product distribution.

Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N2O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha−1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). 15N-N2O emissions from 14NH415NO3 replicates were attributed to denitrification and 15N-N2O from 15NH415NO3 minus that from 14NH415NO3 replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of 15N-NO3−. All of the N2O emitted at 70% WFPS (31.6 mg N2O-N m−2 over 24 days; 1.12 μg N2O-N g dry soil−1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N2O, accounting for 81% of 15N-N2O emitted at 60% WFPS, and 7.9 μg 15N-N2O m−2 (0.28 ng 15N-N2O g dry soil−1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of 15N-N2O from this treatment. Denitrification was the predominant N2O-producing process at 20% WFPS (2.6 μg 15N-N2O m−2 over 7 days; 0.09 ng 15N-N2O g dry soil−1; 85% of 15N-N2O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N2O emissions from different processes and to show that several processes may contribute to N2O emission from agricultural soils depending on soil WFPS.

Effects of transient anaerobic conditions in the presence of acetylene on subsequent aerobic respiration and N 2O emission by soil aggregates

Our objective was to assess the effect of anaerobic conditioning in the presence of acetylene on subsequent aerobic respiration and N 2O emission at the scale of soil aggregates. Nitrous oxide production was measured in intact soil aggregates Δ (compacted aggregates without visible porosity) and Γ (aggregates with visible porosity) incubated under oxic conditions, with or without anaerobic conditioning for 6 d. N 2O emissions were much higher in aggregates that had been submitted to anaerobic conditioning than in aggregates that did not experience this conditioning, although very little NO 3 − remained in soil after the anaerobic period. 15N isotope tracing technique was used to check whether N 2O came from nitrification or denitrification. The results showed that denitrification was the major process responsible for N 2O emissions. The aerobic CO 2 production rate was also measured in intact soil aggregates. It was greater in aggregates submitted to anaerobic conditioning than in those that were not, suggesting that the anaerobic conditioning lead to an accumulation of small compounds including fatty acids that are readily available for microbial decomposition in aerobic conditions. This process increases the aerobic CO 2 production and favours the N 2O emissions through denitrification.

Polymerization of alkynes on nickelocene based catalysts: considerations on polymerization mechanism

Abstract Novel nickelocene based catalysts were used for polymerization of 2-butyne, 1-phenyl-1-propyne, bis(trimethylsilyl)acetylene and trimethylsilylacetylene. The catalyst prepared in the reaction of nickelocene with organolithium compounds in the presence of alkyne was previously isolated and fully characterized. It enabled us to explain the mechanism of the polymerization. Polymerization of alkynes proceeds according to coordination-insertion mechanism. Coordinated acetylene molecule inserts into Ni C bond. An active catalytic species of polymerization appeared to be {CpNiR} stabilized by alkyne molecule. Cyclization is catalyzed by {CpNiH} species.

One‐Pot Synthesis of Monosubstituted Aryl(hetaryl)acetylenes by Direct Introduction of the CCH Residue into Arenes and Hetarenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nickelocene catalysts for polymerization of alkynes: mechanistic aspects

AbstractNovel nickelocene‐based catalysts were used for polymerization of diphenylacetylene and phenylacetylene. The catalyst obtained in the reaction of nickelocene with organolithium compounds in the presence of alkyne was previously isolated and fully characterized. This enabled us to explain the mechanism of the polymerization. Polymerization of alkynes proceeds according to a coordination–insertion mechanism. Coordinated acetylene molecule inserts into an NiC bond. An active catalytic species of polymerization appeared to be {CpNiR} stabilized by alkyne molecule. Cyclization is catalyzed by {CpNiH} species. The mechanism of polymerization and cyclization reaction was determined based on the composition of the reaction products formed, gel‐permeation chromatography and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry measurements. Copyright © 2004 John Wiley & Sons, Ltd.

Metathesis polymerization of norbornene and norbornadiene in the presence of tert-butylacetylene initiated by tungsten(II) and molybdenum(II) complexes

Metathesis polymerization of norbornene and norbornadiene in the presence of tert-butylacetylene initiated by seven-coordinate bimetallic tungsten–tin and molybdenum–tin compounds of the [MCl(SnCl 3)(CO) 3(NCMe) 2] (M = Mo, W) type was investigated. The presence of alkyne in the reaction mixture leads to an increase in cycloolefin conversion and to the formation of blocky polymers with high molecular weights and better solubility in CHCl 3 than polynorbornene and polynorbornadiene homopolymers. The structure of polymers was determined by 1H and 13C{ 1H} NMR spectroscopy. In addition to polymers small amounts of cooligomers and cyclotrimers were also detected in the above reactions. The reactivity of tungsten(II) and molybdenum(II) complexes towards tert-butylacetylene is higher than towards olefins. Monitoring of reactions of [WCl(SnCl 3)(CO) 3(NCEt) 2] with norbornadiene and tert-butylacetylene by 1H NMR spectroscopy made it possible to observe the coordination of alkyne to the metal atom in the step preceding polymerization and copolymerization. In those reactions the alkyne plays the role of an activator.

