Alkyne Insertion Research Articles

Mechanism and Stereoselectivity Control of Terminal Alkyne Dimerization Activated by a Zr/Co Heterobimetallic Complex: A DFT Study.

Heterobimetallic complexes have recently garnered considerable attention in organic synthesis owing to their high activity and selectivity, which surpass those of monometallic complexes. In this study, the detailed mechanisms of terminal alkyne dimerization activated by the heterobimetallic Zr/Co complex, as well as the different stereoselectivities of Me3SiC≡CH and PhC≡CH dimerization, were investigated and elucidated by using density functional theory calculations. After excluding the three-molecule reaction and outer-sphere mechanisms, the inner-sphere mechanism was determined as the most optimal process. The inner-sphere mechanism involves four processes: THF dissociation and coordination of the first alkyne; ligand migration and C-H activation; N2 dissociation and insertion of the second alkyne; and reductive elimination. The stereoselectivity between the E-/Z- and gem-isomers is determined by the C-C coupling mode of the two alkynes and that of the E- and Z-isomers is determined by the sequence of the C-C coupling and hydrogen migration in the reductive elimination process. Me3SiC≡CH dimerization yields only an E-isomer owing to the large differences in the distortion and interaction energies, whereas PhC≡CH dimerization produces an E-, Z-, and gem-isomers owing to the reduced interaction energy differences.

Combined C-H amination and intermolecular alkyne insertion for a three-component cyclization

Combined C-H amination and intermolecular alkyne insertion for a three-component cyclization

A Photoreactor-Interfaced Mass Spectrometer: An Online Platform to Monitor Photochemical Reactions.

An experimental platform is reported that allows for the online characterization of photochemical reactions by coupling a continuous flow photoreactor, equipped with LED light irradiation and a dual-tipped ESI source, directly to a mass spectrometer with electrospray ionization. The capabilities of this platform are demonstrated with two classes of photoreactions: (1) the photopolymerization of methyl methacrylate and (2) photocatalyzed alkyne insertion into a 1,2,3-benzotriazinone. The online technique provides rapid information to inform the underlying photochemical mechanism and evaluate the overall photochemistry.

Mechanistic insights to define the directional role of hydrogen-bonding network between aryl oximes and propargyl alcohols in Rh(III) catalysis

The mechanism of the Cp*Rh(OAc)2-catalyzed C−H activation/annulation of acetophenone oxime and secondary propargyl alcohol was studied theoretically. Four elementary steps are involved as follows: C−H activation via concerted-metalation-deprotonation (CMD) mechanism, regioselective alkyne insertion, β-OH elimination and metal-free 6π electrocyclization. Importantly, density functional theory (DFT) calculations reveal that β-H elimination is inferior to β-OH elimination that is assisted by the directional hydrogen-bonding network between the two hydroxyl groups of the substrates. This noncovalent interaction also contributes to tuning the regioselectivity of alkyne insertion. Furthermore, the metal-free 6π electrocyclization rather than Rh-assisted cyclization gave the final product, isoquinoline N-oxides. The computationally supported mechanistic features were further validated by exploring substrate effects via replacing acetophenone oxime with O-methyl oxime and secondary propargyl alcohol with allyl alcohol, allenol and tertiary propargyl alcohol, respectively.

Ruthenium‐Catalyzed Synthesis of Macrocyclic Isoquinolines and Isoquinolones via a C−H/N−H Annulations Reaction

AbstractAn Ru(II)‐catalyzed C−H annulations of oximes and N‐methoxybenzamides with bridged alkyne is described. This method is useful for the efficient synthesis of various macrocyclic isoquinolines and isoquinolones in good to moderate yields. A possible mechanism, involving a key five‐membered metallacycle intermediate, alkyne insertion followed by reductive elimination to give a final annulated product has been proposed.

