Acceptorless Dehydrogenation Of Alcohols Research Articles

Acceptorless Dehydrogenation of Primary and Secondary Alcohols Catalysed by Phosphine‐free C, C Chelated Ir (III) NHC Complexes

AbstractHerein, an efficient, atom‐economic, environmentally benign catalytic protocol has been developed for synthesising ketones and aldehydes by acceptorless dehydrogenation of alcohols. The protocol employs C, C chelated Ir (III) NHC complexes to promote acceptorless dehydrogenation of both secondary and primary alcohols. Notably, oxidation of primary alcohol selectively yields the corresponding aldehyde where forming ester byproducts is possible. Using C, C chelated Ir (III) NHC complexes (0.1 mol%), and a catalytic amount of base tBuONa (5 mol%), several aliphatic and aromatic aldehydes and ketones including more challenging carbonyl compounds bearing heterocyclic rings were obtained in moderate to high yields. Moreover, the protocol is also efficient for synthesising industrially important molecules like heliotropin and 3,4,5‐trimethoxy acetophenone in moderate yields. Remarkably, the catalytic system allows the straightforward synthesis of the potentially bio‐active compound cholest‐4‐en‐3‐one through acceptorless dehydrogenation followed by double bond isomerisation under the reaction conditions. Low catalyst loading, mild reaction conditions, high selectivity, short reaction time, and broad‐substrate scopes are a few interesting characteristics of the catalytic protocol described herein.

Acceptorless dehydrogenation of biomass-derived aromatic alcohols to aromatic ketones synergistically catalysed by Pd/C and ionic liquids

Acceptorless dehydrogenation of alcohols is a promising pathway for biomass resource utilization and hydrogen supply, but still lacking for simple and effective catalysts. Herein, we report a metal-ionic liquids (ILs) catalyst for synergistical catalytic dehydrogenation of aromatic alcohols to aromatic ketones under mild conditions without any acid/base additives. It is found that the combination of Pd/C with 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) displays high activity for the dehydrogenation of biomass-derived alcohols including 1-phenyl-1-ethanol, 1-phenyl-1-propanol and 1-(4-methoxyphenyl)ethanol. Mechanistic studies reveal that the Pd/C can be anchored and stabilized by the IL via interacting with [BMIm]+ cation, and the [BF4]- anion plays vital role in the activation of hydroxyl-H and α-H protons via hydrogen bonding interactions. These interactions create an effective microenvironmental for Pd/C catalyzing the cleavage of the hydroxyl-H and α-CH bonds of α-hydroxyl aromatic alcohols to generate aromatic ketones and H2. Moreover, the Pd/C-[BMIm][BF4] catalyst could be recovered and reused for 5 runs without obvious activity loss. This strategy provides more evidences for the synergetic catalysis of metal and ionic liquids and exhibits potential application prospects for hydrogen supply from renewable biomass-derived alcohols.

Creation of a Highly Active Small Cu-Based Catalyst Derived from Copper Aluminium Layered Double Hydroxide Supported on α-Al2 O3 for Acceptorless Alcohol Dehydrogenation.

A highly dispersed carbonate-intercalated Cu2+ -Al3+ layered double hydroxide (CuAl LDH) was created on an unreactive α-Al2 O3 surface (CuAl LDH@α-Al2 O3 ) via a simple coprecipitation method of Cu2+ and Al3+ under alkaline conditions in the presence of α-Al2 O3 . A highly reducible CuO nanoparticles was generated, accompanied by the formation of CuAl2 O4 on the surface of α-Al2 O3 (CuAlO@α-Al2 O3 ) after calcination at 1073 K in air, as confirmed by powder X-ray diffraction (XRD) and Cu K-edge X-ray absorption near edge structure (XANES). The structural changes during the progressive heating process were monitored by using in-situ temperature-programmed synchrotron XRD (tp-SXRD). The layered structure of CuAl LDH@α-Al2 O3 completely disappeared at 473 K, and CuO or CuAl2 O4 phases began to appear at 823 K or 1023 K, respectively. Our synthesised CuAlO@α-Al2 O3 catalyst was highly active for the acceptorless dehydrogenation of benzylic, aliphatic, or cyclic aliphatic alcohols; the TON based on the amount of Cu increased to 163 from 3.3 of unsupported CuAlO catalyst in 1-phenylethanol dehydrogenation. The results suggested that Cu0 was obtained from the reduction of CuO in the catalyst matrix during the reaction without separate reduction procedure and acted as a catalytically active species.