Reaction of [Pt 3(μ-CO) 3(P tBu 3) 3] with activated olefins and alkynes: the X-ray-structure of [Pt(CO)(P tBu 3)(CF 3C [tbnd]CCF 3)

The reactions of the triangulo-cluster [Pt 3(μ-CO) 3(P t Bu 3) 3] with activated olefins and alkynes have been examined under various conditions. At low temperature, cluster fragmentation occurs yielding the Pt(0) complexes [Pt(CO)(P t Bu 3)(olefin)] (olefin = maleic anhydride and maleimide), while di( tert-butyl)acetylenedicarboxilate reacts quantitatively giving the dinuclear Pt(0) complex [Pt 2(CO) 2(P t Bu 3) 2(μ–η 2:η 2- t BuO 2CC CCO 2 t Bu)]. At higher temperature and in the presence of alkyne in large excess, the latter dimer converts quantitatively to the monomers [Pt(CO)(P t Bu 3)(alkyne)] (alkyne = CF 3C CCF 3 and t BuO 2CC CCO 2 t Bu). The stereochemistry of these complexes has been established by NMR and IR measurements. The structure of [Pt(CO)(P t Bu 3)(CF 3CCCF 3)] was confirmed by X-ray diffraction analysis.

One-pot synthesis of monosubstituted aryl(hetaryl)acetylenes by direct introduction of the CCH residue into arenes and hetarenes

A convenient one-pot synthesis of aryl(hetaryl)acetylenes by cross-coupling of aryl(hetaryl)iodides with acetylene in presence of PdCl 2(Ph 3) 2, CuI and K 2CO 3 in DMF has been developed.

Influence of maize mucilage on the diversity and activity of the denitrifying community.

In order to understand the effect of the maize rhizosphere on denitrification, the diversity and the activity of the denitrifying community were studied in soil amended with maize mucilage. Diversity of the denitrifying community was investigated by polymerase chain reaction (PCR) amplification of total community DNA extracted from soils using gene fragments, encoding the nitrate reductase (narG) and the nitrous oxide reductase (nosZ), as molecular markers. To assess the underlying diversity, PCR products were cloned and 10 gene libraries were obtained for each targeted gene. Libraries containing 738 and 713 narG and nosZ clones, respectively, were screened by restriction fragment analysis, and grouped based on their RFLP (restriction fragment length polymorphism) patterns. In all, 117 and 171 different clone families have been identified for narG and nosZ and representatives of RFLP families containing at least two clones were sequenced. Rarefaction curves of both genes did not reach a clear saturation, indicating that analysis of an increasing number of clones would have revealed further diversity. Recovered NarG sequences were related to NarG from Actinomycetales and from Proteobacteria but most of them are not related to NarG from known bacteria. In contrast, most of the NosZ sequences were related to NosZ from alpha, beta, and gammaProteobacteria. Denitrifying activity was monitored by incubating the control and amended soils anaerobically in presence of acetylene. The N2O production rates revealed denitrifying activity to be greater in amended soil than in control soil. Altogether, our results revealed that mucilage addition to the soil results in a strong impact on the activity of the denitrifying community and minor changes on its diversity.

Synthesis and reactions of silicon containing cyclic α-amino acid derivatives

A simple synthesis of a new amino acid derivative 5,6-bis(trimethylsilyl)indanylglycine via cobalt mediated [2 + 2 + 2] cycloaddition strategy is described. Co-trimerization of diyne building block containing amino acid moiety with bis(trimethylsilyl)acetylene in presence of CpCo(CO) 2 catalyst afforded the silylated indane-based α-amino acid (AAA) derivative. Electrophilic aromatic substitution reaction, ipso to the trimethylsilyl group gave highly functionalised indane-based AAA derivatives.

Irradiation of imine-group VI carbene complexes in the presence of alkynes: a theoretical and experimental study.

The photoreaction between imine-substituted Fischer carbene complexes and alkynes is studied at both experimental and theoretical levels. 2H-Pyrrole derivatives are easily obtained as main products in moderate to good yields, with complete control of the regiochemistry. High-level theoretical calculations are carried out in order to explore and fully understand the reaction pathway. On the basis of the theoretical results, a mechanism that accounts for the experimental findings is proposed.

