Advances in chemoselective and/or stereoselective semihydrogenation of alkynes

Cooperative H2 activation at a nickel(0)-olefin centre

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H2) intermediate. Here we provide evidence for an alternative H2-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H2. The usefulness of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.

Recent advances in the selective semi-hydrogenation of alkyne to (E)-olefins

Considering the potential importance and upsurge in demand, the selective semi-hydrogenation of alkynes to (E)-olefins has attracted significant interest. This article highlights the recent advances in newer technologies and important methodologies directed to (E)-olefins from alkynes developed from 2015 to 2023. Notable features summarised include the catalyst or ligand design and control of product selectivity based on precious and nonprecious metal catalysts for semi-hydrogenation to (E)-olefins. Mechanistic studies for various catalytic transformations, including synthetic application to bioactive compounds, are summarised.

Spin-Crossing in the (Z)-Selective Alkyne Semihydrogenation Mechanism Catalyzed by Mo3S4 Clusters: A Density Functional Theory Exploration

Semihydrogenation of internal alkynes catalyzed by the air-stable imidazolyl amino [Mo3S4Cl3(ImNH2)3]+ cluster selectively affords the (Z)-alkene under soft conditions in excellent yields. Experimental results suggest a sulfur-based mechanism with the formation of a dithiolene adduct through interaction of the alkyne with the bridging sulfur atoms. However, computational studies indicate that this mechanism is unable to explain the experimental outcome: mild reaction conditions, excellent selectivity toward the (Z)-isomer, and complete deuteration of the vinylic positions in the presence of CD3OD and CH3OD. An alternative mechanism that explains the experimental results is proposed. The reaction begins with the hydrogenation of two of the Mo3(μ3-S)(μ-S)3 bridging sulfurs to yield a bis(hydrosulfide) intermediate that performs two sequential hydrogen atom transfers (HAT) from the S-H groups to the alkyne. The first HAT occurs with a spin change from singlet to triplet. After the second HAT, the singlet state is recovered. Although the dithiolene adduct is more stable than the hydrosulfide species, the large energy required for the subsequent H2 addition makes the system evolve via the second alternative pathway to selectively render the (Z)-alkene with a lower overall activation barrier.

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons

Alkyne semihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e-) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes. Mechanistic studies reveal that the transformation is concerted and Z-selective. Calculations support a ligand-based hydrogen-atom transfer pathway instead of a hydride mechanism, which is commonly invoked for transition metal hydrogenation catalysts. The synthesis of the proposed intermediates demonstrates that the catalytic mechanism proceeds through a reduced formal Ni(I) species. The high yields for terminal alkene products without over-reduction or oligomerization are among the best reported for any homogeneous catalyst. Furthermore, the metal-ligand cooperative hydrogen transfer enabled with this system directs the efficient flow of H atom equivalents toward alkyne reduction rather than hydrogen evolution, providing a blueprint for applying similar strategies toward a wide range of electroreductive transformations.

Selective photocatalytic semihydrogenation of alkynols to alkenols on Pd-C3N4 nanosheets under ambient conditions

Selective hydrogenation of alkynols to alkenols is an essential process for producing fine and intermediate chemicals. Currently, thermocatalytic alkynol hydrogenation faces several challenges, e.g., the safety of high-pressure hydrogen (H2) gas and the need for elevated temperature, and unavoidable side reactions, e.g., overhydrogenation. Here, a novel photocatalytic strategy is proposed for selectively reducing alkynols to alkenols with water as a hydrogen source under ambient temperature and pressure. Under the irradiation of simulated solar light, carbon nitride (C3N4) nanosheets with palladium (Pd) nanoparticles as cocatalysts (Pd-C3N4 NSs) exhibit a 2-methyl-3-butyn-2-ol (MBY) conversion of 98% and 2-methyl-3-buten-2-ol (MBE) selectivity of 95%, outperforming state-of-the-art thermocatalysts and electrocatalysts. After natural-sunlight irradiation (average light intensity of 25.13 mW cm-2) for 36 h, a MBY conversion of 98% and MBE selectivity of 92% was achieved in a large-scale photocatalytic system (2500 cm2). Experimental and theoretical investigations reveal that Pd cocatalysts on C3N4 facilitate the adsorption and hydrogenation of MBY as well as the formation of active hydrogen species, which promote the selective semihydrogenation of alkynols. Moreover, the proposed strategy is applicable to various water-soluble alkynols. This work paves the way for photocatalytic strategies to replace thermocatalytic hydrogenation processes using pressurized hydrogen.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

