Dimerization Of Alkynes Research Articles

Terminal‐Alkyne‐Induced Decomposition of a Phosphine‐Free Ruthenium Alkylidene Catalyst

AbstractThe transformations of the third‐generation Grubbs catalyst [RuCl2(=CHPh)(3‐Br‐py)2(H2IMes)] (py=pyridine, H2IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolidin‐2‐ylidene) in the presence of a terminal alkyne (phenylacetylene) were studied theoretically. The ruthenium η3‐vinylcarbene complex is the most probable product of the reaction between the 14‐electron ruthenium alkylidene species and phenylacetylene. Its subsequent transformation into the ruthenium vinylidene complex and stilbene, both being experimentally observed earlier, is possible. The former is believed to be the active species catalyzing the dimerization of terminal alkynes. The calculated pathway also predicts the formation of [RuCl2(H2IMes)] species, which might initiate the cyclotrimerization of terminal alkynes or transform into the experimentally detected [RuCl2(3‐Br‐py)3(H2IMes)] complex. An alternative mechanism of the Grubbs catalyst decomposition through an intermediate Ru species having alkylidene and vinylidene ligands in the coordination sphere of ruthenium is also proposed.

Continuous-flow oxidative homocouplings without auxiliary substances: Exploiting a solid base catalyst

The catalytic oxidative dimerization of aromatic amines and acetylenes is of outstanding synthetic importance among homocoupling reactions. Both transformations necessitate the use of extraneous bases and ligands, which contains significant disadvantages as concerns environmental impacts and process costs. We exploited the inherent basic character of a copper-containing layered double hydroxide to facilitate the catalytic homocouplings of alkynes and aniline derivatives without the need for any auxiliary substances. The reactions were studied in a continuous-flow system to achieve extended parameter spaces for chemical intensification, and also to avoid undesired reaction pathways by means of strategic control over the residence time. Valuable 1,4-disubstituted 1,3-diynes and diversely substituted aromatic azo compounds were achieved chemoselectively in excellent yields and in short process times even on preparative scales.

How different is the borazine–acetylene dimer from the benzene–acetylene dimer? A matrix isolation infrared and ab initio quantum chemical study

ABSTRACTThe 1:1 dimer of borazine–acetylene has been studied for the first time, both experimentally and computationally. The borazine–acetylene dimer was trapped in Ar and N2 matrices, and studied using infrared spectroscopy. Our experiments clearly revealed two isomers of the borazine–acetylene complex, one in which the N–H of borazine interacted with the carbon of acetylene, and another in which the C–H of acetylene formed a hydrogen bond with a nitrogen atom of borazine. The formation of both isomers in the matrix was evidenced by shifts in the vibrational frequencies of the appropriate modes. Reassuringly, the experimental observations were corroborated by our computations using the second-order Møller–Plesset perturbation theoretic method and coupled-cluster singles, doubles and perturbative triples method in conjunction with different Dunning basis sets, which indicated both these isomers to be stable minima, with the N–H⋅⋅⋅C complex being the global minimum. Atoms-in-molecules and energy decomposition analysis were also carried out for the different isomers of the dimer. These studies reveal that replacing the three C–C linkages in benzene with three B–N linkages in borazine modifies the interaction in the dimer sufficiently, to result in a different potential energy landscape for the borazine–acetylene system when compared with the benzene–acetylene system.

A copper (I or II)/diethylphosphite catalytic system for base-free additive dimerization of alkynes

Copper (I) or copper (II) salts and oxides promote regioselective head-to-head additive dimerization of aromatic and aliphatic terminal alkynes in the presence a catalytic amount of diethylphosphite. The reaction proceeds under ambient conditions without any added base with the formation of 1,4-disubstituted 1,3-enynes with the E isomer as major product in good to excellent yields. A plausible mechanism for additive dimerization of terminal alkynes is proposed.

Novel pyrimidine-2,4-dione–1,2,3-triazole and furo[2,3-d]pyrimidine-2-one–1,2,3-triazole hybrids as potential anti-cancer agents: Synthesis, computational and X-ray analysis and biological evaluation