Modeling the chemistry of plasma polymerization using mass spectrometry.

The goal of the project is a solvent free painting shop. The environmental technologies laboratory is developing processes of plasma etching and polymerization. Polymerized thin films are first-order corrosion protection and primer for painting. Using pure acetylene we get very nice thin films which were not bonded very well. By using air as bulk gas it is possible to polymerize, in an acetylene plasma, well bonded thin films which are stable first-order corrosion protections and good primers. UV/Vis spectroscopy shows nitrogen oxide radicals in the emission spectra of pure nitrogen and air. But nitrogen oxide is fully suppressed in the presence of acetylene. IR spectroscopy shows only C=O, CH(2) and CH(3) groups but no nitrogen species. With the aid of UV/Vis spectra and the chemistry of ozone formation it is possible to define reactive traps and steps, molecule depletion and processes of proton scavenging and proton loss. Using a numerical model it is possible to evaluate these processes and to calculate theoretical mass spectra. Adjustment of theoretical mass spectra to real measurements leads to specific channels of polymerization which are driven by radicals especially the acetyl radical. The estimated theoretical mass spectra show the specific channels of these chemical processes. It is possible to quantify these channels. This quantification represents the mass flow through this chemical system. With respect to these chemical processes it is possible to have an idea of pollutant production processes.

Imidazolium and Imidazolinium Salts as Carbene Precursors or Solvent for Ruthenium-Catalysed Diene and Enyne Metathesis

This review reports the utilisation of catalytic systems prepared in situ from a source of ruthenium, a precursor of 1,3-diaminocarbene, and a base to perform enyne ring-closing metathesis. Silylated enynes and allyl propargyl ethers have been rearranged into cyclic derivatives bearing a conjugated diene structure in the presence of [(p-cymene)RuCl2]2, an imidazolium or imidazolinium chloride, and cesium carbonate. From 1,6-dienes, the dichotomous behaviour of the catalytic system generated from imidazolinium salts makes possible the preparation of cycloisomerisation products, or metathesis compounds when the reaction is carried out in the presence of acetylene. Finally, the possibility of performing ring-closing metathesis in imidazolium salts as an ionic solvent with an ionic catalyst of the type [(p-cymene)RuCl(PCy3)CCCPh2][X] is demonstrated.

Acetylide-bridged organometallic oligomers via the photochemical metathesis of methyl-iron(II) complexes.

The acetylido methyl iron(II) complexes, cis/trans-[Fe(dmpe)(2)(C[triple bond]CR)(CH(3))] (1) and trans-[Fe(depe)(2)(C[triple bond]CR)(CH(3))] (2) (dmpe = 1,2-dimethylphoshinoethane; depe = 1,2-diethylphosphinoethane), were synthesized by transmetalation from the corresponding alkyl halide complexes. Acetylido methyl iron(II) complexes were also formed by transmetalation from the chloride complexes, trans-[Fe(dmpe)(2)(C[triple bond]CR)(Cl)] or trans-[Fe(depe)(2)(C[triple bond]CR)(Cl)]. The structure of trans-[Fe(dmpe)(2)(C[triple bond]CC(6)H(5))(CH(3))] (1a) was determined by single-crystal X-ray diffraction. The methyl acetylido iron complexes, [Fe(dmpe)(2)(C[triple bond]CR)(CH(3))] (1), are thermally stable in the presence of acetylenes; however, under UV irradiation, methane is lost with the formation of a metal bisacetylide. Photochemical metathesis of cis- or trans-[Fe(dmpe)(2)(CH(3))(C[triple bond]CR)] (R = C(6)H(5) (1a), 4-C(6)H(4)OCH(3) (1b)) with terminal acetylenes was used to selectively synthesize unsymmetrically substituted iron(II) bisacetylide complexes of the type trans-[Fe(dmpe)(2)(C[triple bond]CR)(C[triple bond]CR')] [R = Ph, R' = Ph (6a), 4-CH(3)OC(6)H(4) (6b), (t)()Bu (6c), Si(CH(3))(3) (6d), (CH(2))(4)C[triple bond]CH (6e); R = 4-CH(3)OC(6)H(4), R' = 4-CH(3)OC(6)H(4), (6g), (t)()Bu (6h), (CH(2))(4)C[triple bond]CH (6i), adamantyl (6j)]. The structure of the unsymmetrical iron(II) bisacetylide complex trans-[Fe(dmpe)(2)(C[triple bond]CC(6)H(5))(C[triple bond]CC(6)H(4)OCH(3))] (6b) was determined by single-crystal X-ray diffraction. The photochemical metathesis of the bis-acetylene, 1,7-octadiyne, with trans-[Fe(dmpe)(2)(CH(3))(C[triple bond]CPh)] (1a), was utilized to synthesize the bridged binuclear species trans,trans-[(C(6)H(5)C[triple bond]C)Fe(dmpe)(2)(mu-C[triple bond]C(CH(2))(4)C[triple bond]C)Fe(dmpe)(2)(C[triple bond]CC(6)H(5))] (11). The trinuclear species trans,trans,trans-[(C(6)H(5)C[triple bond]C)Fe(dmpe)(2)(mu-C[triple bond]C(CH(2))(4)C[triple bond]C)Fe(dmpe)(2)(mu-C[triple bond]C(CH(2))(4)C[triple bond]C)Fe(dmpe)(2)(C[triple bond]CC(6)H(5))] (12) was synthesized by the photochemical reaction of Fe(dmpe)(2)(C[triple bond]CPh)(C[triple bond]C(CH(2))(4)C[triple bond]CH) (6e) with Fe(dmpe)(2)(CH(3))(2). Extended irradiation of the bisacetylide complexes with phenylacetylene resulted in insertion of the terminal alkyne into one of the metal acetylide bonds to give acetylide butenyne complexes. The structure of the acetylide butenyne complex, trans-[Fe(dmpe)(2)(C[triple bond]CC(6)H(4)OCH(3))(eta(1)-C(C(6)H(5))=CH(C[triple bond]CC(6)H(4)OCH(3)))] (9a) was determined by single-crystal X-ray diffraction.

