Internal Triple Bond Research Articles

How to Break the Law: trans‐Hydroboration and gem‐Hydroboration of Alkynes

AbstractClassical hydroboration is the textbook example of an organometallic reaction under rigorous frontier orbital control: a synergetic bonding mode between substrate and reagent evolves into a four‐membered transition state that enforces syn/suprafacial delivery of H−BR2 to the π‐system. This strict stereochemical “law” remained valid for decades, even after the advent of metal‐catalyzed hydroboration. During the last decade, however, numerous possibilities emerged that allow this paradigm to be challenged. Thus, direct trans‐hydroboration of terminal as well as internal alkynes was achieved through radical, ionic, organocatalytic, and metal‐catalyzed manifolds, which are summarized in this review. Among them, pathways involving either metal vinylidenes (Rh, Ir, Ru, Fe) derived from terminal alkynes or metallacyclopropenes (η2‐vinylmetal complexes) derived from an internal triple bond and a [CpXRu]‐based catalyst represent the currently most widely applicable solutions. The latter type of intermediates is also accountable for the equally perplexing gem‐hydroboration of alkynes: geminal delivery of a borane to a triple bond has no precedent in the classical canon. The only forerunner was gem‐hydrogenation, which represents an unconventional yet highly useful entry into discrete ruthenium carbene complexes. The close mechanistic ties between these transformations are outlined, and brief reference is also made to remotely related reactions such as trans‐diboration and trans‐carboboration.

Unsaturated oligoesters containing internal triple and double bonds based on DL-malic acid: synthesis, characterization and study of crosslinking via click reaction

Unsaturated oligoesters containing internal triple and double bonds based on DL-malic acid: synthesis, characterization and study of crosslinking via click reaction

A palladium iodide catalyzed regioselective carbonylative route to isocoumarin and thienopyranone carboxylic esters

The reactivity of 2-alkynylbenzoic acids under PdI2/KI-catalyzed oxidative alkoxycarbonylation conditions, with oxygen (from air) as external oxidant and an alcohol as external nucleophile, has been studied. It was found that substrates with triple bond substituted with a bulky alkyl group, such as tert-butyl, selectively underwent a 6-endo-dig cyclization - alkoxycarbonylation pathway with ethanol or isopropanol as nucleophile, to give high value added isocoumarin-4-carboxylic esters in good to high yields (67–87%).When applied to alkynylthiophencarboxylic acids bearing an internal triple bond, the reaction turned out to be completely regioselective toward the formation of the corresponding alkyl thienopyranonecarboxylates ensuing from 6-endo-dig cyclization-alkoxycarbonylation, regardless the nature of the substituent on the triple bond and the external alcohol (54–91% yields).To confirm the structure of the products deriving from different kinds of substrates, and therefore to corroborate the proposed mechanistic pathways, the crystalline structures of 11 new compounds have been resolved by XRD analysis.

Inhomogeneities of charge distribution in porous graphitic carbon nitride g-CxNy nanosheets

Porous graphitic carbon nitrides g-CxNy (g-CN, g-C2N and g-C3N4) were studied computationally using semi-empirical molecular orbital method to obtain insights into charge distribution in them. It is found that the charge distribution is inhomogeneous but has good symmetry in the symmetrical structure. And sheet boundary has significant effect on the charge distribution. All N atoms have negative charge due to electrons enrichment. The absolute charge value of the edge atoms is significantly larger than that of the internal atoms in g-CN and g-C2N, which is opposite in g-C3N4. For the C atom, all C atoms show positive charge in g-CN. However, in g-C2N, the double-bond and triple-bond C atoms at the boundary have different gain and loss electrons. The internal triple-bond C atoms show the same positive charge value.

Synthesis of a New Hyperbranched Pyridylphenylene Polymer Based on an АB2 Monomer and a Pair of АB/АB2 Monomers

A hyperbranched pyridylphenylene polymer with the weight average molecular mass of 56000 Da is synthesized for the first time from an AB2 monomer bearing one diene and two internal triple bonds. The introduction of a linear AB comonomer into the reaction system leads to an increase in the yield of the polymer which has the weight average molecular mass of 21400 Da. Due to the limited degree of intramolecular cyclization, the high content of functional groups facilitates the formation of defect-free structures and provides an opportunity to modify the resulting polymers.

