Ethyne dimerization

The pathway 5.72 is the simplest in which a reversible step precedes a competing irreversible one. Such behavior is quite common, especially in catalysis. A non-constant reaction order between one and two with respect to the reactant for which the steps compete is a typical symptom, even though the actual pathway usually is more complex. Two examples—ethyne dimerization and aldol condensation—will be examined later (see Examples 6.4 and 8.2 in Sections 6.4.3 and 8.2.1, respectively). 

Example 6.4. By-product formation in ethyne dimerization. Dimerization of ethyne (acetylene) to vinyl ethyne (vinyl acetylene)... 

Practical examples include nitration of aromatics, olefin hydroformylation with cobalt hydrocarbonyls and phosphine-substituted hydrocarbonyls as catalysts, and ethyne dimerization. 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

Dinitrogen tetroxide reacts with phenylethynyltrimethyltin to give benzoyl cyanide, probably by rearrangement of (dimeric) phenyl(nitroso)ethyne (Equation (84)).245... 

Considering the success of the condensation route to carboranes in the hydride bath (vide supra), other alkynylboranes than diethyl(propyn-l-yl)borane might be equally suitable. By heating a mixture of bis(diethylboryl) ethyne (65) and excess of (Et2BH)2 ( hydride bath ) at 110-120 °C, l,2,3,4-tetraethyl-5,6,7,8-tetracarba-mdo-octaborane(8) 64c was obtained by distillation in ca. 20% yield as a colorless liquid, stable to air and H2O (Scheme 3.2-35) [87]. Possible intermediates in this reaction can be proposed as 67 and 68, where 67 results from double hydroboration of bis(diethylboryl)ethyne (65), which dimerizes to 68 and finally yields the carbo-rane 64c by elimination of Et3B. 

Exercise 11-19 A serious contaminant in butenyne made by dimerization of ethyne with cuprous ion is 1,5-hexadien-3-yne. Show how this substance can be formed. 

The substituted bis-Cp alkynyl-alkenyl (C5Me4R1)2Ti(G=CR2)(GH=GHR2) (Scheme 502) have been synthesized by reaction of the bis(trimethylsilyl)ethyne complex (G5Me4R1)2Ti(Me3SiC2SiMe3) with 1-alkynes the crystal structures have been determined by X-ray diffraction. Photolysis of these complexes involves dimerization processes with coupling of the two cr-ligands to give complexes with 1,4-disubstituted but-l-en-3-ynes.1287... 

Isothiazoles are reported to yield the lactam (101) on reaction with diphenyl ketene <85BSB149>. Ethyne dicarboxylate esters add to 2,1-benzisothiazole to generate quinoline esters (102), and benzyne reacts to yield aeridine <83H(20)489, 88JCS(Pl)2l4l>. 2-Methyl-2,l-benzisothiazolin-3-thione with acetylene esters forms 2-iminobenzoquinone methides (103), which dimerize to give diazocines (104) or further react with the ethyne ester to yield diadducts (105). The initial adduct formed with phenylethyne and 2-methyl-2,l-benzisothiazolin-3-thione rearranges to produce l,2-dithiole[3,26]... 

Moreover, it has been shown (Imamura et al. 1995a) that such rare-earth imide or imide-like species exhibit oligomerization activity of alkynes. Selective cyclic dimerization and trimerization of propyne and ethyne to cyclohexadiene and benzene occur at 453 K during the oligomerization, respectively, in which the active catalysts are characterized as rare-earth imides induced by the thermal treatment of R/C. 

The important observation was made that the dimer (e.g. (218) from 2//-phosphole (217)), whose structure was proved by x-ray analysis, thermally rearranged to the exo isomer (219), implying reversibility in the dimerization process <82CCI272,83JA687I). It was found possible to conduct the retro-dimerization in refluxing toluene, and when dienes or ethynes were present, the 2 f-phosphole could be trapped (Scheme 51). Adducts of alkenes with 2//-phospholes showed similar reversibility <89J(X 4754>. 

