Activation Of Alkynes Research Articles

Rh(III)-catalyzed redox-neutral C-H activation of pyrazolones: an economical approach for the synthesis of N-substituted indoles.

A new strategy is reported for the economical synthesis of indoles bearing an N-(3-aminobut-2-enoyl) substituent through Rh(III)-catalyzed redox-neutral C-H activation of pyrazolones and alkynes. This approach utilizes cheap substrates and mild reaction conditions to access a unique class of indoles via a N-N bond oxidative cleavage without loss of the N-terminus, therefore meeting all the atom/step/redox economy principles.

Mechanism of [3+2] cycloaddition of alkynes to the [Mo3 S4 (acac)3 (py)3 ][PF6 ] cluster.

A study, involving kinetic measurements on the stopped-flow and conventional UV/Vis timescales, ESI-MS, NMR spectroscopy and DFT calculations, has been carried out to understand the mechanism of the reaction of [Mo3 S4 (acac)3 (py)3 ][PF6 ] ([1]PF6 ; acac=acetylacetonate, py=pyridine) with two RCCR alkynes (R=CH2 OH (btd), COOH (adc)) in CH3 CN. Both reactions show polyphasic kinetics, but experimental and computational data indicate that alkyne activation occurs in a single kinetic step through a concerted mechanism similar to that of organic [3+2] cycloaddition reactions, in this case through the interaction with one Mo(μ-S)2 moiety of [1](+) . The rate of this step is three orders of magnitude faster for adc than that for btd, and the products initially formed evolve in subsequent steps into compounds that result from substitution of py ligands or from reorganization to give species with different structures. Activation strain analysis of the [3+2] cycloaddition step reveals that the deformation of the two reactants has a small contribution to the difference in the computed activation barriers, which is mainly associated with the change in the extent of their interaction at the transition-state structures. Subsequent frontier molecular orbital analysis shows that the carboxylic acid substituents on adc stabilize its HOMO and LUMO orbitals with respect to those on btd due to better electron-withdrawing properties. As a result, the frontier molecular orbitals of the cluster and alkyne become closer in energy; this allows a stronger interaction.

Early Steps of Homogeneous Catalysis in Rennes: Carbon Dioxide Incorporation, Alkyne Activation and Ruthenium Catalysis

This review reports the first discoveries in homogeneous catalysis in Rennes. Ruthenium(II) catalysts are shown to promote the activation of terminal alkynes and the synthesis of vinylcarbamates on addition of CO2 and amines. The ruthenium–vinylidene are the key catalytic species. Applications to the addition of carboxylic acids to alkynes have opened the access to various enolesters and dienylesters via controlled catalyzed Markovnikov or anti-Markonikov additions. The catalytic synthesis of α-methylene cyclic carbonates and oxazolidinones from propargylic alcohols and CO2 is presented and shown to lead to allene derivatives via palladium catalysis. Cp*RuX(COD) catalyst promote the head to tail oxidative couplings of alkene with alkyne, and the synthesis of functional dienes via 1,4 addition of H-nucleophiles to the biscarbene-ruthenium intermediate arising from head to head couplings of alkynes, whereas Cp*RuL2 + catalysts favour activation of alkynes via vinylidene intermediate. This historical review presents the first steps in homogeneous catalysis in Rennes starting in 1985 with ruthenium(II) catalysts promoting the activation of terminal alkynes via ruthenium–vinylidene or oxidative couplings for the synthesis of CO2 derivatives: vinylcarbamates, carbonates or oxazolidinones, enolesters and functional dienes.

Rhodium(III) Porphyrin-catalyzed Reactions via Activation of Alkynes

Abstract The cycloisomerization of 1,6-enynes catalyzed by rhodium(III) porphyrin under mild reaction conditions successfully afforded a five-membered ring system. The rhodium porphyrin was found to be a strong π-Lewis acid that could activate alkynes. Thus, rhodium porphyrin-catalyzed intramolecular Friedel–Crafts-type reactions of alkynes with arenes were also accomplished. Furthermore, rhodium porphyrin-catalyzed intermolecular cyclization of alkynes with styrenes afforded the indene derivatives.

ChemInform Abstract: Microwave Assisted [RuCl2(p‐cymene)2]2 Catalyzed Regioselective endo‐Tandem Cyclization Involving Imine and Alkyne Activation: An Approach to Benzo[4,5]imidazo[2,1‐a]pyridine Scaffold.

A microwave assisted efficient route to the synthesis of benzimidazole fused heterocycles through metal catalyzed endo-cyclization strategy involving imine and alkyne activation has been developed. In the presence of [RuCl2(p-cymene)2]2, a variety of 2-ethynyl aldehydes underwent cascade cyclization with substituted benzenediamines to afford the corresponding benzo[4,5]imidazo[2,1-a] pyridine scaffold in moderate to good yields.

