Transfer Semihydrogenation Research Articles

Cobalt-Catalyzed Chemo- and Stereoselective Transfer Semihydrogenation of 1,3-Dienes with Water as a Hydrogen Source.

(Z)-1,2-Disubstituted, trisubstituted, and tetrasubstituted alkenes are not only important units in medicinal chemistry, natural product synthesis, and material science but also useful intermediates in organic synthesis. Development of catalytic stereoselective transformations to access multisubstituted alkenes with various substitution patterns from easily accessible modular starting materials and readily available catalysts is a crucial goal in the field of catalysis. Water is an ideal hydrogen source for catalytic transfer hydrogenation despite of the high difficulty to activate water. Here, we report a cobalt-catalyzed protocol for regio- and stereoselective transfer semihydrogenation of 1,3-dienes to construct a broad scope of (Z)-1,2-disubstituted, (Z)-, (E)-trisubstituted, and tetrasubstituted alkenes in high stereoselectivity with H2O as the hydrogen source. Mechanistic studies revealed that the reactions proceeded through a unique Co(I)/Co(III) cycle and involved a 1,4-cobalt shift process, which is an unprecedented reaction pathway, providing a new platform for modular synthesis of multisubstituted alkenes as well as opportunities for designing novel reaction modes and pushing forward the advancement in organocobalt chemistry.

Alloying and confinement effects on hierarchically nanoporous CuAu for efficient electrocatalytic semi-hydrogenation of terminal alkynes

Electrocatalytic alkynes semi-hydrogenation to produce alkenes with high yield and Faradaic efficiency remains technically challenging because of kinetically favorable hydrogen evolution reaction and over-hydrogenation. Here, we propose a hierarchically nanoporous Cu50Au50 alloy to improve electrocatalytic performance toward semi-hydrogenation of alkynes. Using Operando X-ray absorption spectroscopy and density functional theory calculations, we find that Au modulate the electronic structure of Cu, which could intrinsically inhibit the combination of H* to form H2 and weaken alkene adsorption, thus promoting alkyne semi-hydrogenation and hampering alkene over-hydrogenation. Finite element method simulations and experimental results unveil that hierarchically nanoporous catalysts induce a local microenvironment with abundant K+ cations by enhancing the electric field within the nanopore, accelerating water electrolysis to form more H*, thereby promoting the conversion of alkynes. As a result, the nanoporous Cu50Au50 electrocatalyst achieves highly efficient electrocatalytic semi-hydrogenation of alkynes with 94% conversion, 100% selectivity, and a 92% Faradaic efficiency over wide potential window. This work provides a general guidance of the rational design for high-performance electrocatalytic transfer semi-hydrogenation catalysts.

Transfer semi-hydrogenation of terminal alkynes with a well-defined iron complex.

The synthesis and characterization of a bis-iron(II) complex was accomplished upon treatment of a phosphine free NNN-pincer ligand (L) with FeCl2·4H2O under ambient conditions. The deep greenish colored iron(II) complex (Fe-1) was characterized by a single-crystal X-ray diffraction study along with IR spectroscopy, UV-Vis spectroscopy, mass spectrometry, and elemental analysis. The Fe-1 complex was tested for the transfer semi-hydrogenation of terminal alkynes to the corresponding alkenes through the dehydrogenation of dimethyl amine-borane. This procedure enables the conversion of various structurally different terminal alkynes to alkenes under mild conditions. Control experiments were performed to shed light on the possible intermediates generated during the present protocol.

Electron-deficient Pd nanoparticles on nitrogen-doped carbon nanofibers for high selectivity alkyne transfer semi-hydrogenation

The catalytic activity and stability of Pd nanoparticles on nitrogen-doped carbon nanofibers for the semi-hydrogenation of phenylacetylene.

Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi-Hydrogenation of Alkynes.

We report a selectivity-switchable nickel hydride-catalyzed methodology that enables the stereocontrolled semi-reduction of internal alkynes to E- or Z-alkenes under very mild conditions. The proposed transfer semi-hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo-induced generation of nickel hydride, syn-hydronickelation of alkyne, and alkenylnickel isomerization as key steps. Remarkably, mechanistic experiments indicate that the control of the stereoselectivity is not ensuing from a post-reduction alkene photoisomerization under our conditions. Instead, we demonstrate that the stereoselectivity of the reaction is dependent on the rate of a final protonolysis step which can be tuned by adjusting the pKa of an alcohol additive.