Rhodium-catalyzed enantioselective and diastereodivergent access to diaxially chiral heterocycles

N-N axially chiral biaryls represent a rarely explored class of atropisomers. Reported herein is construction of diverse classes of diaxially chiral biaryls containing N-N and C-N/C-C diaxes in distal positions in excellent enantioselectivity and diastereoselectivity. The N-N chiral axis in the products provides a handle toward solvent-driven diastereodivergence, as has been realized in the coupling of a large scope of benzamides and sterically hindered alkynes, affording diaxes in complementary diastereoselectivity. The diastereodivergence has been elucidated by computational studies which revealed that the hexafluoroisopropanol (HFIP) solvent molecule participated in an unusual manner as a solvent as well as a ligand and switched the sequence of two competing elementary steps, resulting in switch of the stereoselectivity of the alkyne insertion and inversion of the configuration of the C-C axis. Further cleavage of the N-directing group in the diaxial chiral products transforms the diastereodivergence to enantiodivergence.

Rhodium-Catalyzed Selective Nucleophilic Cyclization/Cross-Coupling of Two ortho-Alkynylanilines Bearing Differential N-Substituents.

Herein, we reported an effective selective nucleophilic cyclization/cross-coupling cascade reaction of N-tosyl ortho-alkynylanilines and N-acyl ortho-alkynylanilines using Rh(COD)2BF4/tBuXantPhos as a catalyst. The present protocol features excellent chemo- and regioselectivity, high atom-economy, and a broad range of substrates. The mechanism studies indicated that the key to the success of this reaction is the powerful capacity of the rhodium catalyst to recognize the N-substituent group in the selective nucleophilic cyclization and selective alkyne insertion.

Synthesis of Cyclopenta[c]quinolines by Palladium-Catalyzed Cyclization of 3-Bromoindoles with Internal Alkynes via Spirocyclic Cyclopentadiene Intermediates.

A novel method for the construction of a cyclopenta[c]quinoline ring via cyclization of 3-bromoindoles with internal alkynes in the presence of palladium is described. The formation of the cyclopenta[c]quinoline ring is proposed from a double [1,5] carbon sigmatropic rearrangement of the spirocyclic cyclopentadiene intermediate, which is generated in situ from the cyclization of 3-bromoindoles with internal alkynes involving a sequential double alkyne insertion into the carbon-palladium bond and dearomatization of indole. The present studies have developed a novel ring-expansion reaction of the pyrrole ring to pyridine via one carbon insertion into the C2-C3 bond of indoles and provided a simple and distinct route for the construction of tricyclic fused-quinoline derivatives that are not easy to access with conventional methods.

Regio- and Chemoselective Formal (4+1) Carbocyclization of Chalcones with Internal Alkynes via Rhodium(III) Catalysis.

A rhodium(III)-catalyzed oxidative cyclization of chalcones with internal alkynes is reported, generating biologically important 3,3-disubstituted 1-indanones along with reusable aromatic aldehydes. This transformation features unique (4+1) reaction mode, excellent regioselectivity in alkyne insertion, broad substrate scope, allows for the construction of quaternary carbon centers, and is scalable. Steric hindrance from substrate and ligand probably controls the chemoselectivity of this carbocyclization. Importantly, this discovery enables a practical two-step protocol switching the overall reaction of acetophenones with internal alkynes from a (3+2) to a (4+1) annulation.

Regio‐ and Chemoselective Formal (4+1) Carbocyclization of Chalcones with Internal Alkynes via Rhodium(III) Catalysis

AbstractA rhodium(III)‐catalyzed oxidative cyclization of chalcones with internal alkynes is reported, generating biologically important 3,3‐disubstituted 1‐indanones along with reusable aromatic aldehydes. This transformation features unique (4+1) reaction mode, excellent regioselectivity in alkyne insertion, broad substrate scope, allows for the construction of quaternary carbon centers, and is scalable. Steric hindrance from substrate and ligand probably controls the chemoselectivity of this carbocyclization. Importantly, this discovery enables a practical two‐step protocol switching the overall reaction of acetophenones with internal alkynes from a (3+2) to a (4+1) annulation.