Denitrification activity in sediment surrounding polychaete (Ceratonereis aequisetis) burrows

The effect of burrow-dwelling fauna on sediment denitrification within the Swan River Estuary, Western Australia, was assessed by determining the spatial profile of potential denitrification activity surrounding individual burrows of a polychaete. This activity was described for Ceratonereis aequisetis and compared with uninhabited sediment. Potential porewater denitrification activity was measured as N’2O production in the presence of acetylene (which blocks N2O reduction and NH4+ oxidation) and supplementary NO3-(provided as a substrate for denitrification). Snap-freezing of sediment cores in liquid nitrogen allowed easy sectioning in both the vertical (perpendicular depth from surface sediment) and radial (depth from burrow wall) planes. Overall, potential denitrification activity was significantly greater in inhabited sediment than in uninhabited sediment, although uninhabited sediment had higher surficial (0–10 mm) potential denitrification activity. Potential denitrification activity was also greater closer to the burrow wall (0–9 mm) rather than further into the sediment (9–13 mm). Greater sampling resolution would be required to determine whether a thin oxygenated surface layer (of either the vertical or radial plane) exists in which denitrification is inhibited. Although this study accurately demonstrates the spatial effect of C. aequisetis on sediment potential denitrification, the reported denitrification intensity may not reflect the rate in situ.

Radiation-induced crosslinking of acetylene-impregnated polyesters

Enhanced crosslinking and reduction in chain scission are found in the amorphous regions of polycrystalline polyesters, when they are irradiated in the presence of acetylene. Increasing the chain length of the alkyl moiety is also found to decrease the extent of damaging main chain scission.

Ruthenium Catalyst Dichotomy: Selective Catalytic Diene Cycloisomerization or Metathesis

Ruthenium based catalysts are versatile promoters of a large variety of reactions. A catalytic system active in metathesis has been generated in situ from [RuCl2(p-cymene)]2, 1,3-bismesitylimidazolinium chloride, as precursor of a bulky carbene ligand, and cesium carbonate. We report that this three component catalytic system exhibits dichotomous reactivity for the transformation of dienes, providing an active catalytic system for the cycloisomerization of dienes to methylidene five-membered cyclic molecules, whereas, in the presence of acetylene, a metathesis catalyst is generated that transforms the same dienes into cyclic olefins with loss of ethylene.

Formation of silazirconacyclopentenes from zirconium-silene complex and alkynes and their reactivities.

During the course of our study on the formation of a complex having a zirconium-silicon bond, we found that zirconium-silene complex 7 was formed from Cp2ZrCl2 and Me2PhSiLi. In the presence of alkyne, diarylalkyne reacted with silene coordinated with zirconium to give silazirconacyclopentene 8. On the other hand, dialkylalkyne inserted into a zirconium-silene complex gave silazirconacyclopentene 9. Hydrolysis of 8 or 9 afforded vinylsilane 13 or allylsilane 16. Transmetalation of zirconacycle 8 into copper in the presence of allyl halide gave a bis-allylated compound in high yield, indicating that alkylation occurred on the alkyne carbon and the methyl group of silicon. From bis-allylated compounds, eight-membered ring compounds having silicon were obtained in high yield using olefin metathesis.