A Single Nickel Catalyst for the Acceptorless Dehydrogenation of Alcohols and Hydrogenation of Carbonyl Compounds

Multicomponent synthesis via acceptorless alcohol dehydrogenation: an easy access to tri-substituted pyridines

Herein, we report palladium supported on a hydroxyapatite catalyst for synthesizing tri-substituted pyridines using ammonium acetate as the nitrogen source via acceptorless alcohol dehydrogenation strategy. The strategy offers a broad substrate scope using inexpensive and readily available alcohols as the starting material. The catalyst was prepared using a simple method and analyzed by several techniques, including FE-SEM, EDS, HR-TEM, BET, XRD, FT-IR, UV-visible spectroscopy, and XPS, demonstrating the anchoring of Pd nanoparticles on hydroxyapatite in the zero oxidation state. Moreover, several controlled experiments were carried out to understand the reaction pathway and a suitable mechanism has been proposed.

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.

Hydrogenation and dehydrogenation of N-heterocycles under Cp*Co(III)-catalysis

To realize the goal of a carbon-free energy economy, it is crucial to discover reactions that utilize sustainable resources as alternatives to fossil feedstocks. In this study, a well-defined, air-stable Cp*Co(III)-catalyst for transfer hydrogenation of quinoline derivatives and oxidative dehydrogenation of cyclic amines in water is developed. While the former reaction is promoted by formic acid as a transfer hydrogenation reagent, the latter is mediated by molecular oxygen as the sole oxidant. These processes provide new avenues for the investigation of air-stable cobalt catalysts for environmentally benign hydrogenation and dehydrogenation reactions.

Ruthenium complexes of 1,4-diazabutadiene ligands with a cis-RuCl2 moiety for catalytic acceptorless dehydrogenation of alcohols: DFT evidence of chemically non-innocent ligand participation

The acceptorless dehydrogenative coupling (ADC) of primary alcohols to esters by diazabutadiene-coordinated ruthenium compounds is reported. Treatment of cis-Ru(dmso)4Cl2 in acetone at 56 °C with different 1,4-diazabutadienes [p-XC6H4N[double bond, length as m-dash]C(H)(H)C[double bond, length as m-dash]NC6H4X-p; X = H, CH3, OCH3, and Cl; abbreviated as DAB-X], gives trans-Ru[κ2-N,N-DAB-X]2Cl2 as the kinetic product of substitution. Heating these products in o-xylene at 144 °C gives the thermodynamically favored cis-Ru[κ2-N,N-DAB-X]2Cl2 isomers. Electronic structure calculations confirm the greater stability of the cis diastereomer. The molecular structures for each pair of geometric isomers have been determined by X-ray diffraction analyses. Cyclic voltammetry experiments on the complexes show an oxidative response and a reductive response within 0.50 to 0.93 V and -0.76 to -1.24 V vs. SCE respectively. The cis-Ru[κ2-N,N-DAB-X]2Cl2 complexes function as catalyst precursors for the acceptorless dehydrogenative coupling of primary alcohols to H2 and homo- and cross-coupled esters. When 1,4-butanediol and 1,5-pentanediol are employed as substrates, lactones and hydroxyaldehydes are produced as the major dehydrogenation products, while secondary alcohols afforded ketones in excellent yields. The mechanism for the dehydrogenation of benzyl alcohol to benzyl benzoate and H2 using cis-Ru[κ2-N,N-DAB-H]2Cl2 (cis-1) as a catalyst precursor was investigated by DFT calculations. The data support a catalytic cycle that involves the four-coordinate species Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-OCH2Ph) whose protonated κ1-diazabutadiene moiety functions as a chemically non-innocent ligand that facilitates a β-hydrogen elimination from the κ1-O-benzoxide ligand to give the corresponding hydride HRu[κ2-N,N-DAB-H][κ1-N-DAB-H](κ2-O,C-benzaldehyde). H2 production follows a Noyori-type elimination to give (H2)Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-O-benzaldehyde) as an intermediate in the catalytic cycle.