CS2/KOH System-Promoted Stereoselective Synthesis of (E)-Alkenes from Diarylalkynes and a "Hidden" Zinin-Type Reduction of Nitroarenes into Arylamines

In this work, we present the CS2/KOH system as a practical and efficient reductive medium for obtaining (E)-alkenes from alkynes through a highly stereoselective semireduction reaction. This cost-effective system enabled successful semireduction reactions of diverse alkynes using water as a hydrogen source, yielding moderate to excellent yields. The versatility of this protocol is further demonstrated through the synthesis of relevant compounds such as pinosylvin and resveratrol precursors, along with the notable anticancer agent DMU-212. Furthermore, during the reaction scope investigation, we serendipitously disclosed that this reductive system was also able to promote a Zinin-type reaction to reduce nitroarenes into arylamines.

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

Two bench-stable Fe(II) alkyl complexes [Fe(κ3PCP-PCP-iPr)(CO)2(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(κ3PCP-PCP-iPr)(CO)2(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ3PCP-PCP-iPr)(CO)2]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ3PCP-PCP-iPr)(CO)2(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.

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.

Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel-Bipyridine Complexes

Hydrogenation reactions of carbon-carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C-C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.

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.

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

Nickel(0) nanoparticles coordinated to NHC ligands bearing N-coordinated cinnamyl moieties were readily prepared by reduction of a [NiCpBr(NHC-cinnamyl)] complex with methyl magnesium bromide. The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticle aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.

Pairing d-Band Center of Metal Sites with π-Orbital of Alkynes for Efficient Electrocatalytic Alkyne Semi-Hydrogenation

Electrocatalytic alkyne semi-hydrogenation has attracted ever-growing attention as a promising alternative to traditional thermocatalytic hydrogenation. However, the correlation between the structure of active sites and electrocatalytic performance still remains elusive. Herein, the energy difference (∆ε) between the d-band center of metal sites and π orbital of alkynes as a key descriptor for correlating the intrinsic electrocatalytic activity is reported. With two-dimensional conductive metal organic frameworks as the model electrocatalysts, theoretical and experimental investigations reveal that the decreased ∆ε induces the strengthened d-π orbitals interaction, which thus enhances acetylene π-adsorption and accelerates subsequent hydrogenation kinetics. As a result, Cu₃ (HITP)₂ featuring the smallest ∆ε (0.10 eV) delivers the highest turnover frequency of 0.36 s^-1 , which is about 124 times higher than 2.9 × 10^-3  s^-1 for Co₃ (HITP)₂ with the largest ∆ε of 2.71 eV. Meanwhile, Cu₃ (HITP)₂ presents a high ethylene partial current density of -124 mA cm^-2 and a large ethylene Faradaic efficiency of 99.3% at -0.9 V versus RHE. This work will spark the rapid exploration of high-activity alkyne semi-hydrogenation catalysts.

Parts-Per-Million of Soluble Pd0 Catalyze the Semi-Hydrogenation Reaction of Alkynes to Alkenes

The synthesis of cis-alkenes is industrially carried out by selective semi-hydrogenation of alkynes with complex Pd catalysts, which include the Lindlar catalyst (PdPb on CaCO₃) and c-Pd/TiS (colloidal ligand-protected Pd nanoparticles), among others. Here, we show that Pd⁰ atoms are generated from primary Pd salts (PdCl₂, PdSO₄, Pd(OH)₂, PdO) with H₂ in alcohol solutions, independently of the alkyne, to catalyze the semi-hydrogenation reaction with extraordinarily high efficiency (up to 735 s^-1), yield (up to 99%), and selectivity (up to 99%). The easy-to-prepare Pd⁰ species hold other potential catalytic applications.

E-Selective semi-hydrogenation of alkynes via a sulfur-radical mediation over cyclodextrin-modified nickel nanocatalyst

An efficient cyclodextrin-modified Ni catalyst was developed for E-selective semi-hydrogenation of alkynes that takes into account for the highly active Hδ− and Hδ+, in situ formed Ni nanoparticles, and the host–guest interaction.