Copper-Catalyzed Anti-Markovnikov Hydrosilylation of Terminal Alkynes.

A copper-catalyzed anti-Markovnikov hydrosilylation of alkynes with PhSiH3 was reported. This reaction represents a notable and efficient example on copper-catalyzed hydrosilylatioin of alkynes, which shows excellent recognition between the terminal and internal triple bonds. Various (hetero)aromatic and aliphatic substituted terminal alkynes underwent this reaction to afford the (E)-vinylsilanes in high yields and with excellent regioselectivity.

Palladium-Catalyzed Cyclocarbonylation Approach to Thiadiazafluorenones: A Correction.

The palladium-catalyzed carbonylation of 2-(propynylthio)benzimidazoles bearing a terminal triple bond leads to 2-methyl-1-thia-4a,9-diazafluoren-4-ones instead of the previously reported 3-methyl isomers, as unequivocally established by XRD analysis of a representative product. A correction is therefore provided here in order to rectify the previous erroneous assignment of the position of the methyl group. Moreover, the process has been generalized to substrates bearing an internal triple bond, which lead to 3-alkyl-2-methyl-1-thia-4a,9-diazafluoren-4-ones, whose structure was confirmed by XRD analysis of two representative derivatives.

Synthesis in situ of gold nanoparticles by a dialkynyl Fischer carbene complex anchored to glass surfaces

In this work we present a detailed study of classic reactions such as “click reaction” and nucleophilic substitution reaction but on glass solid surface (slides). We used different reactive center of a dialkynylalcoxy Fischer carbene complex of tungsten(0) to be anchored to modified glass surface with amine, to obtain aminocarbene, and azide terminal groups. These cycloaddition reaction showed regioselectivity to internal triple bond of dialkynyl Fischer carbene complex without Cu(I) as catalyst. Anyway the carbene anchored was able to act as a reducing agent to produce in situ very stable gold nanoparticles fixed on surface. We showed the characterization of modified glasses by contact angle measurements and XPS. Synthesized nanoparticles were characterized by SEM, XPS, EDS and UV–vis. The modified glasses showed an important enhancement Raman-SERS. This simple, fast and robust method to create a polifunctional and hybrid surfaces can be valuable in a wide range of applications such as Raman-SERS substrates and other optical fields.

A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes.

The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [ Xu et al., J. Chem. Phys. 2012, 136, 154303 ], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (l = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from l = 4. These calculations thus confirm the qualitative arguments for the importance of the l = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower l partial waves in these systems are consistent with the broader features observed in the corresponding spectra.

Palladium-catalyzed 6-endo-dig oxycyclization–methoxycarbonylation toward the total synthesis of polymaxenolides

5-exo-Dig versus 6-endo-dig oxycyclizations of alkynols having an internal triple bond catalyzed by PdCl2, followed by methoxycarbonylation of the resulting vinylpalladium intermediates, are described. With an aim toward the total synthesis of polymaxenolides, a hybrid marine natural product containing a dihydropyran substructure, a model alkynol having the africane-type sesquiterpenoid skeleton was treated with PdCl2 and 1,4-benzoquinone in MeOH under an atmosphere of CO (balloon) to afford the desired 6-endo-dig oxycyclization–methoxycarbonylation product.

Stereo‐ and Chemoselective Character of Supported CeO2 Catalysts for Continuous‐Flow Three‐Phase Alkyne Hydrogenation

AbstractTiO2‐, Al2O3‐, and ZrO2‐supported CeO2 catalysts with different Ce loadings were prepared by wet impregnation of the carriers with an acidified solution of cerium ammonium nitrate. The calcined catalysts were characterized by bulk and surface‐sensitive techniques, which included microcalorimetry, and evaluated in the three‐phase hydrogenation of alkynes under continuous‐flow conditions at variable temperature (293–413 K) and pressure (1–90 bar). A number of acetylenic compounds, which contain terminal or internal triple bonds, conjugated unsaturations, and additional functionalities, were systematically assessed. The results revealed the full stereo‐ and chemoselective character of the ceria catalysts, which outperform the well‐known Lindlar catalyst, and open promising perspectives for the revolutionary use of a cost‐effective oxide for the production of olefinic compounds in the vitamin and fine chemical industries.