In Situ Generation of Copper Nanoparticle‐Graphitic Carbon Nitride Composite for Stereoselective Transfer Semihydrogenation of Alkynes: Evaluation of Catalytic Activity Using Fluorescence‐Based High‐Throughput Screening

AbstractHerein, we present the effective synthesis of a copper nanoparticle‐graphitic carbon nitride (g‐C3N4) composite for the efficient (Z)‐selective transfer semihydrogenation of alkynes. A fluorescence‐based high‐throughput screening method was applied to identify the most synergistic support for in situ‐generated copper nanoparticles. The derived Cu(OAc)2/g‐C3N4 system exhibited more than five‐fold increase in the turnover frequency compared to the copper nanoparticle catalyst in the absence of g‐C3N4. Various alkynes were selectively reduced to the corresponding (Z)‐alkenes under mild reactions conditions, affording up to 99% isolated yields. The present work demonstrates a simple and cost‐effective method for the synthesis of a copper nanoparticle‐g‐C3N4 composite for the efficient (Z)‐selective transfer semihydrogenation of alkynes.magnified image

Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water

A base-assisted metal species modulation mechanism enables Ni-catalyzed stereodivergent transfer semihydrogenation of alkynes with water, delivering both olefinic isomers smoothly using cheap and nontoxic catalysts and additives. Different from most precedents, in which E-alkenes derive from the isomerization of Z-alkene products, the isomers were formed in orthogonal catalytic pathways. Mechanistic studies suggest base as a key early element in modulation of the reaction pathways: by adding different bases, nickel species with disparate valence states could be accessed to initiate two catalytic cycles toward different stereoisomers. The practicability of the method is showcased with nearly 70 examples, including internal and terminal triple bonds, enynes and diynes, affording semi-hydrogenated products in high yields and selectivity.

Enhanced Alkene Selectivity for Transfer Semihydrogenation of Alkynes over Electron-Deficient Pt Nanoparticles Encapsulated in Hollow Silica Nanospheres.

In this work, we report that Pt nanoparticles confined in hollow porous silica nanospheres (Pt@HPSNs) function as highly selective catalysts for the transfer hydrogenation of phenylacetylene to styrene with ammonia borane. Relative to the deep hydrogenation of phenylacetylene to ethylbenzene over the supported Pt/SiO2, Pt@HPSNs exhibit above 88% of styrene selectivity at nearly 100% of phenylacetylene conversions, and the high selectivity of Pt@HPSNs can be maintained even at high ammonia borane/phenylacetylene ratios and longer reaction time. The Pt 4f X-ray photoelectron spectrum of Pt@HPSNs shows a remarkable ∼1.5 eV shift to high binding energy, proving the nature of electron deficiency of such encapsulated Pt nanoparticles. Combined with extremely minor transfer hydrogenation of styrene to ethylbenzene when styrene as substrates, the enhanced styrene selectivity of Pt@HPSNs is ascribed to the electron deficiency of encapsulated Pt nanoparticles, which leads to the fast desorption of styrene and thus avoids deep hydrogenation.

A ligand-free in situ-generated cobalt nanoparticle catalyst for (Z)-selective transfer semihydrogenation of alkynes

A simple and efficient approach for the (Z)-selective transfer semihydrogenation of alkynes based on in situ-generated cobalt nanoparticles (CoNPs) and ammonia-borane (NH3·BH3).

Unveiling subsurface hydrogen inhibition for promoting electrochemical transfer semihydrogenation of alkynes with water

AbstractHighly selective electrocatalytic semihydrogenation of alkynes to alkenes with water as the hydrogen source over palladium-based electrocatalysts is significant but remains a great challenge because of the excessive hydrogenation capacity of palladium. Here, we propose that an ideal palladium catalyst should possess weak alkene adsorption and inhibit subsurface hydrogen formation to stimulate the high selectivity of alkyne semihydrogenation. Therefore, sulfur-modified Pd nanowires (Pd-S NWs) are designedly prepared by a solid-solution interface sulfuration method with KSCN as the sulfur source. The introduction of S weakens the alkene adsorption and prevents the diffusion of active hydrogen (H*) into the Pd lattice to form unfavorable subsurface H*. As a result, electrocatalytic alkyne semihydrogenation is achieved over a Pd-S NWs cathode with wide substrate scopes, potential-independent up to 99% alkene selectivity, good fragile groups compatibility, and easily synthesized deuterated alkenes. An adsorbed hydrogen addition mechanism of this semihydrogenation reaction is proposed. Importantly, an easy modification of commercial Pd/C by in situ addition of SCN− enabling the gram-scale synthesis of an alkene with 99% selectivity and 95% conversion highlights the promising potential of our method.

