Organic Hydride Donors Research Articles

A Mechanism Study of Redox Reactions of the Ruthenium-oxo-polypyridyl Complex.

Over the years, RuIV(bpy)2(py)(O)2+([RuIVO]2+) has garnered considerable interest owing to its extensive use as a polypyridine mono-oxygen complex. However, as the active-site Ru=O bond changes during the oxidation process, [RuIVO]2+ can be used to simulate the reactions of various high-priced metallic oxides. In order to elucidate the hydrogen element transfer process between the Ruthenium-oxo-polypyridyl complex and organic hydride donor, the current study reports on the synthesis of [RuIVO]2+, a polypyridine mono-oxygen complex, in addition to 1H and 3H (organic hydride compounds) and 1H derivative: 2. Through 1H-NMR analysis and thermodynamics- and kinetics-based assessments, we collected data on [RuIVO]2+ and two organic hydride donors and their corresponding intermediates and established a thermodynamic platform. It was confirmed that a one-step hydride transfer reaction between [RuIVO]2+ and these organic hydride donors occurs, and here, the advantages and nature of the new mechanism approach are revealed. Accordingly, these findings can considerably contribute to the better application of the compound in theoretical research and organic synthesis.

Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP+ Cofactor Analogue for Electrocatalytic CO2 Reduction.

Biomimetic NAD(P)H-type organic hydride donors have recently been advocated as potential candidates to act as metal-free catalysts for fuel-forming reactions such as the reduction of CO2 to formic acid and methanol, similar to the natural photosynthesis process of fixing CO2 into carbohydrates. Although these artificial synthetic organic hydrides are extensively used in organic reduction chemistry in a stoichiometric manner, translating them into catalysts has been challenging due to problems associated with the regeneration of these hydride species under applied reaction conditions. A recent discovery of the possibility of their regeneration under electrochemical conditions via a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO2 reduction reactions (eCO2RR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCO2RR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10-3-10-2; thus, sub-stoichiometric only! Herein, we report a novel class of structurally engineered heterohelicene-based organic hydride donor with a proof-of-principle demonstration of catalytic electrochemical CO2 reduction reaction showing a significantly improved activity with more than stoichiometric turnover featuring a 100-1000-fold enhancement of the existing TON values. Mechanistic investigations suggested the critical role of the two cationic imidazolium motifs along with the extensive π-conjugation present in the backbone of the heterohelicene molecules in accessing and stabilizing various radical species involved in the generation and transfer of hydride, via multielectron-transfer steps in the electrochemical process.

Total Synthesis of the Fungal Metabolite Trienylfuranol A through Nucleophilic Diastereodivergent Additions to Oxocarbenium Ions

AbstractHerein, we describe the first total synthesis of trienylfuranol A, a fungal triene‐substituted tetrahydrofuran metabolite. The stereoselectivity of the chiral center bearing the trienyl side chain was diastereodivergently controlled by the addition of nucleophilic species on substituted γ‐butyrolactone. Remarkably, theC‐nucleophilic species or hydride addition onto oxocarbenium intermediate leads to the opposite diastereoselectivity reported for Kishi or Woerpel models. The use of Hantzsch ester (HEH) as organic hydride donor has enabled us to obtain the desired stereochemistry. The total synthesis of trienylfuranol A was achieved in 8 steps from acetaldehyde and pyruvic acid.

Reactivity of an air-stable dihydrobenzoimidazole n-dopant with organic semiconductor molecules

Summary 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole, N-DMBI-H, is widely used as an air-stable n-dopant for organic semiconductors. Here, its reactivity is investigated with a variety of imide- and amide-containing semiconductor molecules that have reduction potentials in the range −0.54 to −1.10 V versus ferrocene. Reaction rates correlate poorly with these potentials. The more reactive of the imides form the corresponding radical anions cleanly, but kinetic isotope studies using N-DMBI-D indicate that the reaction proceeds via an initial hydride- or hydrogen-transfer step. For an amide- and ester-rigidified bis(styryl)benzene derivative the hydride-reduced product is stable under an inert atmosphere and can be observed directly; the radical anion can only be obtained in sub-stoichiometric yield and under certain reaction conditions. On the other hand, (N-DMBI)2 rapidly reduces all the imides and amides examined to the corresponding radical anions. The implications of these findings for dopant selection and use are discussed.

