An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkyne cis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis

Methyl Formate, an Alternative Transfer Hydrogenating Agent for Chemoselective Reduction of N-Heteroarenes and Azoarenes

The search for efficient molecular hydrogen precursors and their catalytic exploration is necessary for the evolution of catalytic transfer hydrogenation. Methyl formate (MF) having high hydrogen content still remains unexplored for such transformations. Herein, we disclosed a bifunctional Ir(III)-complex catalyzed chemoselective TH protocol for N-heteroarenes and azoarenes using MF. A variety of substrates including ten bioactive molecules have been synthesized under mild reaction conditions. A probable mechanistic pathway was proposed based on control experiments and mechanistic studies.

Membrane-Free Selective Semi-Hydrogenation of Alkynes Over an In Situ Formed Copper Nanoparticle Electrode

Selective semi-hydrogenation of alkynes is a significant reaction for preparing functionalized alkenes. Electrochemical semi-hydrogenation presents a sustainable alternative to the traditional thermal process. In this research, affordable copper acetylacetonate is employed as a catalyst precursor for the electrocatalytic hydrogenation of alkynes, using MeOH as the hydrogen source in an undivided cell. Good to excellent yields for both aromatic and aliphatic internal/terminal alkynes are obtained under constant current conditions. Notably, up to 99% Z selectivity is achieved for various internal alkynes. Mechanistic investigations revealed the formation of copper nanoparticles (NPs) at the cathode during electrolysis, acting as the catalyst for the selective semireduction of alkynes. The copper NPs deposited cathode demonstrated reusable for further hydrogenation.

Protocol for (E)-selective semihydrogenation of alkynes using iridium-based catalyst

In this protocol, we describe highly (E)-selective alkyne semihydrogenation employing commercially available iridium complex and a bidentate phosphine ligand. We provide steps for (E)-stilbene synthesis, remaining formic acid neutralization, and determination of the (Z)/(E) ratio using gas chromatography analysis. We then detail (E)-stilbene purification using a short pad of silica and NMR analysis. The protocol is compatible with a wide range of functionalities and different types of alkynes. For complete details on the use and execution of this protocol, please refer to Kusy et al. (2022).1.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Fluorescent α-amino acids via Heck-Matsuda reactions of phenylalanine-derived arenediazonium salts

The Heck-Matsuda coupling reaction of arenediazonium salts derived from L-phenylalanine with various alkenes has been developed. A two-step process involving the preparation of a tetrafluoroborate diazonium salt from a 4-aminophenylalanine derivative, followed by a palladium(0)-catalysed Heck-Matsuda coupling reaction allowed access to a range of unnatural α-amino acids with cinnamate, vinylsulfone and stilbene side-chains. Analysis of the photophysical properties of these unnatural α-amino acids demonstrated that the (E)-stilbene analogues exhibited fluorescent properties with red-shifted absorption and emission spectra and larger quantum yields than L-phenylalanine.

H2O as the Hydrogen Donor: Stereo-Selective Synthesis of E- and Z-Alkenes by Palladium-Catalyzed Semihydrogenation of Alkynes

It is very desirable to develop a facile controllable method for selective semihydrogenation of alkynes to alkenes with a cheap and safe hydrogen donor but remains a big challenge. H₂O is one of the best choices of the transfer hydrogenation agent of the world, and the development of methods for synthesizing E- and Z-alkenes using H₂O as the hydrogen source is worthwhile. In this article, a palladium-catalyzed synthesis of E- and Z-alkenes from alkynes using H₂O as the hydrogenation agent was reported. The use of di-tert-butylphosphinous chloride (t-Bu₂PCl) and triethanolamine/sodium acetate (TEOA/NaOAc) was essential for the stereo-selective semihydrogenation of alkynes. The general applicability of this procedure was highlighted by the synthesis of more than 48 alkenes, with good yields and high stereoselectivities.

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.

Mechanistic Investigation into the Regio-Controllable Hydroallylations of Alkynes with Allylborons under Pd-Based Synergetic Catalyses

