Hydrogen Transfer Research Articles

Triplet State Radical Chemistry: Significance of the Reaction of 3SO2 with HCOOH and HNO3.

The triplet excited states of sulfur dioxide can be accessed in the UV region and have a lifetime large enough that they can react with atmospheric trace gases. In this work, we report high level ab initio calculations for the reaction of the a3B1 and b3A2 excited states of SO2 with weak and strong acidic species such as HCOOH and HNO3, aimed to extend the chemistry reported in previous studies with nonacidic H atoms (water and alkanes). The reactions investigated in this work are very versatile and follow different kinds of mechanisms, namely, proton-coupled electron transfer (pcet) and conventional hydrogen atom transfer (hat) mechanisms. The study provides new insights into a general and very important class of excited-state-promoted reactions, opening up interesting chemical perspectives for technological applications of photoinduced H-transfer reactions. It also reveals that atmospheric triplet chemistry is more significant than previously thought.

Synthesis, characterization and stability of phosphonium phenolate zwitterions derived from a (diphenylphosphino)phenol derivative and oxiranes

AbstractPhosphonium phenolate zwitterions have been synthesized from 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and five different oxirane derivatives. The reaction does not proceed at a detectable rate when the two reactants are combined in dichloromethane at room temperature. Despite the substantial ring strain, the reaction proceeds only with the addition of methanol, which acts as a hydrogen transfer shuttle, allowing a slow conversion to the desired zwitterions. The compounds have been fully characterized and single crystal X-ray crystallography has been performed on the methyloxirane and the phenyl glycidyl ether-derived zwitterion. The phosphonium phenolate units exhibit an ylidic bonding situation as evidenced by spectroscopic and crystallographic analysis. Glycidyl ethers were found to react faster than alkyl and aryloxiranes. Decomposition studies of the zwitterions showed high thermal stability in solution under ambient conditions. Under forced conditions (150 °C, 6 h), decomposition to the corresponding phosphine oxide and secondary aliphatic alcohols, the formally hydrogenated oxirane derivative, was observed. Graphical abstract

Liberating C-H Bond Activation: Achieving 56% Quantum Efficiency in Photocatalytic Cyclohexane Dehydrogenation.

The technology of liquid organic hydrogen carriers presents great promise for large-scale hydrogen storage. Nevertheless, the activation of inert C(sp3)-H bonds in hydrocarbon carriers poses formidable challenges, resulting in a sluggish dehydrogenation process and necessitating high operating temperatures. Here, we break the shackles of C-H bond activation under visible light irradiation by fabricating subnanometer Pt clusters on defective Ce-Zr solid solutions. We achieved an unprecedented hydrogen production rate of 2601 mmol gcat.-1 h-1 (turnover frequency >50,000 molH2 molPt-1 h-1) from cyclohexane, surpassing the most advanced thermo- and photocatalysts. By optimizing the temperature-dominated hydrogen transfer process, achievable by harnessing hitherto wasted infrared light in sunlight, an astonishing 56% apparent quantum efficiency and a 5.2% solar-to-hydrogen efficiency are attained at 353 K. Our research stands as one of the most effective photocatalytic processes to date, holding profound practical significance in the utilization of solar energy and the exploitation of alkanes.

Application of phthalocyanine complexes of Cu(II), Co(II), Ni(II), and Zn(II) as catalysts in the transfer hydrogenation of acetophenone and its derivatives

In this study, tetra substituted metallophthalocyanines (M: Ni, Zn, Co, and Cu) containing imine and azo groups were synthesized using microwave irradiation. The structures of all complexes have been totally characterized via 1HNMR , 13CNMR , TGA/DTA, LC-MS/MS, elemental analysis, UV–Vis, FT-IR spectroscopy. The comparison of the catalytic features of phthalocyanine based on metals is also discussed briefly. These phthalocyanine-complexes were also used in the transfer hydrogenation (TH) of acetophenone derivatives in the existence of KOH, utilizing isoPrOH as a hydrogen source. Acetophenone compounds determined a TH of up to 99 % conversion. Copper phthalocyanines were first used as catalysts in transfer hydrogenation reactions. The Cu(II) complexes showed higher catalytic activity, converting up to 97 % at 1.0 mol% catalyst loading. Furthermore, we have discovered that the catalytic characteristics of this class of molecules are significantly influenced by both steric and electronic variables.