Catalytic Acceptorless Dehydrogenation (CAD) of Secondary Benzylic Alcohols into Value-Added Ketones Using Pd(II)-NHC Complexes

For the creation of adaptable carbonyl compounds in organic synthesis, the oxidation of alcohols is a crucial step. As a sustainable alternative to the harmful traditional oxidation processes, transition-metal catalysts have recently attracted a lot of interest in acceptorless dehydrogenation reactions of alcohols. Here, using well-defined, air-stable palladium(II)-NHC catalysts (A-F), we demonstrate an effective method for the catalytic acceptorless dehydrogenation (CAD) reaction of secondary benzylic alcohols to produce the corresponding ketones and molecular hydrogen (H2). Catalytic acceptorless dehydrogenation (CAD) has been successfully used to convert a variety of alcohols, including electron-rich/electron-poor aromatic secondary alcohols, heteroaromatic secondary alcohols, and aliphatic cyclic alcohols, into their corresponding value-added ketones while only releasing molecular hydrogen as a byproduct.

N-Alkylation and N-Methylation of Amines with Alcohols Catalyzed by Nitrile-Substituted NHC-Ir(III) and NHC-Ru(II) Complexes

A series of nitrile-modified N-heterocyclic carbene (NHC) complexes of Ir(III) (2a-e) and Ru(II) (3a-d) have been prepared by transmetallation of [IrCp*Cl₂]₂ and [RuCl₂(p-cymene)]₂ forming an in situ NHC-Ag complex. The structures of all complexes were characterized by ¹H NMR, ¹³C NMR, and Fourier transform infrared (FT-IR) spectroscopies. And the structures were clearly elucidated by performing X-ray diffraction studies on 2b, 3a, and 3c single crystals. The complexes of NHC-Ir(III) (2a-e) and NHC-Ru(II) (3a-d) were investigated in the N-alkylation reaction of aniline derivatives with benzyl alcohols to form N-benzyl amines and in the N-methylation reaction of aniline derivatives with methanol. Both reactions were performed in solvent-free media. The Ir(III) complexes (2a-e) were found to perform essentially better than similar Ru(II) complexes (3a-d) in the N-alkylation and N-methylation reactions. Among the Ir(III) complexes (2a-e), the best results were obtained with 2b. The catalytic mechanisms of both reactions were revealed by ¹H NMR study. Formation of Ir-hydride species was observed for both reactions. This new report provides useful information to evaluate the activity of complexes and the differences in sensitivity between the NHCs.

Ligand-Promoted [Pd]-Catalyzed α-Alkylation of Ketones through a Borrowing-Hydrogen Approach

A new class of palladium complexes bearing bidentate 2-hydroxypyridine based ligands have been prepared and fully characterized. The applications of these new complexes towards ketone alkylation reactions with alcohols through a metal-ligand cooperative borrowing-hydrogen (BH) process were demonstrated.

Acceptorless dehydrogenation of alcohols to carboxylic acids by palladium nanoparticles supported on NiO: delving into metal-support cooperation in catalysis

In this work, we have developed a simple NiO-supported Pd nanocatalyst (Pd@NiO) for oxidant-free dehydrogenative oxidation of primary alcohols to carboxylic acids along with hydrogen gas as a byproduct. The catalyst has been characterized by techniques like XRD, HRTEM, SEM-EDX, XPS and ICP-AES. The nanostructured Pd@NiO material showed excellent dehydrogenative oxidation activity and outperformed the activity of free NiO or Pd nanoparticles supported on silica/carbon as a catalyst, which could be attributed to synergistic effect of Pd and NiO. A diverse range of aromatic and aliphatic primary alcohols could be efficiently converted to their corresponding carboxylates in high yields with a catalyst loading as low as 0.08 mol%. Notably, highly challenging biomass derived heterocyclic alcohols such as furfuryl alcohol and piperonyl alcohol can also be efficiently converted to their corresponding acids. Moreover, our catalyst can convert benzyl alcohol to benzoic acid on a gram scale with 89% yield. Interestingly, the H₂ gas liberated in the reaction can also be used as a substrate for the hydrogenation of 3a to 4a in 65% yield. The nanostructured catalyst is highly reusable and no significant decrease in activity was observed after six reaction cycles. A kinetic study revealed that the reaction followed first-order kinetics with a rate constant of k = 1.47 × 10^-4 s^-1, under optimized conditions. The extent of reactivity of different functionalities towards dehydrogenation was also investigated using a Hammett plot showing good linearity.