Stereoselective Semi-Hydrogenations of Alkynes by First-Row (3d) Transition Metal Catalysts

The chemo- and stereoselective semi-hydrogenation of alkynes to alkenes is a fundamental transformation in synthetic chemistry, for which the use of precious 4d or 5d metal catalysts is well-established. In mankind's unwavering quest for sustainability, research focus has considerably veered towards the 3d metals. Given their high abundancy and availability as well as lower toxicity and noxiousness, they are undoubtedly attractive from both an economic and an environmental perspective. Herein, we wish to present noteworthy and groundbreaking examples for the use of 3d metal catalysts for diastereoselective alkyne semi-hydrogenation as we embark on a journey through the first-row transition metals.

Iridium-Catalyzed Selective trans-Semihydrogenation of 1,3-Enynes with Ethanol: Access to (E,E)-1,4-Diarylbutadienes

A trans-semihydrogenation of 1,3-enynes with ethanol as the hydrogen source was developed using a new (PCN)Ir complex as the precatalyst and tBuNH₂ as the cocatalyst. This catalyst system provides an efficient and atom-economical access to unsymmetrical (E,E)-1,4-diarylbutadienes with high yields and stereoselectivities. Monitoring the process revealed that a sequence of cis-semihydrogenation of the triple bond of 1,3-enynes (to form (E,Z)-butadienes) and (E,Z)-to-(E,E) isomerization occurs to form (E,E)-butadienes.

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Catalytic semihydrogenation of internal alkynes using H₂ is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H₂-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

Effect of Iodide on the pH-Controlled Hydrogenations of Diphenylacetylene and Cinnamaldehyde Catalyzed by Ru(II)-Sulfonated Triphenylphosphine Complexes in Aqueous–Organic Biphasic Systems

The effect of NaI on hydrogenation of diphenylacetylene catalyzed by the water-soluble [{RuCl(mtppms-Na)2}2(µ-Cl)2] (1) (mtppms-Na = meta-monosulfonated triphenylphosphine sodium salt) is reported. Hydrogenations were performed under mild conditions (P(H2) = 1 bar, T = 50–80 ℃) in aqueous–organic biphasic reaction mixtures wherein the catalyst was dissolved in aqueous phase of various pHs. In acidic solutions, addition of NaI to 1 + mtppms-Na increased the selective conversion of diphenylacetylene to stilbenes from 10% to 90% but did not effect the high Z-selectivity (up to 98%). In contrast, in basic solutions the major product was diphenylethane (up to 70%), and the yield of E-stilbene exceeded that of the Z-isomer. 1H and 31P NMR measurements revealed that depending on the absence or presence of NaI, the catalytically active Ru(II)-hydride species in acidic solutions was [RuHCl(mtppms-Na)3], 2, or [RuHI(mtppms-Na)3], 5, respectively, while in basic solutions, both 2 and 5 were hydrogenated further to yield the same hydride species, cis,fac-[RuH2(H2O)(mtppms-Na)3]. [RuHI(mtppms-Na)3] proved superior to [RuHCl(mtppms-Na)3] as a catalyst for the selective hydrogenation of cinnamaldehyde to dihydrocinamaldehyde. This finding was explained by a facile formation of a (putative) dihydrogen complex [Ru(H2)I2(H2O)(mtppms-Na)2] intermediate, resulting in fast heterolytic activation of H2.

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

Catalytic and mechanistic studies of a highly active and E-selective Co(II) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Herein we report the application of a Co(II) PNNH pincer catalyst system (PNNH = 2-(5-(t-butyl)-1H-pyrazol-3-yl)-6-(dialkylphosphinomethyl)pyridine) for the highly E-selective transfer semihydrogenation of internal diaryl alkynes using methanol and ammonia borane...

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.

Base-Promoted Stereoselective Hydrogenation of Ynamides with Sulfonyl Hydrazide to Give Z-Enamides

A base-mediated semihydrogenation of ynamides using p-toluenesulfonyl hydrazide as an inexpensive and easy-to-handle hydrogen donor is reported. This transition-metal-free protocol avoids overhydrogenation and reduction of other functional groups, generating the thermodynamically unfavorable Z-enamides exclusively.

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.