Ligand-to-metal ratio controls stereoselectivity: Highly functional-group-tolerant, iridium-based, (E)-selective alkyne transfer semihydrogenation

Herein, we present ( E )-selective transfer semihydrogenation of alkynes based on an iridium complex in situ generated from [Ir(COD)Cl] 2 and an unsymmetrical (bearing two different phosphorous centers), ferrocene-based phosphine ligand utilizing formic acid as a hydrogen donor. Interestingly, a ligand-to-metal ratio may be used to control the stereoselectivity of the semihydrogenation process: a ratio of 1:1 iridium to ligand led to the formation of ( Z )-alkene as a major product, whereas a ratio of 1:2 gave exclusively ( E )-alkene. The latter 1:2 catalytic system is distinguished by its unprecedented chemoselectivity and exceptional stereoselectivity, which is substantiated by the broad scope of tested substrates, including natural product derivatives. The intriguing difference in catalytic activity between unsymmetrical and symmetrical ferrocene-based ligands was attributed to divergent coordination and steric hindrance. The presented methodology constitutes a solution to the common limitations of the known catalytic systems for semihydrogenation. • Excellent stereoselectivity and functional-group tolerance • Stereoselectivity controlled by metal-to-ligand ratio • Catalytic activity affected by ligand symmetry Olefins are ubiquitous organic compounds which contain unsaturated double C–C bonds, and they may be found in many natural products and pharmaceuticals. Alkenes isomerism plays a vital role in the biological activity of drugs, to give one example. Thus, selective methods of their synthesis are highly coveted. Herein, we provide an unprecedented alternative to the other known catalytic systems for alkynes semihydrogenation, of which stereoselectivity may be controlled by the metal-to-ligand ratio. The ( E )-selective variant is distinguished by excellent stereo- and chemoselectivity. Reducible and problematic functionalities are tolerated under reductive conditions, and this makes our methodology a perfect tool for the late-stage functionalization of organic compounds. The environmentally friendly character of this catalytic system is proven by its efficiency and chemoselectivity, which allow one to avoid the use of protecting groups and allow the application of formic acid as a green and safe hydrogen donor. An iridium-based catalytic system for ( E )-selective transfer semihydrogenation of alkynes was developed. A unique feature of this method is the possibility of stereoselectivity control by the metal-to-ligand ratio. The catalytic system is distinguished by high functional-group tolerance, making it a valuable tool for organic synthesis. The observed intriguing dependence of ligand symmetry on catalytic activity may pave the way for efficient development of catalysts.

Copper-Catalyzed Selective Transfer Semihydrogenation of Alkynes

Copper-Catalyzed Selective Transfer Semihydrogenation of Alkynes

Manganese-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes with iPrOH as hydrogen source

The Mn-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes is reported herein, along with Mn-catalyzed hydration of α-keto alkynes. Mechanistic studies displayed an asymmetrical Mn-hydride species performing the catalytic turnover.

Selenium Vacancy Promotes Transfer Semihydrogenation of Alkynes from Water Electrolysis

Developing an efficient strategy for the highly selective transformation of alkynes to alkenes over a cost-effective catalyst using a cheap and safe hydrogen donor under ambient conditions is markedly desirable. Here self-supported Ni0.85Se nanowires with selenium vacancies (denoted as Ni0.85Se1–x) have been synthesized to enable the transfer semihydrogenation of alkynes with high conversion efficiency and selectivity. Theoretical results demonstrate that the accelerated electron transfer from the catalyst to the alkyne molecules and the much lower activation energy barrier of water electrolysis caused by the Se vacancy are conducive to alkyne semihydrogenation, with up to 99% conversion yield at a lower potential. In addition, the weak adsorption of alkenes and the thermodynamic constraints for overhydrogenation to alkanes are crucial in producing alkenes with 99% selectivity. A broad substrate scope consisting of aryl terminal/internal alkynes and aliphatic alkynes with high conversion yields and selectivity and the facile preparation of deuterated alkenes as versatile building blocks for deuterated synthesis by using D2O highlight the promising potential of our method.