Catalytic Conversion of Oil Shale over Fe or Ni Catalysts under Sub-Critical Water

Sub-critical water is an environment-friendly solvent. It is widely used for the extraction of various organic compounds. It can be used to dissolve and transport organic matter in oil shale. In this study, the conversion of oil shale was synergistically catalyzed by the addition of Fe or Ni to the Fe inherent in samples under sub-critical water conditions. Oil shale can be converted to gas, oil and residues of oil. Thermogravimetric (TG) analysis results presented that the weight loss of raw oil shale was up to 15.85%. After sub-critical water extraction, the weight loss rate of the residues was reduced to 8.41%. With the application of a metal catalyst, Fe or Ni, the weight loss of residues was further reduced to 7.43% and 6.57%, respectively. According to DTG curves, it was found that there were two weight-loss rate peaks. The decomposition process of kerogen in oil shale could be divided into two cracking processes. One is decomposed at a high velocity at around 420 °C, and another is decomposed at a low velocity at around 515 °C. Gas chromatography (GC) results of gas products indicated that Fe or Ni could contribute to producing normal alkanes, such as methane, ethane, propane, etc., which are produced by the hydrogenation of alkenes via hydrogen transfer during the conversion process of kerogen. Gas chromatography-mass spectrometry (GC–MS) was conducted to analyze the components of the liquid products. The results showed that n-alkanes, iso-alkane, oxygenated hydrocarbons and aromatic compounds were the major components of the kerogen cracking products. When Ni was introduced as a catalyst, the contents of aromatic compounds and oxygenated hydrocarbons in the liquid products were increased from 19.55% and 6.87% to 22.38% and 13.77%, respectively. This is due to the synergistic effect of the addition of Ni with the inherent Fe in oil shale under sub-critical water which ensures kerogen is more easily cracked to produce aromatic compounds and oxygenated hydrocarbons.

Quaternary conducting Cs/GO/PANi hydrogel composites: A smart material for room temperature hydrogen sensing

Hydrogen sensors are essential for hydrogen energy technology development and safety monitoring. However, hydrogen sensing is difficult because it requires effective and fast detection of tiny quantities of hydrogen. In situ oxidative radical polymerization is used in this study to combine polyaniline (PANi) with a chitosan/graphene oxide (Cs/GO) hydrogel, which is very good at conducting electricity, to make it sensor. FTIR, XRD, SEM, and UV were used to evaluate the material's functional groups, crystal structure, morphology, and bandgap. This work shows that Cs/GO/PANi can synthesise a hydrogel-based sensor. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry (CA) were used to evaluate electron transfer mobility, redox behaviour, and hydrogen sensing. Using a hydrogen source with a concentration of 0.5–2.5 M KOH, the hydrogen sensor's response time, recovery sensitivity, and limit of detection were found to be 0.4 s, 0.8 s, 54.3748 μAM, and 3.5520 μM, respectively. Under conventional conditions, the experiment demonstrated significant sensing capabilities at ambient temperature. According to the research, the sensor works so well because of the π-π stacking interaction, the irregular structure, and the way the GO-based hydrogel and PANi work together.

