Stereoselective Alkyne Hydrohalogenation by Trapping of Transfer Hydrogenation Intermediates

Electrochemical cobalt-catalyzed semi-deuteration of alkynes to access deuterated Z-alkenes

Deuterium labeling has found extensive applications across various research fields, including organic synthesis, drug design, and molecular imaging. Electrocatalytic semi-hydrogenation of alkynes offers a viable route for the synthesis of Z-alkenes, yet it falls short in achieving the semi-deuteration of these compounds. In this study, we report an electrochemical cobalt-catalyzed transfer deuteration reaction that proficiently accomplishes the semi-deuteration of alkynes, yielding Z-configuration deuterated alkene products. This reaction utilizes cost-effective cobalt salts as catalysts and employs D2O and AcOD (acetic acid-d) as economical and efficient deuterium sources, underscoring its practicality and feasibility. The reaction demonstrates a broad alkyne substrate scope, high reaction efficiency, good functional group compatibility, excellent Z-selectivity, and a remarkable degree of deuteration rate.

Ligand-Controlled Regioselective Alkoxycarbonylation of Nonfunctionalized Unsymmetric Internal Alkynes

Pd-catalyzed alkoxycarbonylation of internal alkynes provides a straightforward access to α,β-disubstituted acrylic esters. Compared with the well-established regioselective alkoxycarbonylation of terminal alkynes, the regioselective hydrocarboxylation of non-functionalized unsymmetric internal alkynes was more challenging owing to the delicate differences of properties between the two substituents. Herein, by using either monophosphine ligand based on 2,3-dihydrobenzo[d][1,3]oxaphosphole motif or bidentate ligand Ph-Phox, the regioselective alkoxycarbonylations of aryl-aryl, aryl-alkyl and alkyl-alkyl disubstituted alkynes were achieved, giving a diversity of trisubstituted α,β-unsaturated carboxylic esters with moderate to excellent yields and high regioselectivity. The synthetic utility of obtained α,β-disubstituted acrylic esters was demonstrated.

Manganese catalysed reduction of nitriles with amine boranes

The room temperature reduction of various nitriles using amine boranes (ABs) catalysed by a manganese(i) alkyl complex is described. Based on experimental findings, a plausible mechanistic scenario is presented. This includes the presence of two catalytic cycles, one for productive reduction of nitriles and one for hydrogen evolution.

Lewis acid catalyzed Markovnikov hydrobromination and hydrochlorination of alkynes using TMSX (X = Br, Cl)

Markovnikov hydrobromination and hydrochlorination of alkynes were achieved using TMSX (X = Br, Cl) instead of corrosive HX (X = Br, Cl) as the bromination and chlorination reagents. Mn(OAc)2·4H2O was used as the hydrobromination catalyst for electron-neutral/rich alkynes. For the hydrobromination of electron-deficient alkynes and hydrochlorination of alkynes, Zn(OAc)2·2H2O was employed as the catalyst. Mechanistic studies suggested that the in situ formed TMS-substituted alkyne might be a reactive intermediate and the proton of the terminal alkyne should be a hydrogen source for the hydrohalogenation reaction.

Stereoselective Synthesis of Trisubstituted Alkenes via Copper Hydride-Catalyzed Alkyne Hydroalkylation

Alkenes are ubiquitous in organic chemistry, yet many classes of alkenes remain challenging to access by current synthetic methodology. Herein, we report a copper hydride-catalyzed approach for the synthesis of Z-configured trisubstituted alkenes with high stereo- and regioselectivity via alkyne hydroalkylation. A DTBM-dppf-supported Cu catalyst was found to be optimal, providing a substantial increase in product yield compared to reactions conducted with dppf as the ligand. DFT calculations show that the DTBM substitution leads to the acceleration of alkyne hydrocupration through combined ground and transition state effects related to preventing catalyst dimerization and enhancing catalyst-substrate dispersion interactions, respectively. Alkyne hydroalkylation was successfully demonstrated with methyl and larger alkyl tosylate electrophiles to produce a variety of (hetero)aryl-substituted alkenes in moderate to high yields with complete selectivity for the Z stereochemically configured products. In the formation of the key C-C bond, computational studies revealed a direct SN2 pathway for alkylation of the vinylcopper intermediate with in situ-formed alkyl iodides.

