A Practical and Stereoselective In Situ NHC-Cobalt Catalytic System for Hydrogenation of Ketones and Aldehydes

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

Amine-functionalized bifunctional CoIII-NHC complexes: highly effective phosphine-free catalysts for the α-alkylation of nitriles

Newly developed amine functionalized NHC-supported CoIII-complexes have been identified as highly effective bifunctional catalysts for the α-alkylation of nitriles using a plethora of alcohols, ranging from aliphatic to aromatic and intriguingly, also secondary ones. Comparison of their activities with the non-bifunctional analogues uncovered their extremely high activities although possessing the high-valent CoIII-center due to metal-ligand cooperativity, which has been established by an array of control experiments.

Properties of Metal Hydrides of the Iron Triad

Metal hydride complexes are essential intermediates in hydrogenation reactions. The hydride-donor ability determines the scope of use of these complexes. We present a new, simple mass-spectrometry method to study the hydride-donor ability of metal hydrides using a series of 18 iron, cobalt, and nickel complexes with N- and P-based ligands (L). The mixing of [(L)MII(OTf)2] with NaBH4 forms [(L)MII(BH4)]+ (M = Fe, Co, Ni) that can be detected by electrospray ionization mass spectrometry. Energy-resolved collision-induced dissociations of [(L)MII(BH4)]+ provide threshold energies (ΔECID) for the formations of [(L)MII(H)]+ that correlate well with the hydride donor ability of the metal hydride complexes. We studied the vibrational and electronic spectra of the generated metal hydrides, assigned their structure and spin state, and demonstrated a good correlation between ΔECID and the M-H stretching vibration frequencies. The ΔECID also correlates with reaction rates for hydride transfer reactivity in the gas phase and known reactivity trends in the solution phase.

Iridium Catalysts with f-Amphbinol Ligands: Highly Stereoselective Hydrogenation of a Variety of Ketones

A series of novel and modular ferrorence-based amino-phosphine-binol (f-amphbinol) ligands have been successfully synthesized. The f-amphbinol ligands exhibited extremely high air stability and catalytic efficiency in the Ir-catalyzed stereoselective hydrogenation of various ketones to afford corresponding stereodefined alcohols with excellent results (full conversions, cis/trans >99:1, and 83% → 99% ee, TON up to 500 000). Control experiments have shown that -OH and -NH groups played a key role in this stereoselective hydrogenation.

Synchronous Proton-Hydride Transfer by a Pyrazole-Functionalized Protic Mn(I) Complex in Catalytic Alcohol Dehydrogenative Coupling

A series of Mn(I) complexes Mn(L1 )(CO)3 Br, Mn(L2 )(CO)3 Br, Mn(L1 )(CO)3 (OAc) and Mn(L3 )(CO)3 Br [L1 =2-(5-tert-butyl-1H-pyrazol-3-yl)-1,8-naphthyridine, L2 =2-(5-tert-butyl-1H-pyrazol-3-yl)pyridine, L3 =2-(5-tert-butyl-1-methyl-1H-pyrazol-3-yl)-1,8-naphthyridine] were synthesized and fully characterized. The acid-base equilibrium between the pyrazole and the pyrazolato forms of Mn(L1 )(CO)3 Br was studied by 1 H NMR and UV-vis spectra. These complexes are screened as catalysts for acceptorless dehydrogenative coupling (ADC) of primary alcohols and aromatic diamines for the synthesis of benzimidazole and quinoline derivatives with the release of H2 and H2 O as byproducts. The protic complex Mn(L1 )(CO)3 Br shows the highest catalytic activity for the synthesis of 2-substituted benzimidazole derivatives with broad substrate scope, whereas a related complex [Mn(L3 )(CO)3 Br], which is devoid of the proton responsive β-NH unit, shows significantly reduced catalytic efficiency validating the crucial role of the β-NH functionality for the alcohol dehydrogenation reactions. Control experiments, kinetic and deuterated studies, and density functional theory (DFT) calculations reveal a synchronous hydride-proton transfer by the metal-ligand construct in the alcohol dehydrogenation step.

Cobalt-Catalyzed Enantioselective Hydrogenation of Diaryl Ketones with Ferrocene-Based Secondary Phosphine Oxide Ligands

A new class of cobalt catalytic system for asymmetric hydrogenation of ketones was herein reported, involving the development of novel ferrocene-based secondary phosphine oxide ligands. An unusual P-O bidentate coordination pattern with cobalt was confirmed by an X-ray diffraction study. The bichelating tetrahedral cobalt(II) complexes afforded high reactivities (up to 99% yield) and good to excellent enantioselectivities (up to 92% ee) in the AH of various ortho-substituted diaryl ketones. In addition, the diferrocenyl cobalt complex was characterized with intriguing UV-vis absorption and electrochemical properties.