Evaluation of indole-based organometallics as transfer hydrogenation catalysts with anticancer activity

Half-sandwich complexes containing transition metals offer a wide range of applications owing to their interactions with a variety of targets/molecules that are not accessible for some organic scaffolds. The use of such complexes is therefore a way to provide new mechanisms of drug action. Here, we report a series of neutral metal complexes of the type [(arene)M(O-cyclohexyl-1H-indole-2-carbothiolate)Cl] that have the ability to reduce the coenzyme NAD+ at physiological conditions in the presence of sodium formate as an hydride donor. The influence of the metal ion (Ru, Os, Rh, Ir) is studied to establish some structure-activity relationships. The [(η6-p-cym)Ru(O-cyclohexyl-1H-indole-2-carbothioate)Cl] is the most active catalyst of this family of complexes with a turnover frequency (TOF) of 14.79 h-1 for the transfer hydrogenation reaction of NAD+ to give 1,4-NADH in the presence of formate. Further in vitro studies were carried out to determine a relationship between catalytic activity and potency as an anticancer drug, with the rhodium complex promisingly showing 7x more selectivity towards cancer cells (A2780) than normal cells (PNT2). Preliminary in vivo studies demonstrated the importance of co-treatment with formate to enhance activity. This work gives insights into the mechanism of action of this family of indole-based organometallic complexes.

Hydrogen Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects.

Probing quantum mechanical tunneling (QMT) in chemical reactions is crucial to understanding and developing new transformations. Primary H/D kinetic isotopic effects (KIEs) beyond the semiclassical maximum values of 7‒10 (room temperature) are commonly used to assess substantial QMT contributions in one-step hydrogen transfer reactions, because of the much greater QMT probability of protium vs. deuterium. Nevertheless, we report here the discovery of a reaction model occurring exclusively by H-atom QMT with residual primary H/D KIEs. A 2-hydroxyphenylnitrene, generated in N2 matrix, was found to isomerize to an imino-ketone via sequential (domino) QMT involving anti to syn OH-rotamerization (rate determining step) and [1,4]-H shift reactions. These sequential QMT transformations were also observed in the OD-deuterated sample, and unexpected primary H/D KIEs between 3 and 4 were measured at 3 to 20 K. Analogous residual primary H/D KIEs were found in the anti to syn OH-rotamerization QMT of 2-cyanophenol in a N2 matrix. Evidence strongly indicates that these intriguing isotope-insensitive QMT reactivities arise due to the solvation effects of the N2 matrix medium, putatively through coupling with the moving H/D tunneling particle. Should a similar scenario be extrapolated to conventional solution conditions, then QMT may have been overlooked in many chemical reactions.

Tandem hydrogenation/hydrogenolysis of furfural to 2-methylfuran over multifunctional metallic Cu nanoparticles supported ZIF-8 catalyst

Catalytic transfer hydrogenation (CTH), that employs protic solvents as hydrogen sources to alleviate the use of molecular hydrogen H2, has gained great attention. This work, reports multifunctional, metallic Cu nanoparticles supported ZIF-8 material for CTH of furfural to a highly valued fuel additive, 2-methylfuran (2-MF) using 2-propanol. Of all as-synthesized xCu(yM)/ZIF-8 catalysts with varied NaBH4 concentration (yM) and Cu loading (x), 11Cu(1.5 M)/ZIF-8 exhibited higher catalytic activity with > 99 % FAL conversion and 93.9 % 2-MF selectivity. This is ascribed to its high specific surface area, and existence of optimum amount of Lewis acid-base sites along with Cu0 species, which are responsible for hydrogenation of furfural to furfuryl alcohol and subsequent hydrogenolysis to produce 2-MF. The present work reports a highly efficient and stable, metal-MOF hybrid material for CTH of FAL to 2-MF, which is one among the best reports available in literature, therewith suggests a promising approach for bio-oil upgradation.

Cyclometalated and NNN Terpyridine Ruthenium Photocatalysts and Their Cytotoxic Activity.