Regioselective 1,4-disubstituted 1,2,3-triazole tethered pyrimidine-2,4-dione derivatives (5–23) were successfully prepared by the copper(I)-catalyzed click chemistry. While known palladium/copper-cocatalyzed method based on Sonogashira cross-coupling followed by the intramolecular 5-endo-dig ring closure generated novel 6-alkylfuro[2,3-d]pyrimidine-2-one–1,2,3-triazole hybrids (24b–37b), a small library of their 5-alkylethynyl analogs (24a–37a) was synthesized and described for the first time by tandem terminal alkyne dimerization and subsequent 5-endo-trig cyclization, which was additionally corroborated with computational and X-ray crystal structure analyses. The nature of substituents on alkynes and thereof homocoupled 1,3-diynes predominantly influenced the ratio of the formed products in both pathways. In vitro antiproliferative activity of prepared compounds evaluated on five human cancer cell lines revealed that N,N-1,3-bis-(1,2,3-triazole)-5-bromouracil (5–7) and 5,6-disubstituted furo[2,3-d]pyrimidine-2-one-1,2,3-triazole 34a hybrids exhibited the most pronounced cytostatic acitivities against hepatocellular carcinoma (HepG2) and cervical carcinoma (HeLa) cells with higher potencies than the reference drug 5-fluorouracil. Cytostatic effect of pyrimidine-2,4-dione-1,2,3-triazole hybrid 7 in HepG2 cells could be attributed to the Wee-1 kinase inhibition and abolishment of sphingolipid signaling mediated by acid ceramidase and sphingosine kinase 1. Importantly, this compound proved to be a non-mitochondrial toxicant, which makes it a promising candidate for further lead optimization and development of a new and more efficient agent for the treatment of hepatocellular carcinoma.

Evaluation of the Reactivity of Metallocatalytic Cavities in the Dimerization of Terminal Alkynes

The effect of a metallocatalytic cavity flanked by aromatic rings on the catalytic dimerization of terminal alkynes was explored through a comparison with model catalysts that weakened the cavity. The diquinoxaline‐spanned resorcin[4]arene provided a definite compartment, in which the two Au centers enticed two alkynes to undergo the coupling reaction. We synthesized two kinds of model compounds in which one lacked two quinoxaline walls and the other had two pyrazine walls and found that these catalysts exhibited much lower reactivity. The two quinoxaline moieties proved to be quintessential for the catalytic event.

ChemInform Abstract: Z‐Selective (Cross‐)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex.

AbstractThe homocoupling of terminal alkynes and a cross‐dimerization of aryl alkynes with ethynylsilane affords the corresponding enynes stereoselectively.

Effect of Cu+/Cu2+ Ratio on the Catalytic Behavior of Anhydrous Nieuwland Catalyst during Dimerization of Acetylene

The relative contents of Cu+ and Cu2+ in solutions of anhydrous Nieuwland catalyst and CuCl2-added anhydrous Nieuwland catalyst were detected by X-ray photoelectron spectroscopy (XPS). Results indicated that Cu+ could be transformed into Cu2+ during the process of acetylene dimerization into monovinylacetylene, leading to deactivation of the catalyst. Moreover, the addition of Cu2+ inhibited the oxidation of Cu+ into Cu2+ and enhanced the performance and lifetime of the catalyst. The optimum molar ratio of Cu+ to Cu2+ was approximately 2:1; at this ratio, up to 40% acetylene conversion may be achieved.

An Introverted Bis‐Au Cavitand and Its Catalytic Dimerization of Terminal Alkynes

AbstractA preparative synthesis of an inwardly oriented phosphoramidite‐Au dinuclear resorcinarene cavitand complex is described, including a description of potent catalytic abilities. The cavitand structure was determined by crystallographic analysis, which revealed that the phosphoramidite P–N bonds point outside placing the two Au atoms inside. We explored the catalytic proclivity of the cavitand and found that it effeciently catalyzes selective and direct dimerization of terminal alkynes to afford conjugated enynes. Mixed dimerizations give rise to chemoselective products, and macrocyclization by intramolecular dimerization are both trademark capabilities of the method.

Z‐Selective (Cross‐)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex

AbstractEfficient iron‐catalyzed homocoupling of terminal alkynes and cross‐dimerization of aryl acetylenes with trimethylsilylacetylene is reported. The complex [Fe(H)(BH4)(iPr‐PNP)] (1) catalyzed the (cross‐)dimerization of alkynes at room temperature, with no need for a base or other additives, to give the corresponding dimerized products with Z selectivity in excellent yields (79–99 %).

Z-Selective (Cross-)Dimerization of Terminal Alkynes Catalyzed by an Iron Complex.

Efficient iron-catalyzed homocoupling of terminal alkynes and cross-dimerization of aryl acetylenes with trimethylsilylacetylene is reported. The complex [Fe(H)(BH4 )(iPr-PNP)] (1) catalyzed the (cross-)dimerization of alkynes at room temperature, with no need for a base or other additives, to give the corresponding dimerized products with Z selectivity in excellent yields (79-99 %).

The Choice Is Yours: Using Liquid-Assisted Grinding To Choose between Products in the Palladium-Catalyzed Dimerization of Terminal Alkynes

Herein, we report on the dimerization of terminal alkynes using various palladium catalysts under solvent-free mechanochemical conditions. When tetrakis(triphenylphosphine)palladium(0) was employed as the catalyst, we observed the 1,3-butadiyne as the major product. However, when we employed bis(triphenylphosphine)palladium(II) dichloride as the catalyst, we observed the trans-enyne as the major product. When we used a polymer-supported bis(triphenylphosphine)palladium(II) dichloride catalyst under liquid-assisted grinding conditions, we discovered the ability to tune the catalyst to generate either the diyne or trans-enyne as the major product, depending on the grinding medium.