Chromium-catalyzed stereodivergent E- and Z-selective alkyne hydrogenation controlled by cyclic (alkyl)(amino)carbene ligands

The hydrogenation of alkynes allows the synthesis of olefins, which are important feedstock for the materials, pharmaceutical, and petrochemical industry. Thus, methods that enable this transformation via low-cost metal catalysis are desirable. However, achieving stereochemical control in this reaction is a long-standing challenge. Here, we report on the chromium-catalyzed E- and Z-selective olefin synthesis via hydrogenation of alkynes, controlled by two carbene ligands. A cyclic (alkyl)(amino)carbene ligand that contains a phosphino anchor enables the hydrogenation of alkynes in a trans-addition manner, selectively forming E-olefins. With an imino anchor-incorporated carbene ligand, the stereoselectivity can be switched, giving mainly Z-isomers. This ligand-enabled geometrical stereoinversion strategy by one metal catalysis overrides common methods in control of the E- and Z-selectivity with two different metal catalysis, allowing for highly efficient and on-demand access to both E- and Z-olefins in a stereo-complementary fashion. Mechanistic studies indicate that the different steric effect between these two carbene ligands may mainly dominate the selective forming E- or Z-olefins in control of the stereochemistry.

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.

Synthesis of Cyclohexanones by a Tandem Photocatalyzed Annulation

The rapid synthesis of cyclic scaffolds is of high importance to the chemistry community. Strategies for the convergent synthesis of substituted carbocycles and heterocycles remain underexplored despite the plethora of applications that these cyclic motifs have in the pharmaceutical and materials industries. Reported herein is a tandem carbene and photoredox-catalyzed process for the convergent synthesis of substituted cycloalkanones via a formal [5 + 1] cycloaddition. Featuring two distinct photoredox cycles and a novel α-oxidation of benzylic ketones, this reaction offers a mild approach to construct two contiguous C-C bonds and eliminates the need for strong bases or expensive metal catalysts. The utility of this method is highlighted through various product diversification reactions that allow access to a range of important cyclic scaffolds.

Cobalt-N-Heterocyclic Carbene Complexes in Catalysis

Cobalt-NHC complexes have emerged as an attractive class of 3d transition metal catalysts for a broad range of chemical processes, including cross-coupling, hydrogenation, hydrofunctionalization and cycloaddition reactions. Herein, we present a comprehensive review of catalytic methods utilizing cobalt-NHC complexes with a focus on catalyst structure, the role of the NHC ligand, properties of the catalytic system, mechanism and synthetic utility. The survey clearly suggests that the recent emergence of well-defined cobalt-NHC catalysts may have a tremendous utility in the design and application of catalytic reactions using more abundant 3d transition metals.

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...

Giant N-heterocyclic carbene-containing macrocycles for cobalt-catalysed hydroboration of alkynes

Giant N-heterocyclic carbene-containing organic macrocycles larger than “Texas-sized” molecular boxes have been synthesized and structurally characterized. The new macrocyles were employed for the Co-NHC promoted syn-selective hydroboration of alkynes with...

Silica-supported Fe/Fe–O nanoparticles for the catalytic hydrogenation of nitriles to amines in the presence of aluminium additives

The hydrogenation of nitriles to amines represents an important and frequently used industrial process due to the broad applicability of the resulting products in chemistry and life sciences. Despite the existing portfolio of catalysts reported for the hydrogenation of nitriles, the development of iron-based heterogeneous catalysts for this process is still a challenge. Here, we show that the impregnation and pyrolysis of iron(II) acetate on commercial silica produces a reusable Fe/Fe–O@SiO2 catalyst with a well-defined structure comprising the fayalite phase at the Si–Fe interface and α-Fe nanoparticles, covered by an ultrathin amorphous iron(III) oxide layer, growing from the silica matrix. These Fe/Fe–O core–shell nanoparticles, in the presence of catalytic amounts of aluminium additives, promote the hydrogenation of all kinds of nitriles, including structurally challenging and functionally diverse aromatic, heterocyclic, aliphatic and fatty nitriles, to produce primary amines under scalable and industrially viable conditions.