The cyclometalated terpyridine complexes [Ru(η2-OAc)(NC-tpy)(PP)] (PP = dppb 1, (R,R)-Skewphos 4, (S,S)-Skewphos 5) are easily obtained from the acetate derivatives [Ru(η2-OAc)2(PP)] (PP = dppb, (R,R)-Skewphos 2, (S,S)-Skewphos 3) and tpy in methanol by elimination of AcOH. The precursors 2, 3 are prepared from [Ru(η2-OAc)2(PPh3)2] and Skewphos in cyclohexane. Conversely, the NNN complexes [Ru(η1-OAc)(NNN-tpy)(PP)]OAc (PP = (R,R)-Skewphos 6, (S,S)-Skewphos 7) are synthesized in a one pot reaction from [Ru(η2-OAc)2(PPh3)2], PP and tpy in methanol. The neutral NC-tpy 1, 4, 5 and cationic NNN-tpy 6, 7 complexes catalyze the transfer hydrogenation of acetophenone (S/C = 1000) in 2-propanol with NaOiPr under light irradiation at 30 °C. Formation of (S)-1-phenylethanol has been observed with 4, 6 in a MeOH/iPrOH mixture, whereas the R-enantiomer is obtained with 5, 7 (50-52% ee). The tpy complexes show cytotoxic activity against the anaplastic thyroid cancer 8505C and SW1736 cell lines (ED50 = 0.31-8.53 µM), with the cationic 7 displaying an ED50 of 0.31 µM, four times lower compared to the enantiomer 6.

Heteropolyacid promoted lignin-MOF derived spherical catalyst for catalytic hydrogen transfer of 5-hydroxymethylfurfural

Catalytic conversion of biomass-derived value-added chemicals was of great significance for the utilization of renewable biomass resources to instead of fossil chemicals. Biomass-derived lignin was regarded as an important support and 5-hydroxymethylfurfural (HMF) was a vital platform chemical derived from cellulose. Herein, a series of lignin-MOF hybrid catalysts were prepared and modified with different heteropolyacids (HPAs), which were then successfully introduced into the selective conversion of HMF to 5-hydroxymethylfurfuryl alcohol (MFA). The effect of different HPA, calcination temperature, etc. were all studied, and all catalysts were well characterized. It was confirmed that silicotungstic acid modified catalyst (Ni3Co-MOF-LS@HSiW) exhibited the best catalytic performance, while the highest conversion of HMF was up to 100%, with the best MFA yield of 86.5%. The finding in this study could provide novel insights for the utilization of lignin and preparation of value-added biomass-derived chemicals.

Bimetallic Ni-Co catalyst for improving selectivity in transfer hydrogenation of phenolic compounds

The study investigated the transfer hydrogenation of guaiacol, anisole, phenol, and positional isomers of dimethoxybenezene using 2-PrOH as the H-donor over Ni, Co, and Ni-Co catalysts. The catalysts, which contained 5 wt% metal, were prepared by the deposition-precipitation method. The results showed that the catalysts had developed a pore structure and fine metal particles. The Ni and Ni-Co catalysts demonstrated exceptional activity in the transfer hydrogenation of phenolic compounds with a substrate to metal molar ratio of 100 to 1. Furthermore, the addition of Co to Ni significantly impacted the selectivity of transfer hydrogenation promoting hydrodeoxygenation of anisole and guaiacol.

Effect of traditional solvent on thermal decomposition mechanism of lignin: A density functional theory study

In order to understand the effect of traditional solvents on lignin pyrolysis, the decarbonylation and decarboxylation reactions of various phenylic lignin model compounds were theoretically investigated using DFT methods at M06-2×/6–31++G(d,p) level. The calculation results show that activation energy of the decarbonylation and decarboxylation reactions of lignin model compounds can be reduced when H2O/CH3OH existed. There are two types of reaction for the H2O/CH3OH during the pyrolysis. For first type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen transfer carrier. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 285.0–300.0 kJ/mol (H2O) and 275.0–290.0 kJ/mol (CH3OH) (decarbonylation), 170.0–210.0 kJ/mol and 155.0–200.0 kJ/mol (decarboxylation). For another type, the synergistic reaction of lignin with H2O/CH3OH as hydrogen source. The energy barriers of the main elemental reaction steps during this type of pyrolysis are about 260.0–278.0 kJ/mol and 240.0–260.0 kJ/mol, 303.0–312.0 kJ/mol and 291.0–297.0 kJ/mol. Furthermore, the reaction temperature has the most significant impact on decomposition reaction of lignin in a methanol medium, suggesting that the reaction in the methanol medium is better than that in the water environment.