Development of robust Pt shell through organic hydride donor in PtCo@Pt core-shell electrocatalysts for highly stable proton exchange membrane fuel cells

Platinum alloys with other transition metals are widely used as catalysts as a promising strategy to improve the activity. However, the vulnerability of transition metals to the liquid electrolyte and fuel gases present during the device operation results in degradation of the catalytic activity. Highly stable catalyst is achieved by encapsulating the carbon-supported PtCo intermetallic core nanoparticles in a robust Pt shell. The hydride from Hantzsch ester reduces the residual Cl on the surface of the Pt skeleton and slowly forms negatively charged defect sites without damaging the core structure. Then, a secondary Pt reduction reaction via the hydride is employed to cover the activated surfaces of the Pt skeleton to optimize the shell thickness without the formation of isolated Pt nanoparticles. The PtCo@Pt with the unique Pt shell shows higher catalytic activity and durability than commercial Pt/C for oxygen reduction reaction electrocatalysts in proton exchange membrane fuel cells.

Organometallic hydride-transfer agents as reductants for organic semiconductor molecules

Both organic hydride donors, DH, and the dimers formed by some organic radicals and by some 19-electron organometallic sandwich compounds, D2, have recently been examined extensively as n-dopants for organic semiconductors. This properties of two classes of organometallic DH derivatives – organic DH derivatives with metallocenyl substituents and species obtained by hydride reduction of 18-electron D+ sandwich cations – are discussed and compared to those of the corresponding D2 molecules. 1,3-Dimethyl-2-metallocenyl-2,3-dihydro-1H-benzo[d]imidazoles, Mc-DMBI-H (Mc = ferrocenyl [Fc], ruthenocenyl [Rc]), and 1,3-dimethyl-2-phenyl-1H-benzo[d]imidazolium tetrakis(3,5-bis(trifluoromethyl))borates, Mc-DMBI+BAr'4– (Mc = Fc, Rc), have recently been reported as intermediates in the synthesis of the dimers (Mc-DMBI)2; here their crystal structures are reported and compared to those of related molecules. The solution reactivity of the Mc-DMBI-H derivatives with a fullerene acceptor is similar to that of purely organic aryl-DMBI-H species. Previously reported (exo-H-η4-pentamethylcyclopentadiene)(η5-cyclopentadienyl)metal complexes, M(exo-H-Cp*H)Cp (M = Rh, Ir) and the new (η5-pentamethylcyclopentadienyl)(η5-1,3,5-trimethylcyclohexa-1,3-dien-5-yl)ruthenium, RuCp*(mesH), have also been examined as potential reductants for organic semiconductors; RuCp*(mesH) is particularly reactive, affording the radical anions of perylene diimide and 6,13-bis(tri(isopropyl)silylethynyl)pentacene. The electrochemistry of DH, D+, and D2 species is compared for the five compounds. Density-functional theory calculations are used to gain insight into the trends in electrochemistry and reactivity.

Evaluation of Organic Hydride Donors as Reagents for the Reduction of Carbon Dioxide and Metal-Bound Formates

A variety of organic hydride donors (OHDs) have been tested as reagents for the transfer of hydride to iron formato complexes in the activation and reduction of carbon dioxide. Theoretical calculat...

N-Heterocycle-Ligated Borocations as Highly Tunable Carbon Lewis Acids.