Ni(OH)2 nanoparticles as a recyclable catalyst in acceptorless dehydrogenation of alcohols to acids/acid salts under aerobic conditions

AbstractWe demonstrate herein the catalytic performance of unsupported Ni(OH)2 nanoparticles towards the dehydrogenation of primary alcohols to corresponding acids under aerobic conditions in refluxing toluene. An assortment of aromatic and aliphatic carboxylic acids/acids salts was prepared from corresponding alcohols in moderate to excellent yields. Interestingly, the protocol can also be employed to synthesize carboxylic acids on a gram scale, demonstrating the practical utility of the method. Mechanistic studies suggest that the reaction proceeds through an acceptorless dehydrogenation strategy and follow both Cannizzaro and catalytic dehydrogenative path. Broad substrate scope, operational simplicity, low cost, and reusability are the most interesting features of the current heterogeneous catalytic protocol described herein.

Binuclear cobalt(ii) complexes: synthesis, structure, characterizations and catalytic applications in acceptor-less dehydrogenation (AD) of primary alcohols into aldehydes

The new binuclear (ii) complexes (1–3) with high catalytic activity for acceptor-less dehydrogenation (AD) of diverse primary alcohols into value-added aldehydes under mild conditions.

Pyrazole-pyridine-pyrazole (NNN) ruthenium(II) complex catalyzed acceptorless dehydrogenation of alcohols to aldehydes.

A new series of cationic ruthenium(II) complexes, [RuCl(PPh3)2(κ3-NNN-L1)]Cl (1), [RuH(PPh3)2(κ3-NNN-L2)]Cl (2), and [RuH(PPh3)2(κ3-NNN-L3)]Cl (3), bearing a 2,6-bis (1-R-5-methyl-pyrazol-3-yl) pyridine ligand (L1: R = H, L2: R = C6H4-p-CF3, L3: R = C6H4-p-OCH3), were synthesized and characterized with NMR, HRMS, and single-crystal X-ray diffraction. Their catalytic application was investigated in the acceptorless dehydrogenation (AD) of primary alcohols. Complex 1 outperformed 2 and 3 in terms of the selectivity towards the aldehydes, and provided the aldehydes with a yield of up to 99%, with good functional group tolerance under mild conditions. In addition, the only by-product of the reaction was dihydrogen, which can be collected as clean energy, and the reaction meets the requirements of environment-friendly chemistry. Complex 1 also proved to be a promising catalyst in the sequential AD/condensation reaction between the primary and secondary alcohols, affording α,β-unsaturated ketones in moderate to good selectivity.

Role of ancillary ligands in selectivity towards acceptorless dehydrogenation versus dehydrogenative coupling of alcohols and amines catalyzed by cationic ruthenium(II)-CNC pincer complexes.