Precision Deuteration Using Cu-Catalyzed Transfer Hydrodeuteration to Access Small Molecules Deuterated at the Benzylic Position

A highly regio- and chemoselective Cu-catalyzed aryl alkyne transfer hydrodeuteration to access a diverse scope of aryl alkanes precisely deuterated at the benzylic position is described. The reaction benefits from a high degree of regiocontrol in the alkyne hydrocupration step, leading to the highest selectivities reported to date for an alkyne transfer hydrodeuteration reaction. Only trace isotopic impurities are formed under this protocol, and analysis of an isolated product by molecular rotational resonance spectroscopy confirms that high isotopic purity products can be generated from readily accessible aryl alkyne substrates.

Structural snapshots of an Al-Cu bond-mediated transformation of terminal acetylenes

The copper(i) alumanyl derivative, [{SiN^Dipp}Al-Cu(NHC^iPr)] (SiN^Dipp = {CH₂SiMe₂NDipp}₂; Dipp = 2,6-di-isopropylphenyl; NHC^iPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), reacts in a stepwise fashion with up to three equivalents of various terminal alkynes. This reactivity results in the sequential formation of cuprous (hydrido)(alkynyl)aluminate, (alkenyl)(alkynyl)aluminate and bis(alkynyl)aluminate derivatives, examples of which have been fully characterised. The process of alkene liberation resulting from the latter reaction step constitutes a unique case of alkyne transfer semi-hydrogenation in which the C-H acidic alkyne itself acts as a source of proton, with the Cu-Al bond providing the requisite electrons to effect reduction. This reaction sequence is validated by DFT calculations, which rationalise the variable stability of the initially formed heterobimetallic hydrides.

Rhodium-Catalyzed Anti-Markovnikov Transfer Hydroiodination of Terminal Alkynes

A rhodium-catalyzed anti-Markovnikov hydroiodination of aromatic and aliphatic terminal alkynes is reported. Depending on the choice of ligand and substrate, either (E)- or (Z)-configured alkenyl iodides are obtained in high to exclusive isomeric purity. The reaction exhibits a broad substrate scope and high functional group tolerance, employing easily accessible or commercially available aliphatic iodides as HI surrogates through a shuttle process. The synthesized vinyl iodides were applied in several C-C and C-heteroatom bond-forming reactions with full retention of the stereoselectivity. The developed method could be used to significantly shorten the total synthesis of a marine cis-fatty acid. Additionally, initial deuterium-labeling experiments and stoichiometric reactions shed some light on the potential reaction mechanism.

Ruthenium-Catalyzed Regioselective Hydrohalogenation of Alkynes Mediated by Trimethylsilyl Triflate

Here we describe a ruthenium-catalyzed regioselective hydrohalogenation reaction of alkynes under mild conditions. Commercially simple halogen sources such as KI, ZnBr₂, and ZnCl₂ were employed to achieve this transformation. Alkynes derived from bioactive molecules such as l-(-)-borneol, l-menthol, and estrone were also suitable for the transformation, demonstrating the potential synthetic value of this new reaction in organic synthesis.

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO2Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion-interaction analysis.

Regio- and Stereoselective Hydroiodination of Internal Alkynes with Ex Situ-Generated HI

Herein, we report an efficient and practical hydroiodination of internal alkynes using HI generated ex situ from the readily available triethylsilane and I₂. This system offers high regio- and stereoselectivity to afford (E)-vinyl iodides in good yields under mild conditions. Furthermore, the hydroiodination reaction shows high functional group tolerance toward alkyl, methoxy, halogen, trifluoromethyl, cyano, ester, halomethyl, acid-sensitive silyl ether, and acetal moieties.