The relative (to BEt3) hydride ion affinity (HIA) of a series of acridine borenium salts has been calculated, with some HIAs found to be similar to that for [Ph3C]+. The HIA at the acridine C9 position is controlled by both acridine and the boron substituents, the latter presumably affecting the strength of the B=N bond in the acridane-BY2 products from hydride transfer. Through a range of hydride abstraction benchmarking reactions against organic hydride donors the experimental HIA of [F5acr-BCat]+ (cat = catechol, F5acr = 1,2,3,4,7-pentafluoroacridine) has been confirmed to be extremely high and closely comparable to that of [Ph3C]+. The high HIA of [F5acr-BCat]+ enables H2 and alkene activation in a FLP with 2,6-di-tert-butylpyridine. Finally, the HIA of pyridine and quinoline borenium cations has been determined, with the HIA at boron in [PinB(amine)]+ (pin = pinacol, amine = pyridine or quinoline) found to be relatively low. This enabled the hydroboration of pyridine and quinoline by HBPin to be achieved through the addition of 5–10 mol % of bench-stable cationic carbon Lewis acids such as 2-phenyl-N,N-dimethylimidazolium salts.

Enantioselective Synthesis of β‐Aminotetralins via Chiral Phosphoric Acid‐catalyzed Reductive Amination of β‐Tetralones

AbstractA new protocol for the synthesis of chiral β‐aminotetralins has been developed via chiral phosphoric acid‐catalyzed asymmetric reductive amination of β‐tetralones using a Hantzsch ester as an organic hydride donor. Various chiral β‐aminotetralins were obtained in good yields with good to high enantioselectivities. Furthermore, the utility of our new protocol was successfully demonstrated in the enantioselective synthesis of rotigotine.magnified image

Elemental steps of the thermodynamics of dihydropyrimidine: a new class of organic hydride donors.

25 Dihydropyrimidine derivatives, a new class of organo-hydrides, were designed and synthesized by the Biginelli reaction. For the first time, the thermodynamic driving forces of the six elemental steps to obtain a hydride in acetonitrile were determined by isothermal titration and electrochemical methods, respectively. The effects of molecular structures and substituents on these thermodynamic parameters were examined, uncovering some interesting structure-reactivity relationships. Both the thermodynamic and kinetic studies show that the hydride transfer from dihydropyrimidines to 9-phenylxanthylium (PhXn+ClO4-) prefers a concerted mechanism.

Bio-inspired transition metal-organic hydride conjugates for catalysis of transfer hydrogenation: experiment and theory.

Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI(+) ) organic hydride-acceptor domain has been coupled with a 1,10-phenanthroline (phen) metal-binding domain to afford a novel multifunctional ligand (L(BI+) ) with hydride-carrier capacity (L(BI+) +H(-) ⇌L(BI) H). Complexes of the type [Cp*M(L(BI) )Cl][PF6 ]2 (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the Ir(III) congener, X-ray crystallography. [Cp*Rh(L(BI) )Cl][PF6 ]2 catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir(L(BI) )Cl][PF6 ] and [Cp*M(phen)Cl][PF6 ] (M=Rh, Ir) complexes are almost inert as catalysts. Possible alternatives for the catalysis pathway are canvassed, and the free energies of intermediates and transition states determined by DFT calculations. The DFT study supports a mechanism involving formate-driven RhH formation (90 kJ mol(-1) free-energy barrier), transfer of hydride between the Rh and BI(+) centres to generate a tethered benzimidazoline (BIH) hydride donor, binding of imine substrate at Rh, back-transfer of hydride from the BIH organic hydride donor to the Rh-activated imine substrate (89 kJ mol(-1) barrier), and exergonic protonation of the metal-bound amide by formic acid with release of amine product to close the catalytic cycle. Parallels with the mechanism of biological hydride transfer in yADH are discussed.