Density functional theory calculations were employed to investigate the Pd-catalyzed regio-selective hydroallylations of alkynes with allylborons: cooperation of Cu(OAc)₂ and dppe resulting in 1,4-dienes while combination of AdCO₂H and PCy₃ leading to 1,5-dienes. A unified rationalization mechanism called "Lewis-acid-base-interaction promoted deprotonation/3,3-rearrangement" was proposed. Compared with the commonly reported metathesis pathway to only afford the metal-allyl intermediate, in the newly established mechanism, an additional Brønsted acid (as an initiator of the Pd⁰ oxidative addition) is generated by the interaction of the allylboron (Lewis acid) B atom with the ⁿBuOH (Lewis base) O atom, and subsequent 3,3-rearrangement ensures the thermodynamic feasibility of the reaction. In addition, it was found that excess Cu(OAc)₂ plays two potential roles in the oxidative addition/alkyne insertion: (i) the participation of one AcO^- of Cu(OAc)₂ ensures a large orbital overlap between the migrating H and Pd atoms, facilitating the formal AcO-H cleavage and (ii) the extra (OAc)₂Cu···O(carboxyl) σ-coordination indirectly contributes to the (Me)C≡C(Ph) insertion into the Pd-H bond. Further analysis showed that the origin of the regioselectivity is closely related to the employed phosphorus ligand. These revealed results, which have been overlooked in the previous documents, would aid the development of new related catalytic reactions.

Advances in Selective Electrocatalytic Hydrogenation of Alkynes to Alkenes

Selective electrochemical hydrogenation of alkynes to alkenes under ambient conditions is a promising alternative to the traditional energy-intensive and high-cost thermocatalytic hydrogenation. However, the systematic summary on the electrocatalysts and electrolyzers remains lacked. Herein, we demonstrate a comprehensive review about recent achievements in the electrocatalysts including noble metal and non-noble-metal materials. Several effective strategies of catalyst design were developed to improve alkyne conversion, and alkene selectivity, for example, accelerating the formation of active hydrogen species, enhancing alkyne adsorption and suppressing the side reactions. Furthermore, the advantages and disadvantages of various electrolyzers are systematically discussed. Accordingly, major challenges and future trends in this field are proposed.

Computational Mechanism Investigation of C=C Bond Hydrogenation Catalyzed by Rhodium Hydride

The hydrogenation of unsaturated carbons is a commonly used synthetic tool in pharmaceutical and industrial production. Recently, the Norton group realized highly selective hydrogenation of C=C bonds catalyzed by a rhodium hydride. Despite the great efforts made by experimentalists, details regarding the mechanism remained unclear. In this work, detailed DFT calculations were carried out to elucidate the principal features of this transformation. For enones we find that two possible competing mechanisms proposed by the experimental groups are computationally excluded, our proposed alternative mechanism with a total barrier of 20.0 kcal mol^-1 is theoretically feasible, solvent methanol to also plays a crucial role in assisting β-hydrogenation in addition to being the hydrogen source for α-hydrogenation, and the cross-polarization of the substrate enone-conjugated system to result in an enhanced charge density of the α-carbon, which favors being hydrogenated first. For isolated alkenes, neither of the two possible competing mechanisms can be excluded computationally and which carbon atom is first hydrogenated depends on the electronic properties of the substrate itself. The combination of rhodium and C=C bonds changes the electronic properties of H on the rhodium hydride and enhances its hydrogenation activity.

Palladium-catalyzed transfer hydrogenation of terminal alkynes using ethanol as the hydrogen donor

A ligand-promoted, palladium-catalyzed transfer hydrogenation of terminal alkynes using ethanol as the hydrogen donor is developed. The chemical selectivity control is achieved based on ligand regulation. The use of triethanolamine and tetrahydrofuran at 80 °C under N2 is found to be critical for the transfer hydrogenation of terminal alkynes. The general applicability of this procedure is highlighted by the synthesis of 27 terminal alkenes in moderate to good yields.

Magnesium Pincer Complexes and Their Applications in Catalytic Semihydrogenation of Alkynes and Hydrogenation of Alkenes: Evidence for Metal-Ligand Cooperation

The development of catalysts for environmentally benign organic transformations is a very active area of research. Most of the catalysts reported so far are based on transition-metal complexes. In recent years, examples of catalysis by main-group metal compounds have been reported. Herein, we report a series of magnesium pincer complexes, which were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ via aromatization/dearomatization metal-ligand cooperation was studied. Utilizing the obtained complexes, the unprecedented homogeneous main-group metal catalyzed semihydrogenation of alkynes and hydrogenation of alkenes were demonstrated under base-free conditions, affording Z-alkenes and alkanes as products, respectively, with excellent yields and selectivities. Control experiments and DFT studies reveal the involvement of metal-ligand cooperation in the hydrogenation reactions. This study not only provides a new approach for the semihydrogenation of alkynes and hydrogenation of alkenes catalyzed by magnesium but also offers opportunities for the hydrogenation of other compounds catalyzed by main-group metal complexes.