Copper(0) nanoparticle catalyzed Z‐Selective Transfer Semihydrogenation of Internal Alkynes

AbstractThe use of copper(0) nanoparticles in the transfer semihydrogenation of alkynes has been investigated as a lead‐free alternative to Lindlar catalysts. A stereo‐selective methodology for the hydrogenation of internal alkynes to the corresponding (Z)‐alkenes in high isolated yields (86% average) has been developed. This green and sustainable transfer hydrogenation protocol relies on non‐noble copper nanoparticles for reduction of both electron‐rich and electron‐deficient, aliphatic‐substituted and aromatic‐ substituted internal alkynes. Polyols, such as ethylene glycol and glycerol, have been proven to act as hydrogen sources, and excellent stereo‐ and chemoselectivity have been observed. Enabling technologies, such as microwave and ultrasound irradiation are shown to enhance heat and mass transfer, whether used alone or in combination, resulting in a decrease in reaction time from hours to minutes.magnified image

Selective Transfer Semihydrogenation of Alkynes with H2O (D2O) as the H (D) Source over a Pd‐P Cathode

AbstractWe reported a selective semihydrogenation (deuteration) of numerous terminal and internal alkynes using H2O (D2O) as the H (D) source over a Pd‐P alloy cathode at a lower potential. P‐doping caused the enhanced specific adsorption of alkynes and the promoted intrinsic activity for producing adsorbed atomic hydrogen (H*ads) from water electrolysis. The semihydrogenation of alkynes could be accomplished at a lower potential with up to 99 % selectivity and 78 % Faraday efficiency of alkene products, outperforming pure Pd and commercial Pd/C. This electrochemical semihydrogenation of alkynes might proceed via a H*ads addition pathway rather than a proton‐coupled electron transfer process. The decreased amount of H*ads at a lower potential and the more preferential adsorption of the Pd‐P to C≡C π bond than C=C moiety resulted in the excellent alkene selectivity. This method was capable of producing mono‐, di‐, and tri‐deuterated alkenes with up to 99 % deuterium incorporation.

Selective Transfer Semihydrogenation of Alkynes with H2 O (D2 O) as the H (D) Source over a Pd-P Cathode.

We reported a selective semihydrogenation (deuteration) of numerous terminal and internal alkynes using H2 O (D2 O) as the H (D) source over a Pd-P alloy cathode at a lower potential. P-doping caused the enhanced specific adsorption of alkynes and the promoted intrinsic activity for producing adsorbed atomic hydrogen (H*ads ) from water electrolysis. The semihydrogenation of alkynes could be accomplished at a lower potential with up to 99 % selectivity and 78 % Faraday efficiency of alkene products, outperforming pure Pd and commercial Pd/C. This electrochemical semihydrogenation of alkynes might proceed via a H*ads addition pathway rather than a proton-coupled electron transfer process. The decreased amount of H*ads at a lower potential and the more preferential adsorption of the Pd-P to C≡C π bond than C=C moiety resulted in the excellent alkene selectivity. This method was capable of producing mono-, di-, and tri-deuterated alkenes with up to 99 % deuterium incorporation.

Cis‐Selective Transfer Semihydrogenation of Alkynes by Merging Visible‐Light Catalysis with Cobalt Catalysis

AbstractHerein, the first example of visible‐light‐driven, cobalt‐catalyzed transfer semihydrogenation of alkynes to alkenes is reported. It is carried out by using Ir[dF(CF3)ppy]2(dtbbpy)]PF6 as photosensitizer, CoBr2/n‐Bu3P as proton‐reducing catalyst, and i‐Pr2NEt/AcOH as the hydrogen source. Under the established catalytic system, the semihydrogenation proceeds with Z as the major selectivity and with inhibition of over‐reduction. Under mild reaction conditions, both internal and terminal alkynes, as well as reducible functional groups such as halogen, cyano, and ester, are tolerated. Preliminary mechanistic studies revealed the dual role of the photosensitizer in initiating the reaction via a single‐electron transfer process and controlling the stereoselectivity via an energy transfer process.magnified image

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.