The saturated five‐membered heterocyclic molecules as organic hydride donors: a computational study

Several five‐membered heterocyclic molecules were studied theoretically as organic hydride donors. The density functional theory and ab initio methods are employed to study the direct one‐step or multistep sequence suggested for the hydride transfer from the selected molecules: H atom/electron, electron/proton/electron or electron/H atom. Out of the three multistep mechanisms, electron/H atom seems to be a probable pathway in the presence of suitable catalyst/photoreaction that can cause ionization. In the lack of such catalyst/photoreaction, the direct hydride transfer seems to be most probable with the presence of suitable hydride acceptor. A detailed mechanism of the hydride transfer from the five‐membered heterocylic compounds is important in understanding chemical and biological reactions and required for scientifically designing and synthesizing new desired five‐membered heterocyclic compounds as organic hydride donor. Copyright © 2014 John Wiley & Sons, Ltd.

Bio-Inspired Catalytic Imine Reduction by Rhodium Complexes with Tethered Hantzsch Pyridinium Groups: Evidence for Direct Hydride Transfer from Dihydropyridine to Metal-Activated Substrate

Inspired by Nature: A conceptually new design for a catalyst, combining a metal center abutted to an organic hydride donor, is demonstrated for the formate-driven transfer hydrogenation of imines under ambient conditions. A key step, transfer of hydride from the organohydride donor to the metal-polarized substrate, mirrors that in metallo-(de)hydrogenase enzymes.

Calculation of thermodynamic hydricities and the design of hydride donors for CO 2 reduction

We have developed a correlation between experimental and density functional theory-derived results of the hydride-donating power, or "hydricity", of various ruthenium, rhenium, and organic hydride donors. This approach utilizes the correlation between experimental hydricity values and their corresponding calculated free-energy differences between the hydride donors and their conjugate acceptors in acetonitrile, and leads to an extrapolated value of the absolute free energy of the hydride ion without the necessity to calculate it directly. We then use this correlation to predict, from density functional theory-calculated data, hydricity values of ruthenium and rhenium complexes that incorporate the pbnHH ligand-pbnHH = 1,5-dihydro-2-(2-pyridyl)-benzo[b]-1,5-naphthyridine-to model the function of NADPH. These visible light-generated, photocatalytic complexes produced by disproportionation of a protonated-photoreduced dimer of a metal-pbn complex may be valuable for use in reducing CO(2) to fuels such as methanol. The excited-state lifetime of photoexcited [Ru(bpy)(2)(pbnHH)](2+) is found to be about 70 ns, and this excited state can be reductively quenched by triethylamine or 1,4-diazabicyclo[2.2.2]octane to produce the one-electron-reduced [Ru(bpy)(2)(pbnHH)](+) species with half-life exceeding 50 μs, thus opening the door to new opportunities for hydride-transfer reactions leading to CO(2) reduction by producing a species with much increased hydricity.

ChemInform Abstract: Transfer Hydrogenation with Hantzsch Esters and Related Organic Hydride Donors

AbstractReview: ca. 150 refs.

Transfer hydrogenation with Hantzsch esters and related organic hydride donors

In recent years, Hantzsch esters and their related organic hydride donors have been widely utilized in biomimetic approaches of asymmetric transfer hydrogenation (ATH) reactions. Various compounds containing C=C, C=N and C=O unsaturated functionalities could be reduced in the presence of organocatalysts or transition metal complexes, affording versatile chiral building blocks in high yields and excellent enantioselectivities under mild conditions. In this critical review, recent advances in this area are summarized and classified according to unsaturated functional groups being reduced and catalytic systems employed (91 references).

Hydride, Hydrogen Atom, Proton, and Electron Transfer Driving Forces of Various Five-Membered Heterocyclic Organic Hydrides and Their Reaction Intermediates in Acetonitrile

