Rethinking Basic Concepts—Hydrogenation of Alkenes Catalyzed by Bench-Stable Alkyl Mn(I) Complexes

Renaissance in Alkyne Semihydrogenation: Mechanism, Selectivity, Functional Group Tolerance, and Applications in Organic Synthesis

Alkenes constitute a significant class of chemical compounds with applications in the bulk, pharmaceutical, or perfume industry. Among the known methods of olefin production, semihydrogenation of the C-C triple bond seems to be the most straightforward one. Nonetheless, the success of this reaction requires full control over diastereoselectivity, eradication of a parasitic process of over-reduction or migration of the C-C double bond formed, and achieving satisfactory functional-group compatibility. The review demonstrates developments in the field of alkyne semihydrogenation over the period 2010-2022, with selected papers published in 2023 and 2024, emphasizing solutions to the above-mentioned limitations. We discuss mechanistic aspects of this transformation, including those related to unconventional systems. The review includes examples of applications of alkyne semihydrogenation in organic synthesis, confirming the considerable utility of this process. Finally, strategies to enhance catalyst selectivity are summarized. For the reader's convenience, we provided a graphical guidebook to catalytic systems, illustrating the efficiency of the particular method.

Thioether-NHC bidentate manganese complexes as efficient phosphine-free catalysts for hydrogenation at room temperature

A series of four original phosphine-free thioether-NHC manganese complexes have been synthesised and fully characterized. These complexes have been applied as efficient catalysts for the hydrogenation of alkenes and ketones at room temperature, with low catalyst loadings (TON up to 900).

Understanding asymmetric hydrogenation of alkenes catalyzed by the first-row transition metal Fe: a first-principles exploration

First-principles analyses were performed for understanding the mechanistic details of Fe-catalysed asymmetric hydrogenation of alkenes in the presence of silane that has recently been experimentally realized. The catalytic hydrogenation is expected to proceed through initial hydride transfer from Fe-H to the CC bond of alkene, followed by σ-bond metathesis of hydrosilane to afford a chiral alkane product and an iron silyl species, which then reacts with H2 to regenerate the iron hydride species via another σ-bond metathesis. The mechanistic details and the origin of the regioselectivity and stereoselectivity of these reactions are understood on the basis of detailed potential energy surface analysis, charge transfer and noncovalent interactions involved therein, strain energy and isodesmic studies in the solvated stage. Finally, general aspects are highlighted for guiding further experimental studies to precisely control the reaction scheme.

Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(P∩P∩P)(CO)2MnH] on their kinetic hydricity

Octahedral Mn(I) complexes bearing tridentate donor ligands [(L∩L'∩L'')(CO)2MnX] have recently emerged as major players in catalytic (de)hydrogenation processes. While most of these systems are still based on structurally rigid pincer scaffolds imposing a meridional coordination mode, for some more flexible tridentate ligands a facial arrangement of donor moieties becomes possible. Accordingly, the geometry of the corresponding Mn(I) hydrides [(L∩L'∩L'')(CO)2MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the trans-position of the hydride (CO and L for mer- and fac-configurations, respectively) may influence their hydride transfer ability. Herein, low-temperature IR and NMR spectroscopy studies of two model Mn(I) complexes, mer-[(L1)(CO)2MnH] and fac-[(L2)(CO)2MnH], bearing similar triphosphine ligands (L1 = PhP(CH2CH2PPh2)2; L2 = MeC(CH2PPh2)3) in the presence of B(C6F5)3 as the H- abstractor revealed for the first time a higher kinetic hydricity of the tripodal system. Even for the pincer complex, hydride transfer proceeds from the non-covalent adduct fac-[(L1)(CO)2MnH]⋯B(C6F5)3 with the facial geometry arising from the mer-to-fac isomerization of the initial mer-[(L1)(CO)2MnH]. The higher reactivity of the fac-hydride derivatives was found to be consistent with the catalytic performance of the corresponding Mn(I) bromide complexes in the benchmark ester hydrosilylation.