Dynamic behavior and alkyne insertion reactions of alkenyl- and thiolate-bridged diiron carbonyl complexes

The alkenyl- and thiolate-bridged diiron carbonyl complexes [Fe2(μ-SC6H4CR1CH)(CO)6] (R1 = H or Me), which are derived from [Fe3(CO)12] and benzothiophenes, reacted with alkynes (R2CCR3) under photoirradiation conditions to form the acyl complexes [Fe2{μ-SC6H4CR1CHCR2C(R3)CO}(CO)5]. In this reaction, an alkyne and CO are inserted into the Fe–alkenyl bond, and the bulky substituents of alkynes, including a redox active ferrocenyl group, occupy the R3 position. To elucidate the reaction pathway, the reaction of the in situ generated acetonitrile complex [Fe2(μ-SC6H4CHCH)(CO)5(NCCH3)] with ethynylbenzene was monitored by 1H NMR spectroscopy. The results demonstrated that the acetonitrile complex undergoes facile insertion of the alkyne, which is followed by CO insertion to give the corresponding acyl complex [Fe2{μ-SC6H4(CH)3C(Ph)CO}(CO)5]. The reactivity of the bridging alkenyl moiety is related to the dynamic behavior of the alkenyl-thiolate ligand (SC6H4CHCH)2- as well as the entering alkyne. Temperature dependent 31P NMR measurements of the dmpm-bridged complex [Fe2(μ-SC6H4CHCH)(μ-dmpm)(CO)4] (dmpm = bis(dimethylphosphino)methane) revealed that the activation parameters for the flip-flop motion of the alkenyl-thiolate ligand are ΔH‡ = 46.7(7) kJ mol-1 and ΔS‡ = −46(2) J mol-1 K-1.

BF3-Catalyzed Intramolecular Fluorocarbamoylation of Alkynes via Halide Recycling

A BF3-catalyzed atom-economical fluorocarbamoylation reaction of alkyne-tethered carbamoyl fluorides is reported. The catalyst acts as both a fluoride source and Lewis acid activator, thereby enabling the formal insertion of alkynes into strong C-F bonds through a halide recycling mechanism. The developed method provides access to 3-(fluoromethylene) oxindoles and γ-lactams with excellent stereoselectivity, including fluorinated derivatives of known protein kinase inhibitors. Experimental and computational studies support a stepwise mechanism for the fluorocarbamoylation reaction involving a turnover-limiting cyclization step, followed by internal fluoride transfer from a BF3-coordinated carbamoyl adduct. For methylene oxindoles, a thermodynamically driven Z-E isomerization is facilitated by a transition state with aromatic character. In contrast, this aromatic stabilization is not relevant for γ-lactams, which results in a higher barrier for isomerization and the exclusive formation of the Z-isomer.

Tailor-made poly(vinylidene sulfide)s by Rh(I)–NHC catalyzed regioselective thiol-yne click polymerization

The [Rh(μ-Cl)(IPr)(η2-coe)]2/pyridine system efficiently catalyzes the polyhydrothiolation of a series of dialkynes with dithiols, producing sulfur-rich poly(vinylidene sulfide)s with a typical Mw in the range 20.000–124.000 and vinylidene content of 75–87%. A combination of flexible aliphatic dithiols, including 1,6-hexanedithiol and 2,2′-(ethylendioxy)diethanethiol, and the rigid aromatic dithiol 4,4′-thiobisbenzenethiol, with rigid aromatic dialkynes, 1,3-diethynylbenzene and 1,4-diethynylbenzene, and flexible dialkynes, including propargyl ether and 1,7-octadiyne, have been used to prepare poly(vinylidene sulfide)s. The copolymerization of flexible dithiols with rigid aromatic dialkynes or vice versa results in high molecular weight polymers, Mw up to 259.000, with low polydispersities. However, polyhidrothiolation of flexible dialkynes with flexible dithiols is much less efficient and usually results in the formation of oligomers. The interplay of the IPr and pyridine ligands on the RhCl(IPr)(py)(η2-coe) catalyst, which controls the regioselectivity of the alkyne insertion step towards the branched vinyl sulfide, is key in the preparation of these poly(vinylidene sulfide)s.