Divergent Palladium Iodide Catalyzed Multicomponent Carbonylative Approaches to Functionalized Isoindolinone and Isobenzofuranimine Derivatives

2-Alkynylbenzamides underwent different reaction pathways when allowed to react under PdI2-catalyzed oxidative carbonylation conditions, depending on the nature of the external nucleophile and reaction conditions. Thus, oxidative carbonylation of 2-ethynylbenzamides, bearing a terminal triple bond, carried out in the presence of a secondary amine as external nucleophile, selectively led to the formation of 3-[(dialkylcarbamoyl)methylene]isoindolin-1-ones through the intermediate formation of the corresponding 2-ynamide derivatives followed by intramolecular nucleophilic attack by the nitrogen of the benzamide moiety on the conjugated triple bond. On the other hand, 3-[(alkoxycarbonyl)methylene]isobenzofuran-1(3H)imines were selectively obtained when the oxidative carbonylation of 2-alkynylbenzamides, bearing a terminal or an internal triple bond, was carried out in the presence of an alcohol R'OH (such as methanol or ethanol) as the external nucleophile and HC(OR')3 as a dehydrating agent, necessary to avoid substrate hydrolysis. In this latter case, the reaction pathway leading to the isobenzofuranimine corresponded to the 5-exo-dig intramolecular nucleophilic attack of the oxygen of the benzamide moiety on the triple bond coordinated to the metal center followed by alkoxycarbonylation. The structures of representative products have been confirmed by X-ray crystallographic analysis.

Copper‐Catalyzed Selective Semihydrogenation of Terminal Alkynes with Hypophosphorous Acid

AbstractA novel copper‐catalyzed selective semihydrogenation of terminal alkynes using hypophosphorous acid as hydrogen donor took place efficiently to afford the corresponding alkenes in high yields. A broad range of substituted terminal aromatic and aliphatic alkenes, including terminal dienes and enynes bearing internal triple bonds, can be efficiently synthesized by this reaction.magnified image

Specific and Reversible Alkynyl Transfer Reactions of an N‐Heterocyclic Phosphane

AbstractReaction of P‐chloro‐1,3,2‐diazaphospholenes with alkynyllithium or Grignard reagents produced N‐heterocyclic alkynylphosphanes 3a–c. Terminal alkynylphosphane 3c reacted in the presence of a catalytic amount of CuCl with formation of bis‐phosphanylalkyne 4, ethyne and chlorodiazaphospholene 2b, which was converted into the appropriate azide 6 upon addition of LiN3. Detailed studies allowed us to explain the formal alkynyl/Cl metathesis and alkynyl scrambling as arising from a combination of Cu‐catalysed cross‐coupling steps and the reverse processes, which involved Cu‐induced P–C bond breaking. The superposition of individual steps allows the formulation of an equilibrium reaction: 2 3c [lrarr2] 4 + HCCH. Shift of the equilibrium by enforced release of acetylene into the gas phase enabled quantitative conversion of 3c into 4. Traces of P–C bond‐activation products were also detected upon action of CuCl on 3b with an internal triple bond, but alkynylphosphanes with Ph2P or (iPr2N)2P moieties were unreactive. Alkynylphosphanes 3a–c and 4 were characterised by spectroscopic data and single‐crystal X‐ray diffraction studies. The presence of notably elongated P–C bonds points to a bond‐weakening effect, which is responsible for explaining the unusual P–C bond reactivity.

Palladium‐Catalyzed Oxidative Regio‐ and Diastereoselective Diarylating Carbocyclization of Dienynes

Transmetallation: A highly regio- and diastereoselective carbocyclization involving a palladium-catalyzed 1,2-addition of two carbon atoms across a conjugated diene has been developed (see scheme). The reaction is performed with dienynes and an oxygen tether containing a terminal or internal triple bond.