Unveiling the Unique Reactivity of Anionic Mn(I) Complexes via Metal-Ligand Cooperation: Nucleophilic Attack on C(sp3)-X Bonds

Metal-ligand cooperation (MLC) has emerged as a pivotal strategy for the catalytic activation of small molecules within both synthetic and biological arenas. Leveraging this approach, a suite of potent catalytic reactions─encompassing hydrogenation, hydroelementation, and dehydrogenative processes─have been realized, with notable advances in manganese catalysis in recent years. However, the activation of alkyl halides by Mn complexes, which typically requires strong reductants to form Mn(-I) complexes that are incompatible with standard cross-coupling conditions, remains a significant challenge. This limitation underscores the urgent need to investigate alternative methods for activating C(sp3)-X bonds using higher valence state Mn complexes. In response to this challenge, we present the synthesis, characterization, and reactivity of a new anionic Mn(I) complex featuring a redox-active dianionic ligand that induces multiple MLC functionalities. We have discovered an innovative mechanism of MLC, characterized by a single ligand transferring two electrons to the metal center. This novel process facilitates an orbital-symmetry-allowed nucleophilic attack on C(sp3)-X bonds, preserving manganese's oxidative state at +1. To the best of our knowledge, this is the first instance where the MLC strategy via a two-electron transfer process has been utilized to execute an SN2 nucleophilic attack at a C(sp3)-X bond by a relatively electron-deficient metal center like Mn(I). Additionally, the dianionic ligand of the anionic Mn(I) complex exhibits ambident nucleophilicity by reacting with different electrophiles, further highlighting its versatile reactivity.

Alkene Isomerization Catalyzed by a Mn(I) Bisphosphine Borohydride Complex

An additive-free manganese-catalyzed isomerization of terminal alkenes to internal alkenes is described. This reaction is implementing an inexpensive nonprecious metal catalyst. The most efficient catalyst is the borohydride complex cis-[Mn(dippe)(CO)2(κ2-BH4)]. This catalyst operates at room temperature, with a catalyst loading of 2.5 mol %. A variety of terminal alkenes is effectively and selectively transformed into the respective internal E-alkenes. Preliminary results show chain-walking isomerization at an elevated temperature. Mechanistic studies were carried out, including stoichiometric reactions and in situ NMR analysis. These experiments are flanked by computational studies. Based on these, the catalytic process is initiated by the liberation of "BH3" as a THF adduct. The catalytic process is initiated by double bond insertion into an M-H species, leading to an alkyl metal intermediate, followed by β-hydride elimination at the opposite position to afford the isomerization product.

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.

Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways

A stereo- and regioselective Mn(I)-catalyzed hydroboration of terminal alkynes with pinacolborane (HBPin) is described. The hydroboration reaction is highly Z-selective in the case of aryl alkynes and E-selective in the case of aliphatic alkynes. The reaction requires no additives or solvents and proceeds with a catalyst loading of 1 mol % at 50-70 °C. The most active precatalyst is the bench-stable alkyl Mn(I) complex cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)]. The catalytic process is initiated by the migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate. This species undergoes C-H and B-H bond cleavage of the alkyne (aromatic alkynes) and HBPin (in the case of aliphatic alkynes) forming the active Mn(I) alkynyl and boryl catalysts [Mn(PCP-iPr)(CO)(C≡CR)] and [Mn(PCP-iPr)(CO)(BPin)], respectively. A broad variety of aromatic and aliphatic alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations. The functionalized alkenes can be used for further applications in cross-coupling reactions.

Chemoselective Hydrogenation of α,β-Unsaturated Ketones Catalyzed by a Manganese(I) Hydride Complex

Here, we report the chemoselective hydrogenation of α,β-unsaturated ketones catalyzed by a well-defined Mn(I) PCNHCP pincer complex [(PCNHCP)Mn(CO)2H] (1). The reaction is compatible with a wide variety of functional groups that include halides, esters, amides, nitriles, nitro, alkynes, and alkenes, and for most substrates occurs readily at ambient hydrogen pressure (1-2 bar). Mechanistic studies and deuterium labeling experiments reveal a non-cooperative mechanism, which is further discussed in this report.

Is Mn(I) More Promising Than Fe(II)-A Comparison of Mn vs Fe Complexes for Olefin Metathesis

Olefin metathesis is one of the most significant transformations in organic chemistry and is an excellent example for efficient homogeneous catalysis. Although most currently used catalysts are primarily based on 4d and 5d metals, cycloaddition and cycloreversion reactions can also be attributed to first-row transition metals, such as Fe. Surprisingly, the potential of Mn(I)-based catalysts for olefin metathesis has been unexplored despite their prominence in homogeneous catalysis and their diagonal relationship to Ru(II). In the present study, we have investigated the prospective capabilities of Mn complexes for cycloaddition and reversion reactions using density functional theory. Therefore, we have initially compared the literature known iron model systems and their isoelectronic Mn counterparts regarding their reactivity and electronic structure. Next, we constructed potential Mn complexes derived from synthetically accessible species, including carbonyl ligands and obeying octahedral geometry. Based on thermodynamic parameters and the calculation of electronic descriptors, we were able to validate the isodiagonal relationship. Our study serves as guidance for the experimental chemist.

Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

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.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt3 adducts. By applying complexes 3 with KHBEt3 as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt3 as cocatalyst.

Synthesis of Polyethyleneimines from the Manganese-Catalysed Coupling of Ethylene Glycol and Ethylenediamine

Polyethyleneimines find many applications in products such as detergents, adhesives, cosmetics, and for processes such as tissue culture, gene therapy, and CO2 capture. The current state-of-the-art technology for the production of the branched polyethyleneimines involves aziridine feedstock which is a highly toxic, volatile and mutagenic chemical and raises significant concern to human health and environment. We report here a novel method for the synthesis of branched polyethyleneimine derivative from ethylene glycol and ethylenediamine which are much safer, environmentally benign, commercially available and potentially renewable feedstock. The polymerisation reaction is catalysed by a complex of an earth-abundant metal, manganese and liberates H2 O as the only by-product. Our mechanistic studies using a combination of DFT computation and experiment suggest that the reaction proceeds by the formation and subsequent hydrogenation of imine intermediates.

Base- and Additive-Free Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Non-Pincer-Type Mn(I) Complexes

Well-defined, bench stable Mn(I) non-pincer-type complexes were tested as earth-abundant transition metal catalysts for the selective reduction of CO₂ to boryl-protected MeOH in the presence of pinacolborane (HBpin). Essentially, quantitative yields were obtained under mild reaction conditions (1 bar CO₂, 60 °C), without the need of any base or additives, in the presence of the alkylcarbonyl Mn(I) bis(phosphine) complexes fac-[Mn(CH₂CH₂CH₃)(dippe)(CO)₃] [Mn1, dippe = 1,2-bis(diisopropylphosphino)ethane] and [Mn(dippe)(CO)₂{(μ-H)₂(Bpin)}] (Mn4), that is obtained by reaction of the bench-stable precatalyst Mn1 with HBpin via elimination of butanal. Preliminary mechanistic details were obtained by a combination of NMR experiments and monitoring of the catalytic reactions.

Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

Performance of homogeneous catalysts viewed in dynamics

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

Manganese-Catalyzed Hydrogenative Desulfurization of Thioamides

Earth-abundant transition metal catalysis has emerged as an important alternative to noble transition metal catalysis in hydrogenation reactions. However, there has been no Earth-abundant transition metal catalyzed hydrogenation of thioamides reported so far, presumably due to the poisoning of catalysts by sulfur-containing molecules. Herein, we described the first manganese-catalyzed hydrogenative desulfurization of thioamides to amines or imines. The key to success is the use of MnBr(CO)₅ instead of commonly-employed pincer-manganese catalysts, together with simple NEt₃ and CuBr. This protocol features excellent selectivity on sole cleavage of the C=S bond of thioamides, in contrast to the only known Ru-catalyzed hydrogenation of thioamides, and unprecedented chemo-selectivity tolerating vulnerable functional groups such as nitrile, ketone, aldehyde, ester, sulfone, nitro, olefin, alkyne and heterocycle, which are usually susceptible to common hydride-type reductive protocols.

Room temperature chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds by using a well-defined mixed donor Mn(i) pincer catalyst

Chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds in α,β-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(i) catalyst and 5.0 bar H₂. Amongst the three mixed-donor Mn(i) complexes developed, κ³-(^R2PN³N^Pyz)Mn(CO)₂Br (R = Ph, ⁱPr, ^t Bu); the ^t Bu-substituted complex ( ^tBu2PN³N^Pyz)Mn(CO)₂Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (-F, -Cl, -Br, and -I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The C[double bond, length as m-dash]C bond is chemoselectively hydrogenated in α,β-unsaturated ketones, while the aldehyde's C[double bond, length as m-dash]O bond and imine's C[double bond, length as m-dash]N bond are preferentially reduced over the C[double bond, length as m-dash]C bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the ( ^tBu2PN³N^Pyz)Mn(i) complex and indicated a metal-ligand cooperative catalytic pathway. The molecular hydrogen (H₂) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H₂ activation.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Manganese Alkyl Carbonyl Complexes: From Iconic Stoichiometric Textbook Reactions to Catalytic Applications