A New Paradigm in Pincer Iridium Chemistry: PCN Complexes for (De)Hydrogenation Catalysis and Beyond

The discovery and development of organometallic catalysts is of paramount importance in modern organic synthesis, among which the ligand scaffolds play a crucial role in controlling the activity and selectivity. Over the past several decades, d⁸ transition-metal complexes of pincer ligands have been developed extensively thanks to their easy structural modification, versatile reactivities, and high stability. One paradigm is the bis(phosphine)-based pincer iridium complexes PCP-Ir, which are highly active for alkane dehydrogenation, partly due to their high thermostability. However, except for alkane dehydrogenation and related transformations, few applications of pincer iridium catalysis have been seen in organic synthesis. This mainly arises from the low functional-group compatibility and poor substrate scope and the limited catalytic chemistry that invariably involves Ir(I/III) redox processes initiated by oxidative addition of substrates to 14-electron (PCP)Ir fragments (the proposed catalytically active intermediates). In this Account, we describe our endeavor on the development of a new family of PCN-Ir complexes with initial intention on creating more efficient alkane dehydrogenation catalysts. The replacement of a soft, σ-donor phosphine arm in the PCP ligands by a harder, π-acceptor N-heteroarene (pyridine or oxazoline) not only provides an additional platform to modify the structural properties but also offers new modes of bond activation and novel reactivities and catalysis. One uniqueness of the PCN-Ir system lies in the formation, via ortho-C(sp²)-H cyclometalation of the pyridine unit in the PCN_Py ligand, of the neutral monohydride (PCC)Ir^IIIHL (L = neutral ligand), which catalyzes positional and stereoselective 1-alkene-to-(E)-2-alkene isomerization. Moreover, the PCN-Ir catalysts effect ethanol dehydrogenation without decarbonylation, allowing for transfer hydrogenation of unactivated alkenes and trans-selective semihydrogenation of internal alkynes with user-friendly ethanol as the H-donor. Another feature originates from the ability of the pentacoordinate hydrido chloride complex (PCN)Ir^IIIHCl to undergo reversible solvent-coordination-induced-ionization (SCII), furnishing a cationic monohydride [(PCN)Ir^IIIH(Sol)]⁺Cl^- bearing an uncoordinated Cl anion that effects selective hydrometalation of internal alkynes over the corresponding (Z)-alkenes; the resulting (PCN)Ir^III(vinyl)Cl complex undergoes amine-assisted formal alcoholysis involving the protonation of the Cl anion by the activated Ir^III-bound EtOH, again via the SCII pathway. Together these elementary reactions lay the foundation for cis-selective semihydrogenation of alkynes with EtOH. Further, the design of the oxazoline-containing chiral complexes (PCN_Oxa)Ir^IIIHCl enables asymmetric transfer hydrogenation of alkenes/ketones with ethanol. The efficient catalytic α-alkylation of unactivated esters/amides with alcohols is another case showing the benefit that the PCN-Ir catalyst can offer. These examples illustrate the profound impact of the pincer ligands on the reactivities and catalysis. We hope this Account will provide an in-depth view into the fundamentals of pincer iridium chemistry and ultimately broaden its applications in organic synthesis.

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.

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.

Transfer hydrogenation of pyridinium and quinolinium species using ethanol as a hydrogen source to access saturated N-heterocycles

Catalytic transfer hydrogenation (TH) for the reduction of heterocycles is an emerging strategy for accessing biologically active saturated N-heterocycles. Herein, we report a TH protocol that utilizes ethanol as a...

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.

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Highly Selective Hydrogenation of C═C Bonds Catalyzed by a Rhodium Hydride

Under mild conditions (room temperature, 80 psi of H₂) Cp*Rh(2-(2-pyridyl)phenyl)H catalyzes the selective hydrogenation of the C═C bond in α,β-unsaturated carbonyl compounds, including natural product precursors with bulky substituents in the β position and substrates possessing an array of additional functional groups. It also catalyzes the hydrogenation of many isolated double bonds. Mechanistic studies reveal that no radical intermediates are involved, and the catalyst appears to be homogeneous, thereby affording important complementarity to existing protocols for similar hydrogenation processes.

Enantioselective α-Arylation of Primary Alcohols under Sequential One-Pot Catalysis

Secondary benzylic alcohols and diarylmethanols are common structural motifs of biologically active and medicinally relevant compounds. Here we report their enantioselective synthesis by α-arylation of primary aliphatic and benzylic alcohols under sequential catalysis integrating a Ru-catalyzed hydrogen transfer oxidation and a Ru-catalyzed nucleophilic addition. The method can be applied to various alcohols and aryl nucleophiles tolerating a range of functional groups, including secondary alcohols, ketones, alkenes, esters, NH amides, tertiary amines, aryl halides, and heterocycles.

Transfer hydrogenation of N-heteroarenes with 2-propanol and ethanol enabled by manganese catalysis

A convenient well-defined manganese catalyzed transfer hydrogenation of N-heteroarenes using 2-propanol and ethanol as hydrogen sources is developed. DFT calculations support an outer sphere hydrogenation mechanism.