Synthesis, characterisation, and catalytic application of a soluble molecular carrier of sodium hydride activated by a substituted 4-(dimethylamino)pyridine

Recently main group compounds have stepped into the territory of precious transition metal compounds with respect to utility in the homogeneous catalysis of fundamentally important organic transformations. Inspired by the need to promote more sustainability in chemistry because of their greater abundance in nature, this change of direction is surprising since main group metals generally do not possess the same breadth of reactivity as precious transition metals. Here, we introduce the dihydropyridylsodium compound, Na-1,2-tBu-DH(DMAP), and its monomeric variant [Na-1,2-tBu-DH(DMAP)]·Me6TREN, and demonstrate their effectiveness in transfer hydrogenation catalysis of the representative alkene 1,1-diphenylethylene to the alkane 1,1-diphenylethane using 1,4-cyclohexadiene as hydrogen source [DMAP = 4-dimethylaminopyridine; Me6TREN = tris(N,N-dimethyl-2-aminoethyl)amine]. Sodium is appealing because of its high abundance in the earth’s crust and oceans, but organosodium compounds have been rarely used in homogeneous catalysis. The success of the dihydropyridylsodium compounds can be attributed to their high solubility and reactivity in organic solvents.

Borane-Trimethylamine Complex: A Versatile Reagent in Organic Synthesis.

Borane-trimethylamine complex (Me3N·BH3; BTM) is the most stable of the amine-borane complexes that are commercially available, and it is cost-effective. It is a valuable reagent in organic chemistry with applications in the reduction of carbonyl groups and carbon-nitrogen double bond reduction, with considerable examples in the reduction of oximes, hydrazones and azines. The transfer hydrogenation of aromatic N-heterocycles and the selective N-monomethylation of primary anilines are further examples of recent applications, whereas the reduction of nitrobenzenes to anilines and the reductive deprotection of N-tritylamines are useful tools in the organic synthesis. Moreover, BTM is the main reagent in the regioselective cleavage of cyclic acetals, a reaction of great importance for carbohydrate chemistry. Recent innovative applications of BTM, such as CO2 utilization as feedstock and radical chemistry by photocatalysis, have extended their usefulness in new reactions. The present review is focused on the applications of borane-trimethylamine complex as a reagent in organic synthesis and has not been covered in previous reviews regarding amine-borane complexes.

Coke formation on different metal-modified (Co, Mo and Zr) ZSM-5 in the catalytic pyrolysis of cellulose to light aromatics

Catalytic pyrolysis of biomass is regarded as a promising method for preparing value-added products. However, the main factors limiting the development of the process at present are the low yield of value-added products and poor selectivity, especially for rapid catalyst deactivation due to coke deposition. The present study focuses on the catalytic pyrolysis of cellulose over metal Co-, Mo-, and Zr-loaded catalysts using a falling fixed bed at 600 °C to evaluate the type of coking of the catalysts as well as the effect of the different metal active sites on the formation of coke deposits. The introduction of Co and Mo metal sites increased the light aromatics yield of ZSM-5 from 128.9 mg/g to 147.2 mg/g and 143.0 mg/g. And the yield of coke deposits decreased from 8.3 to 6.8 and 5.1 wt%. The stability evaluation of the catalyst showed that the deactivation degree of ZSM-5 of the supported metal Co was lower than that of commercial ZSM-5, while the ZSM-5 of the supported metal Mo was rapidly deactivated. The deactivation of the catalysts depended more on the morphology or location of the accumulated coke than on the total amount deposited. The clogging of metal and acidic sites on the surface of the zeolite and the pore channels by the amorphous encapsulating coke was the main cause of catalyst deactivation. The metal Mo active sites promoted dehydrogenation coupling and hydrogen transfer during coke formation, forming a more highly disordered multiring structure, and the metal Co active sites were more capable of decomposing macromolecular compounds, olefins and alkanes, and promoted the formation of graphite-like structural carbon.