The activation of weakly polarized bonds represents a challenging, yet highly valuable process. In this context, precious metal catalysts have been used as reliable compounds for the activation of rather inert bonds for the last several decades. Nevertheless, base-metal complexes including cobalt, iron, or nickel are currently promising candidates for the substitution of noble metals in order to develop more sustainable processes. In the past few years, manganese(I)-based complexes were heavily employed as efficient catalysts for (de)hydrogenation reactions. However, the vast majority of these complexes operate via a metal-ligand bifunctionality as already well implemented for precious metals decades ago. Although high reactivity can be achieved in various reactions, this concept is often not applicable to certain transformations due to outer-sphere mechanisms. In this Account, we outline the potential of alkylated Mn(I)-carbonyl complexes for the activation of nonpolar and moderately polar E-H (E = H, B, C, Si) bonds and disclose our successful approach for the utilization of complexes in the field of homogeneous catalysis. This involves the rational design of manganese complexes for hydrogenation reactions involving ketones, nitriles, carbon dioxide, and alkynes. In addition to that, the reduction of alkenes by dihydrogen could be achieved by a series of well-defined manganese complexes which was not possible before. Furthermore, we elucidate the potential of our Mn-based catalysts in the field of hydrofunctionalization reactions for carbon-carbon multiple bonds. Our investigations unveiled novel insights into reaction pathways of dehydrogenative silylation of alkenes and trans-1,2-diboration of terminal alkynes, which was not yet reported for transition metals. Due to rational catalyst design, these transformations can be achieved under mild reaction conditions. Delightfully, all of the employed complexes are bench-stable compounds. We took advantage of the fact that Mn(I) alkyl complexes are known to undergo migratory insertion of the alkyl group into the CO ligand, yielding an unsaturated acyl intermediate. Hydrogen atom abstraction by the acyl ligand then paves the way to an active species for a variety of catalytic transformations which all proceed via an inner-sphere process. Although these textbook reactions have been well-known for decades, the application in catalytic transformations is still in its infancy. A brief historical overview of alkylated manganese(I)-carbonyl complexes is provided, covering the synthesis and especially iconic stoichiometric transformations, e.g., carbonylation, as intensively examined by Calderazzo, Moss, and others. An outline of potential future applications of defined alkyl manganese complexes will be given, which may inspire researchers for the development of novel (base-)metal catalysts.

Switching between Hydrogenation and Olefin Transposition Catalysis via Silencing NH Cooperativity in Mn(I) Pincer Complexes

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives has been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein, we report a Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This reactivity is shown to emerge once the N–H functionality within the Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such masking of the cooperative N–H functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity toward a single positional isomerization was also retained for long-chain alkenes, resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of the reaction between the activated Mn catalyst and olefins points to catalysis operating via a metal–alkyl mechanism—one of the three conventional transposition mechanisms previously unknown in Mn complexes.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Manganese-Catalyzed Asymmetric Hydrogenation of 3H-Indoles

The asymmetric hydrogenation (AH) of 3H-indoles represents an ideal approach to the synthesis of useful chiral indoline scaffolds. However, very few catalytic systems based on precious metals have been developed to realize this challenging reaction. Herein, we report a Mn-catalyzed AH of 3H-indoles with excellent yields and enantioselectivities. The kinetic resolution of racemic 3H-indoles by AH was also achieved with high s-factors to construct quaternary stereocenters. Many acid-sensitive functional groups, which cannot be tolerated when using a state-of-the-art ruthenium catalyst, were compatible with manganese catalysis. This new process expands the scope of this transformation and highlights the uniqueness of earth-abundant metal catalysis. The reaction could proceed with catalyst loadings at the parts per million (ppm) level with an exceptional turnover number of 72 350. This is the highest value yet reported for an earth-abundant metal-catalyzed AH reaction.

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H]

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)₃H] and Lewis acids (LA = Ph₃C⁺, B(C₆F₅)₃) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)₃H⋯LA] and unusual dearomatization of the Ph₃C⁺ cation upon hydride transfer.

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.

The Rise of Manganese-Catalyzed Reduction Reactions

Recent developments in manganese-catalyzed reducing transformations—hydrosilylation, hydroboration, hydrogenation, and transfer hydrogenation—are reviewed herein. Over the past half a decade (i.e., 2016 to the present), more than 115 research publications have been reported in these fields. Novel organometallic compounds and new reduction transformations have been discovered and further developed. Significant challenges that had historically acted as barriers for the use of manganese catalysts in reduction reactions are slowly being broken down. This review will hopefully assist in developing this research area, by presenting a clear and concise overview of the catalyst structures and substrate transformations published so far.