Nickel-catalyzed coupling of R2P(O)Me (R = aryl or alkoxy) with (hetero)arylmethyl alcohols

α-Alkylation of methyldiarylphosphine oxides with (hetero)arylmethyl alcohols was performed under nickel catalysis. Various arylmethyl and heteroarylmethyl alcohols can be used in this transformation. A series of methyldiarylphosphine oxides were alkylated with 30-90% yields. Functional groups on the aromatic rings of methyldiarylphosphine oxides or arylmethyl alcohols including OMe, NMe2, SMe, CF3, Cl, and F groups can be tolerated. The conditions are also suitable for the α-alkylation reaction of dialkyl methylphosphonates.

Identification of key oxidative intermediates and the function of chromium dopants in PKU-8: catalytic dehydrogenation of sec-alcohols with tert-butylhydroperoxide

t-BuOO* activated by Cr-PKU-8 from TBHP is the key intermediate to the highly selective dehydrogenation of sec-alcohols.

Paramagnetic Pyrazolylborate Complexes Tp2M and Tp*2M: 1H, 13C, 11B, and 14N NMR Spectra and First-Principles Studies of Chemical Shifts

The paramagnetic pyrazolylborates Tp₂M and Tp*₂M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp₂M]⁺ and [Tp*₂M]⁺ (M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The ¹H signal shift ranges vary from ∼30 ppm (Cu(II) and V(III)) to ∼220 ppm (Co(II)), and the ¹³C signal shift ranges from ∼180 ppm (Fe(III)) to ∼1150 ppm (Cr(II)). The ¹¹B and ¹⁴N shifts are ∼360 and ∼730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved ¹³C,¹H couplings. All chemical shifts have been calculated from first-principles on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splitting, and g tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

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.

Nickel(ii)-catalyzed direct olefination of benzyl alcohols with sulfones with the liberation of H2

A nickel(ii)-catalyzed direct olefination of benzyl alcohols with sulfones to access various terminal and internal olefins with the liberation of hydrogen gas is reported. The present protocol has been used for E-selective synthesis of DMU-212, and Resveratrol.

Nickel-Catalyzed Alkylation of Ketone Enolates: Synthesis of Monoselective Linear Ketones

Herein we have developed a Ni-catalyzed protocol for the synthesis of linear ketones. Aryl, alkyl, and heteroaryl ketones as well as alcohols yielded the monoselective ketones in up to 90% yield. The catalytic protocol was successfully applied in to a gram-scale synthesis. For a practical utility, applications of a steroid derivative, oleyl alcohol, and naproxen alcohol were employed. Preliminary catalytic investigations involving the isolation of a Ni intermediate and defined Ni-H species as well as a series of deuterium-labeling experiments were performed.

Cobalt-Catalyzed Acceptorless Dehydrogenative Coupling of Primary Alcohols to Esters

A novel catalytic system with a tripodal cobalt complex is developed for efficiently converting primary alcohols to esters. KO ^tBu is found essential to the transformation. A preliminary mechanistic study suggests a plausible reaction route that involves an initial Co-catalyzed dehydrogenation of alcohol to aldehyde, followed by a Tishchenko-type pathway to ester mediated by KO ^tBu.

Dehydrogenation of alcohols and polyols from a hydrogen production perspective

The production of hydrogen from renewable resources is still a major challenge in our way to reach a foreseen hydrogen economy. Abstracting the hydrogen contained in alcohols by means of acceptorless dehydrogenation reactions has emerged as a viable method with high potential. This is particularly true when applied to bio-based alcohols such as ethanol, glycerol or sugars, whose hydrogen extrusion is covered in this contribution. A general overview of the development of aceptorless alcohol dehydrogenation reactions and its potential implementation into future biorefineries are discussed.