Iridium-catalyzed highly stereoselective deoxygenation of tertiary cycloalkanols: stereoelectronic insights and synthetic applications

Excellent and unique diastereoselectivity is observed in the iridium-catalyzed deoxygenation of tertiary cyclohexanols and cyclopentanols. The substituent effect on the diastereoselectivity and detailed control models are analyzed case by case, using tertiary monocyclic and polycyclic cyclohexanols, bicyclic bridged cycloalkanols, and cyclopentanols as the model substrates. The selectivity is decided by the steric environment of the carbocation intermediates and is independent of the catalyst loading. Stereoelectronically, the iridium hydride approaches the carbocation in directions perpendicular to the carbocation plane. The sterically large iridium hydride delivers its hydride in the sterically least hindered direction to the carbocation. The deoxygenation has found important applications in the stereospecific arylations of sterically complex compounds. Our deoxygenation is stereochemically very different from the coupling reactions and can be used to specifically synthesize stereoisomers that are not available via cross-couplings.

Recent advances in the synthesis and derivatization of N-heterocyclic carbene metal complexes

N-heterocyclic carbene (NHC) metal complexes have gained an incredible amount of attention in the course of the last two decades and have become indispensable as an intricate part of a plethora of applications. The areas of their synthesis and derivatization are constantly evolving and bring new, more sustainable, cost-effective and simpler approaches to the design of existing and next generation catalysts and materials. This article provides an overview of the latest developments, focusing on those which have appeared during the last two years.

Thermodynamic Network Cards of Hantzsch Ester, Benzothiazoline, and Dihydrophenanthridine Releasing Two Hydrogen Atoms or Ions on 20 Elementary Steps

In this work, thermodynamic driving forces on 20 possible elementary steps of Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydrophenanthridine (PDH₂) releasing two hydrogen atoms or ions were measured or derived from the related thermodynamic data using Hess' law in acetonitrile. Furthermore, thermodynamic network cards of HEH₂, BTH₂, and PDH₂ releasing two hydrogen atoms or ions on 20 elementary steps were first established. Based on the thermodynamic network cards, hydride-donating, hydrogen-atom-donating, and electron-donating abilities of XH₂ and XH^-, and two hydrogen-atom(ion)-donating abilities of XH₂ are discussed in detail. Obviously, the thermodynamic network cards of HEH₂, BTH₂, and PDH₂ not only offer rational data guidance for organic synthetic chemists to properly choose an appropriate reducer among the three reducing agents to hydrogenate various unsaturated compounds but also strongly promote elucidatation of the detailed hydrogenation mechanisms.

Proton-responsive naphthyridinone-based RuII complexes and their reactivity with water and alcohols

We report the synthesis and reactivity of RuII complexes with a new naphthyridinone-substituted phosphine ligand, 7-(diisopropylphosphinomethyl)-1,8-naphthyridin-2(1H)-one (L-H), which contains two reactive sites that can potentially be deprotonated by a strong base: an NH proton of naphthyridinone and a methylene arm attached to the phosphine. In the absence of a base, the stable bis-ligated complex Ru(L-H)2Cl2 (1) containing two NH groups in the secondary coordination sphere is formed. Upon further reaction with a base, a doubly deprotonated, dimeric complex is obtained, [Ru2(L*-H)2(L)2] (2), in which two of the four ligands undergo deprotonation at the NH (L), while the other two ligands are deprotonated at the methylene groups (L*-H) as confirmed by an X-ray diffraction study; intramolecular hydrogen bonding is present between the NH group of one ligand and an O-atom of another ligand in the dimeric structure, which stabilizes the observed geometry of the complex. Complex 2 reacts with protic solvents such as water or methanol generating aqua Ru(L)2(OH2)2 (3) or methanol complexes Ru(L)2(MeOH)2 (4), respectively, both exhibiting intramolecular H-bonded patterns with surrounding ligands at least in the solid state. These complexes react with benzyl alcohols to give aldehydes via base-free acceptorless dehydrogenation.

A General Regioselective Synthesis of Alcohols by Cobalt-Catalyzed Hydrogenation of Epoxides

A straightforward methodology for the synthesis of anti‐Markovnikov‐type alcohols is presented. By using a specific cobalt triphos complex in the presence of Zn(OTf)₂ as an additive, the hydrogenation of epoxides proceeds with high yields and selectivities. The described protocol shows a broad substrate scope, including multi‐substituted internal and terminal epoxides, as well as a good functional‐group tolerance. Various natural‐product derivatives, including steroids, terpenoids, and sesquiterpenoids, gave access to the corresponding alcohols in moderate‐to‐excellent yields.