Oxidative cyclopalladation triggers the hydroalkylation of alkynes

This report describes the oxidative cyclopalladation activation of a CC bond during the Pd-catalyzed hydroalkylation of alkynes and analyzes potential reaction pathways based on density functional theory calculations. The more favorable pathway in-volves an oxidative cyclopalladation to generate a palladacyclopropene intermediate, which is rarely examined in Pd-catalyzed alkyne transformations. The reaction pathway proposed herein is kinetically favorable relative to the commonly proposed alkyne insertion mode. Furthermore, the Laplacians of the electron density, interaction region indicators, Mayer bond orders, and localized orbital bonding are evaluated to determine the reaction processes and characterize the key intermediates. Theoretical calculations indicate covalent bonding between a Pd(II) center and the two C-atoms in three-membered palladacycle species. Finally, electrostatic potential analysis reveals that the regioselectivity is governed by the charge distribution on the palladacycle moiety during the protonation step.

Alkyne Insertion Enabled Vinyl to Acyl 1,5-Palladium Migration: Rapid Access to Substituted 5-Membered-Dihydrobenzofurans and Indolines.

"Through space" palladium/hydrogen shift is an efficient strategy to achieve selective functionalization of a specific remote C-H bond. Compared with relatively extensive exploited 1,4-palladium migration process, the relevant 1,5-Pd/H shift was far less investigated. We herein report a novel 1,5-Pd/H shift pattern between a vinyl and an acyl group. Through the pattern, rapid access to 5-membered-dihydrobenzofuran and indoline derivatives has been achieved. Further studies have unveiled an unprecedented trifunctionalization (vinylation, alkynylation and amination) of a phenyl ring through 1,5-palladium migration relayed decarbonylative Catellani type reaction. A series of mechanistic investigations and DFT calculations have provided insights into the reaction pathway. Notably, it was unveiled that the 1,5-palladium migration in our case prefers a stepwise mechanism involving a PdIV intermediate.

Alkyne Insertion Enabled Vinyl to Acyl 1,5‐Palladium Migration: Rapid Access to Substituted 5‐Membered‐Dihydrobenzofurans and Indolines

Abstract“Through space” palladium/hydrogen shift is an efficient strategy to achieve selective functionalization of a specific remote C−H bond. Compared with relatively extensive exploited 1,4‐palladium migration process, the relevant 1,5‐Pd/H shift was far less investigated. We herein report a novel 1,5‐Pd/H shift pattern between a vinyl and an acyl group. Through the pattern, rapid access to 5‐membered‐dihydrobenzofuran and indoline derivatives has been achieved. Further studies have unveiled an unprecedented trifunctionalization (vinylation, alkynylation and amination) of a phenyl ring through 1,5‐palladium migration relayed decarbonylative Catellani type reaction. A series of mechanistic investigations and DFT calculations have provided insights into the reaction pathway. Notably, it was unveiled that the 1,5‐palladium migration in our case prefers a stepwise mechanism involving a PdIV intermediate.

Theoretical Investigation on the Mechanistic Pathways of Cp*Cobalt(III)- Catalyzed Transformation of N-Substituted Carbamoyl Indoles to Pyrroloindolone Derivatives