Hydroalkylation of Alkynes: Functionalization of the Alkenyl Copper Intermediate through Single Electron Transfer Chemistry

We have developed a method for the stereoselective coupling of terminal alkynes and α-bromo carbonyls to generate functionalized E-alkenes. The coupling is accomplished by merging the closed-shell hydrocupration of alkynes with the open-shell single electron transfer (SET) chemistry of the resulting alkenyl copper intermediate. We demonstrate that the reaction is compatible with various functional groups and can be performed in the presence of aryl bromides, alkyl chlorides, alkyl bromides, esters, nitriles, amides, and a wide range of nitrogen-containing heterocyclic compounds. Mechanistic studies provide evidence for SET oxidation of the alkenyl copper intermediate by an α-bromo ester as the key step that enables the cross coupling.

CuCl2-catalyzed highly stereoselective and chemoselective reduction of alkynyl amides into α,β-unsaturated amides using silanes as hydrogen donors

A CuH-catalyzed Z-selective partial reduction of alkynyl amides to afford α,β-unsaturated amides using silane as the hydrogen donor is developed. This reaction is carried out under mild conditions and able to accommodate a broad scope of alkynyl amides including those bearing a terminal carbon-carbon double bond or triple bond, affording alkenyl amides with high stereoselectivity and excellent yields.

Zirconium-hydride-catalyzed transfer hydrogenation of quinolines and indoles with ammonia borane

Herein, by applying zirconium-hydride complex as the catalyst, the transfer hydrogenation of quinoline and indole derivatives with ammonia borane as a proton and hydride source is achieved.

Radical 1,2-addition of bromoarenes to alkynes via dual photoredox and nickel catalysis

A 1,2-addition of aryl bromides to alkynes enabled by the photocatalytic generation of bromine radicals via photoredox and nickel catalysis is reported.

Catch It If You Can: Copper-Catalyzed (Transfer) Hydrogenation Reactions and Coupling Reactions by Intercepting Reactive Intermediates Thereof

The key reactive intermediate of copper(I)-catalyzed alkyne semihydrogenations is a vinylcopper(I) complex. This intermediate can be exploited as a starting point for a variety of trapping reactions. In this manner, an alkyne semihydrogenation can be turned into a dihydrogen­-mediated coupling reaction. Therefore, the development of copper-catalyzed (transfer) hydrogenation reactions is closely intertwined with the corresponding reductive trapping reactions. This short review highlights and conceptualizes the results in this area so far, with H2-mediated carbon–carbon and carbon–heteroatom bond-forming reactions emerging under both a transfer hydrogenation setting as well as with the direct use of H2. In all cases, highly selective catalysts are required that give rise to atom-economic multicomponent coupling reactions with rapidly rising molecular complexity. The coupling reactions are put into perspective by presenting the corresponding (transfer) hydrogenation processes first.

1 Introduction: H2-Mediated C–C Bond-Forming Reactions

2 Accessing Copper(I) Hydride Complexes as Key Reagents for Coupling Reactions; Requirements for Successful Trapping Reactions

3 Homogeneous Copper-Catalyzed Transfer Hydrogenations

4 Trapping of Reactive Intermediates of Alkyne Transfer Semi­hydrogenation Reactions: First Steps Towards Hydrogenative Alkyne Functionalizations

5 Copper(I)-Catalyzed Alkyne Semihydrogenations

6 Copper(I)-Catalyzed H2-Mediated Alkyne Functionalizations; Trapping of Reactive Intermediates from Catalytic Hydrogenations

6.1 A Detour: Copper(I)-Catalyzed Allylic Reductions, Catalytic Generation of Hydride Nucleophiles from H2

6.2 Trapping with Allylic Electrophiles: A Copper(I)-Catalyzed Hydro­allylation Reaction of Alkynes

6.3 Trapping with Aryl Iodides

7 Conclusion

Iridium-Catalyzed Hydrochlorination and Hydrobromination of Alkynes by Shuttle Catalysis

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium-catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4-chlorobutan-2-one or tert-butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional-group tolerance and safety profile of these reactions compared to the state-of-the-art. This method has granted access to alkenyl halide compounds containing acid-sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram-scale synthesis where low catalyst loading was achieved.

Homogeneous catalytic transfer semihydrogenation of alkynes – an overview of hydrogen sources, catalysts and reaction mechanisms

Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks.

Ammonia borane as a reducing agent in organic synthesis

Ammonia borane is gaining increasing attention as a sustainable and atom-economical winning reagent for the reduction of several substrates.