1 Introduction

2 Hydrosilylation

3 Hydroboration

4 Hydrogenation

5 Transfer Hydrogenation

6 Conclusion and Perspective

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.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Utilizing Design of Experiments Approach to Assess Kinetic Parameters for a Mn Homogeneous Hydrogenation Catalyst

Homogeneous hydrogenation catalysts based on metal complexes provide a diverse and highly tunable tool for the fine chemical industry. To fully unleash their potential, fast and effective methods for the evaluation of catalytic properties are needed. In turn, this requires changes in the experimental approaches to test and evaluate the performance of the catalytic processes. Design of experiment combined with statistical analysis can enable time- and resource-efficient experimentation. In this work, we employ a set of different statistical models to obtain the detailed kinetic description of a highly active homogeneous Mn (I) ketone hydrogenation catalyst as a representative model system. The reaction kinetics were analyzed using a full second order polynomial regression model, two models with eliminated parameters and finally a model which implements "chemical logic". The coefficients obtained are compared with the corresponding high-quality kinetic parameters acquired using conventional kinetic experiments. We demonstrate that various kinetic effects can be well captured using different statistical models, providing important insights into the reaction kinetics and mechanism of a complex catalytic reaction.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex

A Mn^I‐catalyzed hydroboration of terminal alkenes and a 1,2‐diboration of terminal alkynes with pinacolborane (HBPin) is described. For alkenes, anti‐Markovnikov hydroboration takes place; for alkynes the reaction proceeds with excellent trans‐1,2‐selectivity. The most active pre‐catalyst is bench‐stable alkyl bisphosphine Mn^I complex fac‐[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate, which undergoes B−H bond cleavage of HBPin (for alkenes) and rapid C−H bond cleavage (for alkynes), forming the active Mn^I boryl and acetylide catalysts [Mn(dippe)(CO)₂(BPin)] and [Mn(dippe)(CO)₂(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex

A MnI-catalyzed hydroboration of terminal alkenes and a 1,2-diboration of terminal alkynes with pinacolborane (HBPin) is described. For alkenes, anti-Markovnikov hydroboration takes place; for alkynes the reaction proceeds with excellent trans-1,2-selectivity. The most active pre-catalyst is bench-stable alkyl bisphosphine MnI complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate, which undergoes B-H bond cleavage of HBPin (for alkenes) and rapid C-H bond cleavage (for alkynes), forming the active MnI boryl and acetylide catalysts [Mn(dippe)(CO)2(BPin)] and [Mn(dippe)(CO)2(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

Manganese-Catalyzed Dehydrogenative Silylation of Alkenes Following Two Parallel Inner-Sphere Pathways

We report on an additive-free Mn(I)-catalyzed dehydrogenative silylation of terminal alkenes. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate which undergoes rapid Si–H bond cleavage of the silane HSiR₃ forming the active 16e^– Mn(I) silyl catalyst [Mn(dippe)(CO)₂(SiR₃)] together with liberated butanal. A broad variety of aromatic and aliphatic alkenes was efficiently and selectively converted into E-vinylsilanes and allylsilanes, respectively, at room temperature. Mechanistic insights are provided based on experimental data and DFT calculations revealing that two parallel reaction pathways are operative: an acceptorless reaction pathway involving dihydrogen release and a pathway requiring an alkene as sacrificial hydrogen acceptor.

Reduction of carbonyl compounds via hydrosilylation catalyzed by well-defined PNP-Mn(I) hydride complexes

Reduction reactions of unsaturated compounds are fundamental transformations in synthetic chemistry. In this context, the reduction of polarized double bonds such as carbonyl or C=C motifs can be achieved by hydrogenation reactions. We describe here a highly chemoselective Mn(I)-based PNP pincer catalyst for the hydrosilylation of aldehydes and ketones employing polymethylhydrosiloxane (PMHS) as inexpensive hydrogen donor.

Graphic abstract

Selective Manganese-Catalyzed Dimerization and Cross-Coupling of Terminal Alkynes

Herein, efficient manganese-catalyzed dimerization of terminal alkynes to afford 1,3-enynes is described. This reaction is atom economic, implementing an inexpensive, earth-abundant nonprecious metal catalyst. The precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate that undergoes rapid C–H bond cleavage of alkyne, forming an active Mn(I) acetylide catalyst [Mn(dippe)(CO)₂(C≡CPh)(η²-HC≡CPh)] together with liberated butanal. A range of aromatic and aliphatic terminal alkynes were efficiently and selectively converted into head-to-head Z-1,3-enynes and head-to-tail gem-1,3-enynes, respectively, in good to excellent yields. Moreover, cross-coupling of aromatic and aliphatic alkynes selectively yields head-to-tail gem-1,3-enynes. In all cases, the reactions were performed at 70 °C with a catalyst loading of 1–2 mol %. A mechanism based on density functional theory (DFT) calculations is presented.