Aim: A theoretical study on the Cp*cobalt(III) catalyzed C-H functionalization transformation of N-substituted carbamoyl indoles with alkyne derivatives to pyrroloindolone derivatives has been performed using DFT method. Background: Indole skeleton is an important part for developing regioselective C-H bond functionalisation reactions. Pyrrolo[1,2-a]indole bearing a 6-5-5 tricyclic skeleton (Scheme-1) is an important structural component found in many biologically active natural products and pharmaceuticals, (such as antitumor mitomycin C, antimalarial flinderole B etc.).Thus C−H activation process has great demand methods for efficiently synthesizing a pyrrolo[1,2-a]indole unit from readily available starting materials. One such attractive report was published by Matsunaga & Kanai et.al., Which shows pyrroloindolone derivatives can be synthesized (Scheme-2) through Cp*Co(III) catalysed C-H alkenylation and annulations from N-substituted carbamoyl indoles. Objective: Our current project deals with the determination of the rate determining step of the reaction, to determine the energetically favourable pathway. Investigation of the actual reason behind the regioselectivity of the products whether it is steric or electronic effects of the substituents. Method: The geometry optimization and energy calculations of all the systems were carried out with the Gaussian 09 software. Construction of trial geometries, monitoring the progress of calculations and visualization of the final output were done by several graphical user interface softwares like Gauss View, Molden etc. Geometry optimization of all species is performed using M06-2X functional in DFT method as a method for performing optimization of structures. 6-31G (d, p) basis set was employed for all non-metal atoms and the LANL2DZ basis set was employed for cobalt. To find out the geometries of several transition structures on the potential energy surface (PES) of the reaction pathway, relaxed scan method was used. Result: DFT study on a reaction of carbamoyl indoles with alkyne derivatives by activate Co(III)Cp* catalytic condition is done here. The target scheme followed the energetics of two possible pathways. The initial step of the reaction is the alkyne insertion step which needs 21.27 kcal mol-1 amount of energy. In the first case the final annulated product has been generated through a carbon-carbon bond formation, proton transfer and demetallation processes in path-a. The global activation barrier has been found to be quite high (30.17 kcal mol-1). However the study of the second pathway, which generated the simple alkenylated product through the proto-demetalation process in path-b, reveals a more reliable activation barrier (21.27 kcal mol-1). Conclusion: Comparison of the energy requirement clearly reveals that the formation of the final product should go through favourable simple alkenylation process that is path-b for dimethyl amine. Due to lower energy barrier the simple alkenylation pathway (i.e Path-b) is more favourable one. Initial alkyne addition step is the rate determining step. The regioselectivity of the alkyne addition is purely governed by the steric crowding around the substrate. Thus the favourable mode of alkyne addition has lower energy barrier due to lesser steric interactions. This theoretical investigation may guide the future researchers for developing some other economical route for generating similar derivatives.

Electronic and Steric Effects Control Regioselectivity in Alkyne Migratory Insertion during Ni-Catalyzed Indenone Synthesis

Many useful transition-metal-catalyzed reactions depend on the migratory insertion of an alkyne into a metal–carbon bond. Steric effects, electronic effects, and coordinating directing groups have all been shown to control product distributions in reactions in which this is the regioselectivity-determining step. This study describes how both steric and electronic parameters influence regioselectivity within a previously disclosed Ni-catalyzed indenone synthesis reaction. Steric repulsion determines the overall sense of regioselectivity, and to some extent the magnitude, while electronic effects can also significantly alter the magnitude of the observed regioselectivity. The relationship between alkyne insertion regioselectivity and the electronic character of the migrating aryl group is demonstrated via comparisons with established electronic substituent constants (σ+ values). These observations will aid in the understanding and development of other regioselective alkyne carbofunctionalization reactions.

Merging C–H Bond Activation, Alkyne Insertion, and Rearrangements by Rh(III)-Catalysis: Oxindole Synthesis from Nitroarenes and Alkynes

We report a Rh(III)-catalyzed ortho-C-H bond functionalization of nitroarenes with 1,2-diarylalkynes and carboxylic anhydrides. The reaction unpredictably affords 3,3-disubstituted oxindoles with the formal reduction of the nitro group under redox-neutral conditions. Besides good functional group tolerance, this transformation allows the preparation of oxindoles with a quaternary carbon stereocenter using nonsymmetrical 1,2-diarylalkynes. This protocol is facilitated by the use of a functionalized cyclopentadienyl (CpTMP*)Rh(III) [CpTMP* = 1-(3,4,5-trimethoxyphenyl)-2,3,4,5-tetramethylcyclopentadienyl] catalyst we developed, which combines an electron-rich character with an elliptical shape. Mechanistic investigations, including the isolation of three rhodacyle intermediates and extensive density functional theory calculations, indicate that the reaction proceeds through nitrosoarene intermediates via a cascade of C-H bond activation─O-atom transfer─[1,2]-aryl shift─deoxygenation─N-acylation.

Arylation and Alkyne Insertion to C-Acylimines: Rapid Access to 2-Trifluoromethylated and Other Fully Substituted Pyrroles in One Pot

AbstractA one-pot, four-component strategy for the synthesis of trifluoromethylated, and other fully-substituted pyrroles is reported using minimum loading of calcium catalyst at room temperature. A variety of arenes, α-keto aldehydes, amines, and activated alkynes took part in the reaction to produce high-yielding products. Friedel–Crafts arylation and aza-Michael addition are the key reactions in this approach.