An unexpected reversal in catalytic activity for acceptorless dehydrogenative coupling compared to acceptorless alcohol dehydrogenation has been observed using a series of cationic Ru(II)-CNC pincer complexes with different ancillary ligands. In continuation of our study of cationic Ru(II)-CNC pincer complexes 1a-6a, new complexes with bulky N-wingtips [Ru(CNCiPr)(CO)(PPh3)Br]PF6 (1b), [Ru(CNCCy)(CO)(PPh3)Cl]PF6 (1c), [Ru(CNCCy)(CO)(PPh3)H]PF6 (2c), [Ru(CNCiPr)(PPh3)2Cl]PF6 (3b), [Ru(CNCCy)(PPh3)2Cl]PF6 (3c), [Ru(CNCiPr)(PPh3)2H]PF6 (4b), [Ru(CNCCy)(PPh3)2H]PF6 (4c), [Ru(CNCiPr)(DMSO)2Cl]PF6 (6b), and [Ru(CNCCy)(DMSO)2Cl]PF6 (6c) [CNCR = 2,6-bis(1-alkylimidazol-2-ylidene)-pyridine] have been synthesized and the catalytic activities of the new complexes have been compared with their N-methyl analogues for transfer hydrogenation of cyclohexanone and acceptorless dehydrogenation of benzyl alcohol. Furthermore, all complexes have been utilized as catalysts in the dehydrogenative coupling reaction of benzyl alcohol with amines. While the catalytic activities of the new complexes for transfer hydrogenation and acceptorless alcohol dehydrogenation were found to be in line with the previously observed trend based on the ancillary ligands (CO > COD > DMSO > PPh3), for the acceptorless dehydrogenative coupling reaction, complexes containing PPh3 and DMSO ligands performed better compared to complexes containing CO and COD ligands. Based on NMR and mass investigation of catalytic reactions, a plausible mechanism has been suggested to explain the difference in catalytic activity and its reversal during the dehydrogenative coupling reaction. Furthermore, the substrate scope for the dehydrogenative coupling reaction of benzyl alcohol with a wide range of amines has been explored, including synthesizing some pharmaceutically important imines. All new complexes have been characterized by various spectroscopic techniques, and the structures of 4b and 6b have been confirmed by the single-crystal X-ray diffraction technique.

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.

Development and mechanistic investigation of the dehydrogenation of alcohols with an iron(III) salen catalyst.

The iron(III) salen complex (R,R)-N,N'-bis(salicylidene)-1,2-cyclohexanediamineiron(III) chloride has been developed as a catalyst for the acceptorless dehydrogenation of alcohols. The complex catalyzes the direct synthesis of imines in good yields from different primary alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with density functional theory calculations. In contrast to the corresponding manganese(III) salen-catalyzed dehydrogenation, it has not been possible to identify a homogeneous catalytic pathway with the iron complex. Instead, poisoning experiments with trimethylphosphine and mercury indicated that the catalytically active species are heterogeneous small iron particles.

Zeolitic imidazolate framework-8 as an efficient and facile heterogeneous catalyst for the acceptorless alcohol dehydrogenation to carboxylates

Zeolitic imidazolate framework-8 (ZIF-8) is a representative Zn-based metal–organic framework (MOF) featuring high porosity, good stability, and large surface area. Owing to these fascinating properties, ZIF-8 exhibits promising potential in catalysis. With our continuous interest in catalyst development of alcohol-involving green transformations, we intended to exploit the capability of ZIF-8 in catalyzing the acceptorless dehydrogenation of alcohols to carboxylates. The preparation of ZIF-8 at room temperature possesses advantages such as environmental friendliness and easy operation. Subsequently, various techniques were adopted to characterize the structures, morphology, and chemical states of both fresh and reused catalysts. Notably, the catalyst displayed good reusability since it could be reused at least 5 times without considerable loss of catalytic activity. It was also perceived that KOH was an indispensable component of this catalytic process. More specifically, KOH not only worked as one of the starting materials but also provided the basic environment to promote this catalysis. Furthermore, the aldehyde was confirmed as a key intermediate for this catalytic process. Based on additional experiments and literature reports, we proposed [Zn–N] as the active sites and presented a plausible catalytic cycle.