Synthesis of [3]Rotaxanes by the Combination of Copper-Mediated Coupling Reaction and Metal-Template Approach.

[3]Rotaxanes with two axle components and one ring component were synthesized by the combination of a coupling reaction using a transition-metal catalyst and a metal-template approach. Thus, [2]rotaxanes were prepared by the oxidative dimerization of alkyne promoted by macrocyclic phenanthroline-CuI complexes. The [2]rotaxane was reacted with a Cu(I) salt and an acyclic ligand to generate a tetrahedral Cu(I) complex. Metal-free [3]rotaxane was isolated by the end-capping reaction of the acyclic ligand, followed by the removal of Cu(I) ion. The stability of the tetrahedral Cu(I) complexes depended on the size of both the ring component and the acyclic ligand, which was correlated with the yield of the corresponding [3]rotaxane.

Model studies on the dimerization of 1,3-diacetylenes.

By means of high-level quantum chemical calculations (B2PLYPD and CCSD(T)), the dimerization of 1,3-diacetylenes was studied and compared to the dimerization of acetylene. We found that substituted 1,3-diacetylenes are more reactive than the corresponding substituted acetylenes having an isolated triple bond. The most reactive centers for a dimerization are always the terminal carbon atoms. The introduction of a test reaction allows the calculation of the relative reactivity of individual carbon centers in phenylacetylene, phenylbutadiyne, and phenylhexatriyne. A comparison shows that the reactivity of the terminal carbon atoms increases with increasing numbers of alkyne units, whereas the reactivity of the internal carbon atoms remains very low independent of the number of alkyne units.

ChemInform Abstract: Activation of Dioxygen in Air by a Phenol/Selectfluor System: An Application in the Oxidation‐Dimerization of Alkynes to 2‐Ene‐1,4‐diones.

AbstractA mild procedure to activate dioxygen by a combined phenol/selectfluor system is presented which allows the oxidative dimerization of terminal alkynes to give 1,4‐enediones.

ChemInform Abstract: Microwave‐Assisted Palladium‐Catalyzed Highly Regio‐ and Stereoselective Head to Head Dimerization of Terminal Aryl Alkynes in Water.

A highly regio- and stereoselective oxime palladacycle/imidazolinium-catalyzed head to head dimerization of terminal aryl alkynes in water is presented. The reaction, which is carried out at 130 °C under microwave irradiation in the presence of 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride as ligand, triethylamine as base, and TBAB as surfactant, allows the synthesis of (E)-1,4-enynes as single stereoisomers in good isolated yields.

Dimerization of two alkyne units: model studies, intermediate trapping experiments, and kinetic studies.

By means of high level quantum chemical calculations (B2PLYPD and CCSD(T)), the dimerization of alkynes substituted with different groups such as F, Cl, OH, SH, NH2, and CN to the corresponding diradicals and dicarbenes was investigated. We found that in case of monosubstituted alkynes the formation of a bond at the nonsubstituted carbon centers is favored in general. Furthermore, substituents attached to the reacting centers reduce the activation energies and the reaction energies with increasing electronegativity of the substituent (F > OH > NH2, Cl > SH, H, CN). This effect was explained by a stabilizing hyperconjugative interaction between the σ* orbitals of the carbon-substituent bond and the occupied antibonding linear combination of the radical centers. The formation of dicarbenes is only found if strong π donors like NH2 and OH as substituents are attached to the carbene centers. The extension of the model calculations to substituted phenylacetylenes (Ph-C≡C-Y) predicts a similar reactivity of the phenylacetylenes: F > OCH3 > Cl > H. Trapping experiments of the proposed cyclobutadiene intermediates using maleic anhydride as dienophile as well as kinetic studies confirm the calculations. In the case of phenylmethoxyacetylene (Ph-C≡C-OCH3) the good yield of the corresponding cycloaddition product makes this cyclization reaction attractive for a synthetic route to cyclohexadiene derivatives from alkynes.

ChemInform Abstract: Carboxylate Switch Between Hydro‐ and Carbopalladation Pathways in Regiodivergent Dimerization of Alkynes.

AbstractThe title process is studied in detail with theoretical and experimental investigations.

CuSO4-H-phosphonate catalyzed highly stereo- and regioselective dimerization of terminal alkynes

The readily available CuSO4-H-phosphonate catalytic system can catalyze the head-to-head dimerization of terminal alkynes to give the corresponding (E) conjugated enynes selectively in high yield.

Computational insight into the mechanism of the Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes

The Pd(0)–Brønsted acid cooperatively catalysed head-to-tail dimerization of terminal alkynes was computationally studied to gain a mechanistic insight.