Gold-catalyzed dearomative spirocyclization of aryl alkynoate esters.

Aryl alkynoate esters undergo gold-catalyzed spirocyclization under mild conditions, affording spirolactones in high yields. This approach obviates the need for stoichiometric halogenating reagents typically employed for alkyne activation in related transformations. Water was found to play a critical role in governing the product selectivity. Anhydrous conditions lead selectively to coumarin products, as has previously been observed for aryl alkynoate esters, while the addition of 1 equiv of water leads selectively to spirocycle formation.

ChemInform Abstract: Activation of Alkynes with B(C6F5)3 — Boron Allylation Reagents Derived from Propargyl Esters.

Novel allyl boron compounds are readily synthesized via rearrangement reactions between Lewis acidic B(C6F5)3 and propargyl esters. These reactions proceed through an initial cyclization followed by ring-opening and concurrent C6F5-group migration. In the absence of disubstitution adjacent to the ester oxygen atom, an allyl boron migration rearrangement leads to formal 1,3-carboboration products. These allyl boron compounds act as allylation reagents with aldehydes introducing both a C3-allyl fragment and a C6F5-unit as a single anti-diastereomer. In these reactions, B(C6F5)3 activates the alkynes, prompting the rearrangement processes and enabling installations of C6F5 and R-groups.

Pd-catalyzed synthesis of 9,9'-bifluorenylidene derivatives via dual C-H activation of bis-biaryl alkynes.

We report a novel Pd-catalyzed alkyne-directed dual C-H activation of bis-biaryl alkynes, which produced important and useful products, 9,9'-bifluorenylidene (9,9'BF) derivatives, in high yields with a broad range of functional group compatibility. The combination of the PdCl2 catalyst with the MnO2 oxidant and PivOH additive is vital for realization of the present catalytic transformation. Mechanistic evidence suggests that this intramolecular arene/alkyne annulation may take place through unusual dual C-H activation followed by annulation with alkynes.

Catalyst-free activation of methylene chloride and alkynes by amines in a three-component coupling reaction to synthesize propargylamines.

Propargylamines are synthesized via metal-free activation of the C-halogen bond of dihalomethanes and the C-H bond of terminal alkynes in a three-component coupling without catalyst or additional base and under mild reaction conditions. The dihalomethanes are used both as solvents as well as precursors for the methylene fragment (C1) in the final product. The scope of the reaction and the influence of various reaction variables has been investigated. A plausible reaction mechanism is proposed and the involvement of various intermediates that can be generated in situ in the process is discussed. The metal-free conditions also make this protocol environmentally benign and atom economical.

Metal and carbene organocatalytic relay activation of alkynes for stereoselective reactions.

Transition metal and organic catalysts have established their own domains of excellence. It has been expected that merging the two unique domains should provide complimentary or unprecedented opportunities in converting simple raw materials to functional products. N-heterocyclic carbenes alone are excellent organocatalysts. When used with transition metals such as copper, N-heterocyclic carbenes are routinely practiced as strong-coordinating ligands. Combination of an N-heterocyclic carbene and copper therefore typically leads to deactivation of either or both of the two catalysts. Here we disclose the direct merge of copper as a metal catalyst and N-heterocyclic carbenes as an organocatalyst for relay activation of alkynes. The reaction involves copper-catalysed activation of alkynes to generate ketenimine intermediates that are subsequently activated by an N-heterocyclic carbene organocatalyst for stereoselective reactions. Each of the two catalysts (copper metal catalyst and N-heterocyclic carbene organocatalyst) accomplishes its own missions in the activation steps without quenching each other.

ChemInform Abstract: The Use of Alkoxides and Related π‐Donors in Organotransition Metal Chemistry as Illustrated by M2(OR)6 Compounds (M = Mo or W)

AbstractReview: [46 refs.

Silicon carbide-derived carbon nanocomposite as a substitute for mercury in the catalytic hydrochlorination of acetylene.

Acetylene hydrochlorination is an important coal-based technology for the industrial production of vinyl chloride, however it is plagued by the toxicity of the mercury chloride catalyst. Therefore extensive efforts have been made to explore alternative catalysts with various metals. Here we report that a nanocomposite of nitrogen-doped carbon derived from silicon carbide activates acetylene directly for hydrochlorination in the absence of additional metal species. The catalyst delivers stable performance during a 150 hour test with acetylene conversion reaching 80% and vinyl chloride selectivity over 98% at 200 °C. Experimental studies and theoretical simulations reveal that the carbon atoms bonded with pyrrolic nitrogen atoms are the active sites. This proof-of-concept study demonstrates that such a nanocomposite is a potential substitute for mercury while further work is still necessary to bring this to the industrial stage. Furthermore, the finding also provides guidance for design of carbon-based catalysts for activation of other alkynes.