Low-temperature trapping experiments of the photolysis products of disilacycloheptene (75), which was prepared by insertion of ethyne into the Si—Si bond of l,l,2,2-tetramethyl-l,2-disilacyclopentane <75JA931>, provided evidence for the trans isomer of (75) <90JA6601>. GC-MS analyses indicated that, as the decay of trans-(75) progressed, a mixture of six dimers of mjz 368 was formed. Only one of the dimeric products was successfully isolated in pure form by preparative GC this dimer was assigned the unsymmetrical c/s,fra 5-fused cyclobutene structure (76) on the basis of NMR data. Trapping experiments of metastable trans-(75) by Diels-Alder cycloadditions with cyclopentadiene and 9,10-dihydro-l l,12-dimethylene-9,10-ethanoanthracene afforded (77) and (78), respectively. The crystal and molecular structures of (78) were determined. 

ICI catalyst showed that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200 Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2- When the pressure of ethyne was varied in the presence of excess ethene, its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation feU to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski and colleagues led " to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (E ) that may play a role on certain supports. Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>C=CH2),... 

Low-energy electron bombardment of monolayers of ethylene on a silver(lll) surface brings about C—H bond fission. The H is absorbed on the surface and the vinyl radical dimerizes to yield buta-1,3-diene. Higher doses of electrons lead to the formation of ethyne Olefins can undergo catalytic oxidation when they are irradiated in the presence of silver catalysts either as silver powder or supported silver metals on anatase titania, sihca and porous glass. Irradiation under these conditions increases the reaction rate. When the surface is coated with ethene and deuterium, partly deuteriated ethene is formed on irradiation, presumably as a result of a vinyl radical reacting with deuterium. UV... 

Copper(l) chloride is essentially covalent and its structure is similar to that of diamond l.e. each copper atom is surrounded tetrahedrally by four chlorine atoms and vice versa. In the vapour phase, dimeric and trimeric species are present Copper(l) chloride is used in conjunction with ammonium chloride as a catalyst in the dimerization of ethyne to but-1 -ene-3-yne (vinyl acetylene), which is used in the production of synthetic rubber. In the laboratory a nuxture of cop-per(I) chloride and hydrochloric acid is used for converting benzene diazonlum chloride to chlorobenzene - the Sandmeyer reactioit... 

The iridium dimer tIr2l2(CO)(jt-CO)(dppm)2l reacted with ethyne to afford363 [Ir2l2(CO)( i-HCX HKdppm)2] in which the alkyne was parallel to the Ir-Ir vector. The compound underwent a transformation in solution at ambient temperature to afford the vinylidene compound [Ii2l2(00)(fi-CXIH2)(dppm)2]. A mechanism for the reaction was proposed. 

Alkyne molecules are often linked together in their reactions with transition metal compounds. An early example of this, discovered by Niewland in 1931, was the linear dimerization of ethyne to but-l-en-3-yne, catalysed by copper(I) chloride. The product was for many years an intermediate in the manufacture of chloroprene for synthetic rubber. 

Finally,but-3-en-l-yne (vinylacetylene, HC C-CH=CH2) can be prepared (Table 6.8, example 5) by the catalyzed dimerization of ethyne (acetylene, HC CH) itself. Interestingly, this is only one of the possiblities of reaction of this alkyne (and substituted alkynes) with itself (or themselves). For example, as shown in Scheme 6.75, treatment of a disubstituted alkyne, such as 2-butyne (dimethylacetylene, CHsC CCHs) with zirconcene hydridochloride ([Cp]2ZrHCl, where Cp = cyclopen-tadienyl [see. Table 6.6]) apparently causes the formation of a vinylic zirconium intermediate. When the intermediate is treated with methyllithium (CHsLi) and then a second alkyne (or a second equivalent of the original) added, and this is followed by aqueous acid, a conjugated diene results. 

The self-dimerization of ethyne (acetylene, HOCH) would give rise to the 4n antiaromatic compound cyclobutadiene and, although the metal-catalyzed trimer-ization and tetramerization (vide supra) occur, the reaction (Equation 6.72) is not observed. Interestingly, cyclobutadiene and substituted cyclobutadienes have been prepared by several more circuitous routes and one of the ways of producing cyclobutadiene (albeit temporarily) is shown in Scheme 6.77. 

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