Asymmetric catalysis by chiral FLPs: A computational mini-review.

Steric hindrance in Lewis acid (LA) and Lewis base (LB) obstruct the Lewis acid-base adduct formation, and the pair was termed as frustrated Lewis pair (FLP). In the past 16years, the field of enantioselective catalysis by chiral FLPs has been slowly growing. It was shown that chiral LAs are significant as they are involved in the hydrogen transfer (HT) step to the imine, resulting in enantioselectivity. After H2 activation, the borohydride can exist in a number of plausible conformations and their stability is governed by the presence of noncovalent interaction through C-H····π and π····π interactions. However, LBs are not ideal for asymmetric induction as they compete with the imine substrate as a counter LB. Further, the proton transfer from chiral LB to the imine does not induce any chirality as chirality develops in the HT step. However, intramolecular FLPs with chiral scaffold are very efficient as they possess an optimum distance between LA and LB, which facilitates the H2 activation but precludes the adduct formation of the small molecules substrate with the LA component. This mini-review summarizes computational investigation involving chiral LA and LB, and discusses intramolecular FLPs in the enantioselective catalysis.

Cobalt-and-Nitrogen-Doped Carbon Nanoparticle Catalysts for Selective Hydrogen Transfer Reactions

Cobalt-and-Nitrogen-Doped Carbon Nanoparticle Catalysts for Selective Hydrogen Transfer Reactions

Polarizability matters in enantio-selection

The prevalence of chirality, or, handedness in biological world is a fundamental phenomenon and a characteristic hallmark of life. Thus, understanding the origin of enantio-selection, i.e., the sense and magnitude of asymmetric induction, has been a long-pursued goal in asymmetric catalysis. Herein, we demonstrated a polarizability-derived electronic effect that was shown to be capable of rationalizing a broad range of stereochemical observations made in the field of asymmetric catalysis. This effect provided a consistent enantio-control model for the prediction of major enantiomers formed in a ruthenium-catalyzed asymmetric transfer hydrogenations of ketones. Direct and quantitative linear free energy relationships between substrates’ local polarizabilities and observed enantio-selectivity were also revealed in three widely known asymmetric catalytic systems covering both reductions and oxidations. This broadly applicable polarizability-based electronic effect, in conjunction with conventional wisdom mainly leveraging on steric effect considerations, should aid rational design of enantio-selective processes for better production of chiral substances.

Synthesis of Chiral 1,2-Amino Alcohol-Containing Compounds Utilizing Ruthenium-Catalyzed Asymmetric Transfer Hydrogenation of Unprotected α-Ketoamines.

Herein, we disclose a facile synthetic strategy to access an important class of drug molecules that contain chiral 1,2-amino alcohol functionality utilizing highly effective ruthenium-catalyzed asymmetric transfer hydrogenation of unprotected α-ketoamines. Recently, the COVID-19 pandemic has caused a crisis of shortage of many important drugs, especially norepinephrine and epinephrine, for the treatment of anaphylaxis and hypotension because of the increased demand. Unfortunately, the existing technologies are not fulfilling the worldwide requirement due to the existing lengthy synthetic protocols that require additional protection and deprotection steps. We identified a facile synthetic protocol via a highly enantioselective one-step process for epinephrine and a two-step process for norepinephrine starting from unprotected α-ketoamines 1b and 1a, respectively. This newly developed enantioselective ruthenium-catalyzed asymmetric transfer hydrogenation was extended to the synthesis of many 1,2-amino alcohol-containing drug molecules such as phenylephrine, denopamine, norbudrine, and levisoprenaline, with enantioselectivities of >99% ee and high isolated yields.