Hydrogenation of carbonyl compounds of relevance to hydrogen storage in alcohols

Alcohols are a promising source for the sustainable production of hydrogen that may also serve as rechargeable liquid organic hydrogen carriers (LOHCs). Metal-catalyzed acceptorless dehydrogenation of alcohols produces carbonyl derivatives as H2-depleted by-products, which by means of a hydrogenation reaction can be reconverted to the initial alcohols. Hence, reversible H2-storage systems based on pairs of secondary alcohols/ketones and primary alcohols/carboxylic acid derivatives may be envisaged. In this contribution, the hydrogenation of carbonyl derivatives, including ketones, esters, amides and carboxylic acids, is reviewed from the perspective of the hydrogen storage in alcohols.

Transfer Hydrogenation of Alkenes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex: Scope and Mechanism

The first general catalytic approach to effecting transfer hydrogenation (TH) of unactivated alkenes using ethanol as the hydrogen source is described. A new NCP-type pincer iridium complex (^BQ-NC^OP)IrHCl containing a rigid benzoquinoline backbone has been developed for efficient, mild TH of unactivated C-C multiple bonds with ethanol, forming ethyl acetate as the sole byproduct. A wide variety of alkenes, including multisubstituted alkyl alkenes, aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal alkynes, are suitable substrates. Importantly, the (^BQ-NC^OP)Ir/EtOH system exhibits high chemoselectivity for alkene hydrogenation in the presence of reactive functional groups, such as ketones and carboxylic acids. Furthermore, the reaction with C₂D₅OD provides a convenient route to deuterium-labeled compounds. Detailed kinetic and mechanistic studies have revealed that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic difference. The OH group of ethanol displays a normal kinetic isotope effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene or 1-octene with relatively strong binding affinity to the Ir(I) center has (^BQ-NC^OP)Ir^I(alkene) adduct as an off-cycle catalyst resting state, and the rate law shows a positive order in EtOH, inverse first-order in styrene, and first-order in the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst resting state of (^BQ-NC^OP)Ir^III(H)[O(Et)···HO(Et)···HOEt] that features a six-membered iridacycle consisting of two hydrogen-bonds between one EtO ligand and two EtOH molecules, one of which is coordinated to the Ir(III) center. The rate law shows a negative order in EtOH, zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium isotope effect for the pre-equilibrium formation of (^BQ-NC^OP)Ir^III(H)(OEt) from the catalyst resting state via ethanol dissociation. Regardless of the substrate, ethanol dehydrogenation is the slow segment of the catalytic cycle, while alkene hydrogenation occurs readily following the rate-determining step, that is, β-hydride elimination of (^BQ-NC^OP)Ir(H)(OEt) to form (^BQ-NC^OP)Ir(H)₂ and acetaldehyde. The latter is effectively converted to innocent ethyl acetate under the catalytic conditions, thus avoiding the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.

Nickel-Catalyzed Phosphine Free Direct N-Alkylation of Amides with Alcohols

Herein, we developed an operational simple, practical, and selective Ni-catalyzed synthesis of secondary amides. Application of renewable alcohols, earth-abundant and nonprecious nickel catalyst facilitates the transformations, releasing water as byproduct. The catalytic system is tolerant to a variety of functional groups including nitrile, allylic ether, and alkene and could be extended to the synthesis of bis-amide, antiemetic drug Tigan, and dopamine D2 receptor antagonist Itopride. Preliminary mechanistic studies revealed the participation of a benzylic C-H bond in the rate-determining step.

Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

A molecularly defined SNS-cobalt(ii) catalyst for the acceptorless dehydrogenative coupling (ADC) of unprotected amino alcohols with secondary alcohols leading to pyrrole and pyridine derivatives is reported.

Nickel-catalysed alkylation of C(sp3)–H bonds with alcohols: direct access to functionalised N-heteroaromatics

Base-metal catalysed C(sp³)–H bond functionalisation of methyl-N-heteroaromatics with alcohols is reported for the first time.