Visible-Light-Initiated Acceptor-Less Dehydrogenation of Alcohols to Vicinal Diols over UiO-66(Zr): Surface Complexation and Role of Bridging Hydroxyl

Visible-Light-Initiated Acceptor-Less Dehydrogenation of Alcohols to Vicinal Diols over UiO-66(Zr): Surface Complexation and Role of Bridging Hydroxyl

Acceptorless Dehydrogenation of Aliphatics, Amines, and Alcohols with Homogeneous Catalytic Systems

AbstractThe dehydrogenation of saturated substrates is fundamentally essential for producing value-added unsaturated organic molecules both in academia and industry. In recent years, homogeneously catalyzed acceptorless C–C, C–N, and C–O bond desaturations have attracted increasing attention due to high atom economy, environmentally benign nature, and wide availability of the starting materials. This short review discusses the acceptorless dehydrogenation of aliphatics, alcohols, and amines by homogeneous catalytic systems based on two categories of reaction mechanisms: thermal transition-metal-catalyzed two-electron pathway and photoredox catalyzed or electrochemically driven one-electron pathway.1 Introduction2 Catalytic Acceptorless Dehydrogenation of Aliphatics3 Catalytic Acceptorless Dehydrogenation of Amines4 Catalytic Acceptorless Dehydrogenation of Alcohols5 Conclusion

Acceptorless Dehydrogenation of Alcohols and Transfer Hydrogenation of Aldehydes in Water Using a Single Bifunctional, Bistate, Water-soluble Ruthenium Catalyst.

A water-soluble bifunctional, bistate ruthenium catalyst has been developed using the 8-aminoquinoline ligand which responds to an acid-base stimuli and is catalytically active for two complementary reactions. The "Ru-amino state" is highly active for catalytic transfer hydrogenation of various aldehydes using formic acid as the hydrogen source resulting in a range of primary alcohols. On the other hand, the "Ru-amido state" is active for the acceptorless dehydrogenation of alcohols to yield carboxylate salts or ketones. The reactions can be carried out in air without an inert atmosphere and the products can be purified by simple extraction without the use of any chromatographic techniques. A range of functional groups which include electron-rich and deficient (hetero)arenes and alkenes, alkynes, halides, esters, cyano, and nitro groups, are all well tolerated, indicating excellent chemoselectivity for transfer hydrogenation.

Arene Ru (II)‐catalyzed facile synthesis of N‐acylhydrazones via acceptorless dehydrogenative coupling strategy

A facile catalytic one‐pot synthesis of N‐acylhydrazones via acceptorless dehydrogenative coupling of readily available alcohols and benzohydrazides (R2CONHNH2) using arene ruthenium (II) complexes has been described. The coupling of alcohols with various benzohydrazides using ruthenium catalysts provide a wide range of N‐acylhydrazones in good to excellent yields (63–93%; 32 examples). The present protocol offers high selectivity of hydrazones without any alkylated products and tolerates a range of functional groups. Control experiments indicate that the mechanism proceeds via acceptorless dehydrogenation of alcohols, and hydrogen and water are the sole by‐products. The gram scale synthesis illustrates the usefulness of the present strategy.

Ancillary ligand effects and microwave‐assisted enhancement on the catalytic performance of cationic ruthenium(II)‐CNC pincer complexes for acceptorless alcohol dehydrogenation

The application of cationic Ru(II)‐CNC pincer complexes [Ru(CNC)(CO)(PPh3)Cl]PF6 (1), [Ru(CNC)(PPh3)2Cl]PF6 (2), [Ru(CNC)(η2:η2‐COD)Cl]PF6 (4), and [Ru(CNC)(DMSO)2Cl]PF6 (5) in acceptorless dehydrogenation of alcohols is reported under conventional “oil‐bath” and microwave heating conditions. The effect of different ancillary ligands (CO, PPh3, COD, and DMSO) have been demonstrated during catalysis. Complexes with more π‐acid ligands CO and COD perform better compared with those with PPh3 and DMSO ligands. A plausible mechanism is proposed based on NMR and mass investigation during the catalysis. The catalytic performance matches with the order of trans effect among the four ancillary ligands studied in these complexes. Further, a significant reduction in reaction time, as well as reaction temperature, was observed for catalytic reactions under the microwave‐assisted reaction compared with the oil‐bath heating reaction.