Gold(I)‐, Palladium(II)‐, Platinum(II)‐, and Mercury(II)‐Catalysed Spirocyclization of 1,3‐Enynediols: Reaction Scope

AbstractThe spirocyclization of different 1,3‐enynediols was investigated. The reaction was only efficient for the synthesis of [5,6]‐spiroacetals. In this case, the reaction was characterized by almost quantitative yields, short reaction times, and low catalyst loadings (0.5–1 %). When the synthesis of [6,6]‐spiroacetals was attempted, the reaction suffered from poor regioselectivity and a higher propensity of the intermediate dienol ethers to decompose under the acidic conditions, and it became no longer viable. But it is possible to generate the dienol ethers cleanly under milder conditions as a mixture of regioisomers. This striking difference in reaction efficiency was explained by the unstable dienol ethers cyclizing more quickly to give [5,6]‐spiroacetals than to give [6,6]‐spiroacetals. In this study, the successful application of a new cationic palladium pincer complex for electrophilic alkyne activation at room temperature has been demonstrated for the first time.

Silver‐Catalyzed Alkyne Activation: The Surprising Ligand Effect

AbstractAn unexpected ligand effect was discovered in the silver(I)‐catalyzed alkyne activation. For both aldehyde‐alkyne‐amine (A3) condensation and intermolecular alkyne hydroamination, the type B complex (P:Ag=1:1) effectively promoted the reaction, while no reaction occurred with either no ligand or excess ligands under the identical conditions.magnified image

ChemInform Abstract: Intriguing Mechanistic Labyrinths in Gold(I) Catalysis

AbstractReview: 131 refs.

ChemInform Abstract: Palladium‐Catalyzed Carbene Migratory Insertion Using Conjugated Ene—Yne—Ketones as Carbene Precursors.

AbstractThe reaction is proposed to proceed via oxidative addition, alkyne activation—cyclization, palladium carbene migratory insertion and β—hydride elimination followed by catalyst regeneration.

Ground-State Elevation Approach To Suppress Side Reactions in Gold-Sensing Systems Based on Alkyne Activation

A novel approach to suppress the side reactions observed in the reaction-based gold-sensing systems based on the alkyne activation is disclosed. By elevating steric strain around the reaction site, the gold ion promoted ring-opening process in rhodamine-lactam probes is significantly accelerated, which also leads to suppression of those possible side reactions. As a result, the probes show very high sensitivity in addition to excellent selectivity toward gold species. Furthermore, bioimaging of gold species in live cells was demonstrated with a FRET version.

A Computational Study of Allene Synthesis via the ZnI2‐Promoted Alleylation of Terminal Alkynes (ATA Reaction)

AbstractA detailed mechanism of allene formation via the ZnI2‐promoted alleylation of terminal alkynes (ATA reaction) was investigated through DFT. The originally proposed pathway for this process was evaluated and accords perfectly with the calculated results. The formation of a propargylamine, the subsequent [1,5]‐hydride migration, and β‐elimination are the key steps of the whole process. The [1,5]‐hydride migration is the rate‐determining step. The main role of ZnI2 in the whole process is alkyne activation to facilitate the [1,5]‐hydride migration and the stabilization of the zwitterionic intermediate.

ChemInform Abstract: Post Ugi Gold(I)‐ and Platinum(II)‐Catalyzed Alkyne Activation: Synthesis of Diversely Substituted Fused Azepinones and Pyridones.

AbstractThe Ugi four‐component reaction of 2‐pyrrole aldehydes and analogous indole and benzothiophene aldehydes with primary amines, isonitriles, and alkynoic acids offers the expected adducts (V), (XI), and (XIV) which can be readily used for the synthesis of various fused pyridones and azepinones.

Activation of Alkynes with B(C6F5)3 – Boron Allylation Reagents Derived from Propargyl Esters

Novel allyl boron compounds are readily synthesized via rearrangement reactions between Lewis acidic B(C6F5)3 and propargyl esters. These reactions proceed through an initial cyclization followed by ring-opening and concurrent C6F5-group migration. In the absence of disubstitution adjacent to the ester oxygen atom, an allyl boron migration rearrangement leads to formal 1,3-carboboration products. These allyl boron compounds act as allylation reagents with aldehydes introducing both a C3-allyl fragment and a C6F5-unit as a single anti-diastereomer. In these reactions, B(C6F5)3 activates the alkynes, prompting the rearrangement processes and enabling installations of C6F5 and R-groups.