Diazaphosphinanes as hydride, hydrogen atom, proton or electron donors under transition-metal-free conditions: thermodynamics, kinetics, and synthetic applications

Exploration of new hydrogen donors is in large demand in hydrogenation chemistry. Herein, we developed a new 1,3,2-diazaphosphinane 1a, which can serve as a hydride, hydrogen atom or proton donor without transition-metal mediation. The thermodynamics and kinetics of these three pathways of 1a, together with those of its analog 1b, were investigated in acetonitrile. It is noteworthy that, the reduction potentials (E_red) of the phosphenium cations 1a-[P]+ and 1b-[P]+ are extremely low, being −1.94 and −2.39 V (vs. Fc^+/0), respectively, enabling corresponding phosphinyl radicals to function as neutral super-electron-donors. Kinetic studies revealed an extraordinarily large kinetic isotope effect KIE(1a) of 31.3 for the hydrogen atom transfer from 1a to the 2,4,6-tri-(tert-butyl)-phenoxyl radical, implying a tunneling effect. Furthermore, successful applications of these diverse P–H bond energetic parameters in organic syntheses were exemplified, shedding light on more exploitations of these versatile and powerful diazaphosphinane reagents in organic chemistry.

A new 1,3,2-diazaphosphinane, serving as a formal hydride, hydrogen-atom or proton donor without transition-metal mediation was exploited thermodynamically and kinetically. And, its promising potentials in versatile syntheses have been demonstrated.

Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)-2-alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base-catalyzed isomerization of terminal alkynes and cobalt-catalyzed hydrogenation of either 2-alkynes or conjugated diene intermediates. Notably, this practical non-noble metal catalytic system enables efficient control of the chemo-, regio-, and stereoselectivity of this transformation.

Homogeneous Hydrogenation with a Cobalt/Tetraphosphine Catalyst: A Superior Hydride Donor for Polar Double Bonds and N-Heteroarenes

The development of catalysts based on earth abundant metals in place of noble metals is becoming a central topic of catalysis. We herein report a cobalt/tetraphosphine complex-catalyzed homogeneous hydrogenation of polar unsaturated compounds using an air- and moisture-stable and scalable precatalyst. By activation with potassium hydroxide, this cobalt system shows both high efficiency (up to 24 000 TON and 12 000 h^-1 TOF) and excellent chemoselectivities with various aldehydes, ketones, imines, and even N-heteroarenes. The preference for 1,2-reduction over 1,4-reduction makes this method an efficient way to prepare allylic alcohols and amines. Meanwhile, efficient hydrogenation of the challenging N-heteroarenes is also furnished with excellent functional group tolerance. Mechanistic studies and control experiments demonstrated that a Co^IH complex functions as a strong hydride donor in the catalytic cycle. Each cobalt intermediate on the catalytic cycle was characterized, and a plausible outer-sphere mechanism was proposed. Noteworthy, external inorganic base plays multiple roles in this reaction and functions in almost every step of the catalytic cycle.

Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed hydrogenation reactions have attracted broad interest since the first report in 2016. Among the reported catalytic systems, Mn catalysts supported by tridentate PNP- and NNP-pincer ligands have most commonly been used. For example, a number of PNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn pincer catalysts have been shown to be active in the hydrogenation of less-reactive substrates such as amides, carbonates, carbamates, and urea derivations. These observations indicated that Mn catalysts supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed hydrogenation reactions has yet to be confirmed. Herein, we investigated the origin and applicability of this ligand effect. A combination of experimental and theoretical investigations showed that NNP-pincer ligands on the Mn complexes were more electron-rich and less sterically hindered than their PNP counterparts, leading to higher reactivity in a series of Mn-catalyzed hydrogenation reactions. Inspired by the ligand effect on Mn-catalyzed hydrogenations, we developed the first Mn-catalyzed hydrogenation of N-heterocycles. Specifically, NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding PNP-Mn pincer catalysts afforded low reactivity under the same conditions. This verified that such a ligand effect is generally applicable in hydrogenation reactions of both carbonyl and noncarbonyl substrates based on Mn catalysis.

Theoretical study of the ligand effect on NHC–cobalt-catalyzed hydrogenation of ketones

Theoretical study of ketone hydrogenation revealed that the catalyst regeneration step is more sensitive to the ligand effect.