Geometrical isomerization and acceptorless dehydrogenative alcohol oxidation based on pyrrole-based Ru(Ⅱ) complexes

Under the condition of room temperature, trans-pyr-Ru-H Ru(pp)H(CO)(PPh3)2] (complex 1) can be obtained by treating RuHCl(CO)(PPh3)3 with deprotonated 2-(1H-pyrrol-2-yl) pyridine (Hpp) ligand in the tetrahydrofuran(THF) solvent, whereas under refluxing condition, cis-pyr-Ru-H Ru(pp)H(CO)(PPh3)2] (complex 2) can be generated. The full characterizations of the above two complexes include 1H NMR, 31P NMR, IR and X-ray single crystal diffraction, which indicates that complexes 1 and 2 are geometric isomer. With two PPh3 as axial ligands, and pp-, hydride and CO as equatorial ligands, Ru is located at the geometric center of octahedron, As observed, the almost complete conversion of complex 1 to complex 2 under refluxing THF conditions indicates that complex 2 is thermodynamically more stable. In this paper, the catalytic effects of complexes 1 and 2 on the acceptorless dehydrogenation of alcohols to carboxylic acids were studied.

Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System.

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.

Acceptorless dehydrogenation of primary alcohols to carboxylic acids by self-supported NHC-Ru single-site catalysts

The acceptorless dehydrogenation of diverse aromatic and aliphatic primary alcohols to corresponding carboxylic acids has been accomplished by self-supported NHC-Ru single-site catalysts under mild reaction conditions. Besides broad substrates with excellent activity, selectivity and good tolerance to sensitive functional groups, the solid single-site catalyst could be recovered and reused for more than 20 runs without deactivation. Remarkably, up to 1.8 × 104 turnover numbers could be achieved by this newly developed sustainable protocol in gram scale at low catalyst loading, highlighting its potential in industry.

Bioinspired desaturation of alcohols enabled by photoredox proton-coupled electron transfer and cobalt dual catalysis

In the biosynthesis sterols an enzyme-catalyzed demethylation is achieved via a stepwise oxidative transformation of alcohols to olefins. The overall demethylation proceeds through two sequential monooxygenation reactions and a subsequent dehydroformylative saturation. To mimic the desaturation processes observed in nature, we have successfully integrated photoredox proton-coupled electron transfer (PCET) and cobaloxime chemistry for the acceptorless dehydrogenation of alcohols. The state-of-the-art remote and precise desaturation of ketones proceeds efficiently through the activation of cyclic alcohols using bond-dissociation free energy (BDFE) as thermodynamic driving force. The resulting transient alkoxyl radical allows C-C bond scission to generate the carbon-centered radical remote to the carbonyl moiety. This key intermediate is subsequently combined with cobaloxime photochemistry to furnish the alkene. Moreover, the mild protocol can be extended to desaturation of linear alcohols as well as aromatic hydrocarbons. Application to bioactive molecules and natural product derivatives is also presented.

In Situ Generation of Catalytically Active Cu0 Species Derived from Cu-Al Layered Double Hydroxides for Acceptorless Alcohol Dehydrogenation

Abstract A CuO-CuAl2O4 nanocomposite (CuAl NC) synthesized by high temperature calcination of Cu-Al layered double hydroxides (CuAl LDHs) acted as an effective heterogeneous catalyst for acceptorless dehydrogenation of various alcohols. The catalytically active Cu0 species was formed by in situ reduction with alcohol. The catalytic activity strongly depended on the Cu/Al ratio in preparation and calcination temperature. The synthesized catalyst was reusable without loss of its high activity and selectivity.