A simple and efficient in situ generated copper nanocatalyst for stereoselective semihydrogenation of alkynes

Development of a simple, effective, and practical method for (Z)-selective semihydrogenation of alkynes has been considered necessary for easy-to-access applications at organic laboratory scales. Herein, (Z)-selective semihydrogenation of alkynes was achieved using a copper nanocatalyst which was generated in situ simply by adding ammonia borane to an ethanol solution of copper sulfate. Different types of alkynes including aryl-aryl, aryl-alkyl, and aliphatic alkynes were selectively reduced to (Z)-alkenes affording up to 99% isolated yield. The semihydrogenation of terminal alkynes to alkenes and gram-scale applications were also reported. In addition to eliminating catalyst preparation, the proposed approach is simple and practical and serves as a suitable alternative method to the conventional Lindlar catalyst.

A Smart Heterogeneous Catalyst for Efficient, Chemo- and Stereoselective Hydrogenation of 3-Hexyn-1-ol

We examine the easy preparation of mono- and bi-metallic heterogeneous catalysts with low Pd and Cu contents on alumina and provide a detailed study of many reaction parameters in the catalyzed selective semihydrogenation of 3-hexyn-1-ol to (Z)-3-hexen-1-ol, a very important fragrance with an herbaceous note. In particular, two different protocols of Pd catalyst preparation, substrate/catalyst molar ratio, the effect of time and temperature, introduction of some additives to the reaction mixture, and the nature of the solvent were investigated. These factors are not independent variables. The results show that it is possible to control the reaction outcome to obtain the target (Z)-alkenol using different experimental conditions. The best result, as an appropriate compromise between conversion and selectivity, may be obtained by working with a very high substrate/catalyst molar ratio (>6000/1), with one type of Pd catalyst, in a short time (about 150 min) at 60 °C.

Unusual behavior of bimetallic nanoparticles in catalytic processes of hydrogenation and selective oxidation

Recent results obtained in studying mono- and bimetallic catalysts for selective hydrogenation of unsaturated carbonyl compounds, even unsaturated ones, acetylenic and nitro compounds as well as CO and bio-available alcohols oxidation are reviewed from the standpoint of the strong interaction between the metal nanoparticles, on the one hand, and two metals in the composition of bimetallic nanoparticles, on the other hand. Such interactions were demonstrated to result in partial positive or negative charging of metal nanoparticles, which, in turn, changes their adsorption and catalytic properties, especially with respect to the reactions involving hydrogen. Among the systems studied, Au–Pt, Au–Pd, Au–Cu, Au–Fe, Pt–WO x , Fe–Pd, Fe–Pt, Fe–Cu nanoparticles prepared by the redox procedure are considered to be most perspective in diverse catalytic applications because of the proper combination of the particle size and the electronic state of the metals.

Stereo-controlled anti-hydromagnesiation of aryl alkynes by magnesium hydrides

A concise protocol for anti-hydromagnesiation of aryl alkynes was established using 1 : 1 molar combination of sodium hydride (NaH) and magnesium iodide (MgI₂) without the aid of any transition metal catalysts. The resulting alkenylmagnesium intermediates could be trapped with a series of electrophiles, thus providing facile accesses to stereochemically well-defined functionalized alkenes. Mechanistic studies by experimental and theoretical approaches imply that polar hydride addition from magnesium hydride (MgH₂) is responsible for the process.

Anti-hydromagnesiation of aryl alkynes was facilitated solely by magnesium hydride. The resulting alkenylmagnesium intermediates were functionalized with various electrophiles to afford stereochemically defined tri-substituted alkenes.

Rhodium(III) Catalyzed Regioselective and Stereospecific Allylic Arylation in Water by β-Fluorine Elimination of the Allylic Fluoride: Toward the Synthesis of Z-Alkenyl-Unsaturated Amides

A direct coupling of arylboronic acids with allylic fluorides was carried out in water without additives using a rhodium(III) catalyst (Cp*RhCl₂)₂. The transformation proceeded with excellent γ-selectivity to afford major allyl-aryl coupling products (Z) γ-substituted α,β-unsaturated amides. The reactions of α-chiral allylic fluorides took place with excellent α-to-γ chirality transfer to give allylated arenes with a stereogenic center at the benzylic and allylic position.