Carbon Dioxide Hydrogenation to Formate Catalyzed by a Bench-Stable, Non-Pincer-Type Mn(I) Alkylcarbonyl Complex

The catalytic reduction of carbon dioxide is a process of growing interest for the use of this simple and abundant molecule as a renewable building block in C1-chemical synthesis and for hydrogen storage. The well-defined, bench-stable alkylcarbonyl Mn(I) bis(phosphine) complex fac-[Mn(CH₂CH₂CH₃)(dippe)(CO)₃] [dippe = 1,2-bis(diisopropylphosphino)ethane] was tested as an efficient and selective non-precious-metal precatalyst for the hydrogenation of CO₂ to formate under mild conditions (75 bar total pressure, 80 °C), in the presence of a Lewis acid co-catalyst (LiOTf) and a base (DBU). Mechanistic insight into the catalytic reaction is provided by means of density functional theory (DFT) calculations.

Manganese-Catalyzed Hydrogenation of Ketones under Mild and Base-free Conditions

In this paper, several Mn(I) complexes were applied as catalysts for the homogeneous hydrogenation of ketones. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe) (CO)₃(CH₂CH₂CH₃)]. The reaction proceeds at room temperature under base-free conditions with a catalyst loading of 3 mol % and a hydrogen pressure of 10 bar. A temperature-dependent selectivity for the reduction of α,β-unsaturated carbonyls was observed. At room temperature, the carbonyl group was selectively hydrogenated, while the C=C bond stayed intact. At 60 °C, fully saturated systems were obtained. A plausible mechanism based on DFT calculations which involves an inner-sphere hydride transfer is proposed.

Robust and efficient hydrogenation of carbonyl compounds catalysed by mixed donor Mn(I) pincer complexes

Any catalyst should be efficient and stable to be implemented in practice. This requirement is particularly valid for manganese hydrogenation catalysts. While representing a more sustainable alternative to conventional noble metal-based systems, manganese hydrogenation catalysts are prone to degrade under catalytic conditions once operation temperatures are high. Herein, we report a highly efficient Mn(I)-CNP pre-catalyst which gives rise to the excellent productivity (TOF° up to 41 000 h^-1) and stability (TON up to 200 000) in hydrogenation catalysis. This system enables near-quantitative hydrogenation of ketones, imines, aldehydes and formate esters at the catalyst loadings as low as 5-200 p.p.m. Our analysis points to the crucial role of the catalyst activation step for the catalytic performance and stability of the system. While conventional activation employing alkoxide bases can ultimately provide catalytically competent species under hydrogen atmosphere, activation of Mn(I) pre-catalyst with hydride donor promoters, e.g. KHBEt₃, dramatically improves catalytic performance of the system and eliminates induction times associated with slow catalyst activation.

Manganese-Catalyzed Hydroboration of Terminal Olefins and Metal-Dependent Selectivity in Internal Olefin Isomerization-Hydroboration

In the past decade, the use of earth-abundant metals in homogeneous catalysis has flourished. In particular, metals such as cobalt and iron have been used extensively in reductive transformations including hydrogenation, hydroboration, and hydrosilylation. Manganese, on the other hand, has been considerably less explored in these reductive transformations. Here, we report a well-defined manganese complex, [Mn(^iPrBDI)(OTf)₂] (2a; BDI = bipyridinediimine), that is an active precatalyst in the hydroboration of a variety of electronically differentiated alkenes (>20 examples). The hydroboration is specifically selective for terminal alkenes and occurs with exclusive anti-Markovnikov selectivity. In contrast, when using the analogous cobalt complex [Co(^iPrBDI)(OTf)₂] (3a), internal alkenes are hydroborated efficiently, where a sequence of isomerization steps ultimately leads to their hydroboration. The contrasting terminal versus internal alkene selectivity for manganese and cobalt was investigated computationally and is further discussed in the herein-reported study.

The cooperative role of innocent ligand in N-heterocyclic carbene manganese catalyzed carbon dioxide hydrogenation

The concept of Lewis acidic π* cooperation was proposed for innocent CO ligand in NHC–Mn catalyzed CO2 hydrogenation by systematic DFT studies.