ChemInform Abstract: A General Synthesis of Indole‐3‐carboxylic Esters by Palladium‐Catalyzed Direct Oxidative Carbonylation of 2‐Alkynylaniline Derivatives.

A general synthesis of indole-3-carboxylic esters by direct Pd-catalyzed oxidative carbonylation of readily available 2-alkynylaniline derivatives is reported. In particular, 2-alkynylanilines bearing an internal triple bond and a secondary amino group (1) were directly converted into indole-3-carboxylic esters 2 in fair to good yields (50–84 %) when let to react with CO, O2, and an alcohol in the presence of the PdI2/KI catalytic system, under relatively mild conditions [100 °C and 20 atm (at 25 °C) of a 4:1 mixture CO/air]. On the other hand, under similar conditions, but in the presence of HC(OMe)3, 2-alkynylanilines bearing an internal triple bond and a primary amino group (7) afforded 1-(dimethoxymethyl)indole-3-carboxylic esters 9 through the intermediate formation of N-(dimethoxymethyl)-2-alkynylaniline derivatives II. Compounds 9 could be conveniently converted into N-unsubstituted indole-3-carboxylic esters 10 by a simple acidic treatment carried out in MeOH/H2O at 80 °C.

A General Synthesis of Indole‐3‐carboxylic Esters by Palladium‐Catalyzed Direct Oxidative Carbonylation of 2‐Alkynylaniline Derivatives

AbstractA general synthesis of indole‐3‐carboxylic esters by direct Pd‐catalyzed oxidative carbonylation of readily available 2‐alkynylaniline derivatives is reported. In particular, 2‐alkynylanilines bearing an internal triple bond and a secondary amino group (1) were directly converted into indole‐3‐carboxylic esters 2 in fair to good yields (50–84 %) when let to react with CO, O2, and an alcohol in the presence of the PdI2/KI catalytic system, under relatively mild conditions [100 °C and 20 atm (at 25 °C) of a 4:1 mixture CO/air]. On the other hand, under similar conditions, but in the presence of HC(OMe)3, 2‐alkynylanilines bearing an internal triple bond and a primary amino group (7) afforded 1‐(dimethoxymethyl)indole‐3‐carboxylic esters 9 through the intermediate formation of N‐(dimethoxymethyl)‐2‐alkynylaniline derivatives II. Compounds 9 could be conveniently converted into N‐unsubstituted indole‐3‐carboxylic esters 10 by a simple acidic treatment carried out in MeOH/H2O at 80 °C.

Thermal [2+2] Cycloaddition of CF3-Substituted Allenynes: Access to Novel Cyclobutene-Containing α-Amino Acids

An efficient approach for the preparation of novel CF3-substituted cyclic α-amino acid derivatives fused with cyclobutene ring has been developed. The method is based on intramolecular thermal [2+2] cycloaddition of amino acid containing allenynes with internal triple bond obtained by successive [2,3]-sigmatropic rearrangement of propargyl-containing nitrogen CF3-ylides and ­Sonogashira derivatization of the triple bond.

Synthesis of CF3-containing α-alkynyl-α-aminophosphonates by Sonogashira cross-coupling reaction

An efficient route to new α-CF3-α-aminophosphonic acid derivatives bearing an arylalkynyl moiety at the α-carbon atom has been developed. The method is based on palladium-catalyzed cross-coupling of the corresponding α-propargyl (ethynyl) α-aminophosphonates with aryl iodides to afford the aminophosphonic acid derivatives with an internal triple bond that is suitable for further modifications.

Copper(I)‐Catalyzed Regioselective Monoborylation of 1,3‐Enynes with an Internal Triple Bond: Selective Synthesis of 1,3‐Dienylboronates and 3‐Alkynylboronates

The regioselectivity is primarily determined by the substrate structure, whereas for moderately substituted 1,3-enynes (IV), the catalytic phosphine ligand displays a significant role.