The enthalpy changes of 47 five-membered heterocyclic compounds (ZH) [33 substituted 2,3-dihydro-2-phenylbenzo[d]imidazoles (1H-5H), 9 substituted 2,3-dihydro-2-phenylbenzo[d]thiazoles (6H), and 5 substituted 2,3-dihydro-2-phenylbenzo[d]oxazoles (7H)] as a class of very important organic hydride donors to release hydride anion were determined by using titration calorimetry. The result shows that the enthalpy change scale of the 47 ZH in acetonitrile ranges from 49.0 to 93.4 kcal/mol. Such a long energy scale evidently shows that the 47 ZH can construct a large and useful library of organic hydride donors, which can provide various organic hydride donors that the hydride-releasing enthalpies are known. The enthalpy changes of the 47 ZH to release hydrogen atom and the 47 ZH+* to release proton and hydrogen atom were also evaluated by using relative thermodynamic cycles according to Hess' law. The results show: (1) the enthalpy change scale of the 47 ZH to release hydrogen atom covers a range from 71.8 to 91.4 kcal/mol, indicating that the 47 ZH all should be weak hydrogen atom donors. (2) The enthalpy change scales of the 47 ZH+* to release proton and to release hydrogen atom range from 17.5 to 25.7 and from 27.2 to 52.4 kcal/mol, respectively, implying that the proton-donating abilities of ZH+* are generally quite larger than the corresponding hydrogen atom-donating abilities. The standard redox potentials of the 47 ZH and the 47 corresponding salts (Z+) were measured by using cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV), the results display that the standard oxidation potential scale of ZH ranges from -0.254 to -0.002 V for 1H-5H and from 0.310 to 0.638 V for 6H-7H, implying that 1H-5H should be strong one-electron reducing agents and 6H-7H should be weak one-electron reducing agents; the standard reduction potential scale of Z+ ranges from -1.832 to -2.200 V for 1+-5+ and from -1.052 to -1.483V for 6+-7+, meaning that 1+-5+ belong to very weak one-electron oxidation agents. The energies of the intramolecular hydrogen bond in 3H, 3H+*, and 3* with a hydroxyl group at ortho-position on the 2-phenyl ring were estimated by using experimental method, the results disclose that the hydrogen bond energy is 3.2, 2.8-3.0, and 3.9-4.0 kcal/mol for 3H, 3H+*, and 3* in acetonitrile, respectively, which is favorable for hydrogen atom transfer but unfavorable for hydride transfer from 3H. The relative effective charges on the active center in ZH, ZH+*, Z*, and Z+, which is an efficient measurement of electrophilicity or nucleophilicity as well as dimerizing ability of a chemical species, were estimated by using experimental method; the results indicate that 1*-5* belong to electron-sufficient carbon-radicals, 6*-7* belong to electron-deficient carbon radicals, they are all difficult to dimerize, and that 1+-5+ belong to weak electrophilic agents, 6+-7+ belong to strong electrophilic agents. All these information disclosed in this work could not only supply a gap of the chemical thermodynamics of the five-membered heterocyclic compounds as organic hydride donors, but also strongly promote the fast development of the chemistry and applications of the five-membered heterocyclic organic hydrides.

Photochemical and Radiolytic Production of an Organic Hydride Donor with a RuII Complex Containing an NAD+ Model Ligand

Photochemical and Radiolytic Production of an Organic Hydride Donor with a RuII Complex Containing an NAD+ Model Ligand

Photochemical and Radiolytic Production of an Organic Hydride Donor with a RuII Complex Containing an NAD+ Model Ligand

Photochemical and Radiolytic Production of an Organic Hydride Donor with a Ru^II Complex Containing an NAD⁺ Model Ligand

Reduction of Benzils Promoted by a New Class of Bifunctional Organocatalysts

Procuranti and Connon developed a new class of bifunctional organocatalysts (1) incorporating both a substrate-activating thiourea moiety and an organic hydride donor as NADH analogue, which is (re)-generated in situ using sodium dithionate as a reductant. Racemic catalyst 1a was successfully applied for the reduction of a range of symmetrical and unsymmetrical substituted benzils 2 of variable steric and electronic characteristics to the corresponding benzoins 3 in good to excellent yields. NMR studies confirmed that the transfer hydrogenation proceeds via intramolecular hydride donation from the in situ generated hydropyridine moiety to the thiourea-bound diketone. One example of enantio­selective reduction of benzyl 2 promoted by the enantiopure catalyst 1b is also given, although the enantiomeric ratio (62.5:37.5) is modest.