Hydride Transfer Reactions Research Articles

Aqueous organometallic chemistry. 3. Catalytic hydride transfer reactions with ketones and aldehydes using [Cp∗Rh(bpy)(H 2O)](OTf) 2 as the precatalyst and sodium formate as the hydride source: Kinetic and activation parameters, and the significance of steric and electronic effects

The reaction of a precatalyst, [Cp∗Rh(bpy)(H 2O)](OTf) 2 ( 1), with sodium formate provided the hydride complex, [Cp∗Rh(bpy)(H)] + ( 2), in situ, at pH 7.0, which was then evaluated in an aqueous, catalytic hydride transfer process with water soluble substrates that encompass 2-pentanone ( 3), cyclohexanone ( 4), acetophenone ( 5), propionaldehyde ( 6), benzaldehyde ( 7), and p-methoxybenzaldehyde ( 8). The initial rates, r i, of appearance of the reduction product alcohols at 23 °C provided a relative rate scale: 8 > 7 ≈ 6 > 5 > 4 > 3, while the effect of concentration of substrate, precatalyst, and sodium formate on r i, using 7 as an example, implicates [Cp∗Rh(bpy)(H)] + formation as the rate-limiting step. The experimental kinetic rate expression was found to be: d[alcohol]/ dt = k cat[ 1][HCO 2Na]; substrate being pseudo zero order in water. The steric effects were also analyzed and appeared to be of less importance intra both the ketone and aldehyde series, but an inter series comparison appeared to show that the aldehydes had less of a steric effect on the initial rate, i.e., 7 > 4 by a factor of 3.6, while the aldehyde series appeared to have some moderate electronic influence on rates, presumably via electron donation to increase binding to the Cp∗Rh metal ion center, in accordance with these proposed concerted binding/hydride transfer reactions. A proposed catalytic cycle will also be presented.

The Effect of Electrostatic Shielding on H Tunneling in R67 Dihydrofolate Reductase

Dihydrofolate reductase (DHFR) catalyzes the hydride (H) transfer reaction between NADPH and dihydrofolate, and produces tetrahydrofolate and NADP+. R67 DHFR is a plasmid encoded enzyme, and is considered a “primitive enzyme” due to its genomic, structural, and kinetic properties.[1, 2] Interestingly, kinetic studies of R67 DHFR show an enhancement in H-transfer rate with increasing ionic strength.[3] To evaluate the source of this rate enhancement, the temperature dependency of intrinsic kinetic isotope effects (KIEs) was measured and the nature of the H-transfer step was evaluated at low and high ionic strengths. At high ionic strength, the KIEs were less temperature dependent than at lower ionic strength. These findings were evaluated using a Marcus-like model, which suggests that at higher ionic strength, the donor and acceptor of the hydride were better oriented for H-tunneling than the same system at lower ionic strength. This comparison addresses the level of system preparation that brings the reaction coordinate into a tunneling-ready conformation. While the effect is small, it is statistically significant, as apparent from the comparative data and standard deviations presented in the Supplementary Information (SI – Table S2). These data demonstrate the high sensitivity of the methodology that was developed to study this system (see detailed methods in the SI). The differences in electrostatic potential surface between low and high ionic strengths were calculated, and the theoretical findings add a molecular perspective to the experimental data.

Thermodynamic Studies and Hydride Transfer Reactions from a Rhodium Complex to BX3 Compounds

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H(2) gas to form B-H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX(3) (X = OR, SPh, F, H; R = Ph, p-C(6)H(4)OMe, C(6)F(5), (t)Bu, Si(Me)(3)) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (DeltaG(o)(H(-))) were determined for HRh(dmpe)(2) and HRh(depe)(2), where dmpe = 1,2-bis(dimethylphosphinoethane) and depe = 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt(3)](-) on this scale. Isodesmic reactions between [HBEt(3)](-) and selected BX(3) compounds to form BEt(3) and [HBX(3)](-) were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX(3) compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX(3) compounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)(3) using HRh(dmpe)(2) in the presence of triethylamine to form Et(3)N-BH(3) in high yields.

Kinetics of the Hydride Reduction of an NAD+ Analogue by Isopropyl Alcohol in Aqueous and Acetonitrile Solutions: Solvent Effects, Deuterium Isotope Effects, and Mechanism

The rate constants of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to an NAD(+) model, 9-phenylxanthylium ion (PhXn(+)), in acetonitrile (AN) and in water containing AN (80% H(2)O/20% AN) were determined over a temperature range from 36 to 67 degrees C. The reactions follow second-order rate laws. In the latter solution, formation of the water adduct of PhXn(+) was observed as a side-equilibrium (K). The observed inverse solvent kinetic isotope effect (k(H(2)O)(obs)/k(D(2)O)(obs) = 0.54), the larger than unity equilibrium isotope effect (K(H(2)O)/K(D(2)O) = 2.69), and the results of acid effect on the observed rate constants of the reactions are consistent with the "side-equilibrium mechanism". Kinetic isotope effects at all three H/D positions of i-PrOH for the net hydride-transfer process were determined in both solutions at 60 degrees C: KIE(alpha-D)(H) = 3.2(AN), 3.2(H(2)O); KIE(beta-D6)(H) = 1.05(AN), 1.16(H(2)O); KIE(OD)(H) = 1.08(AN), 1.04(H(2)O). These KIE values are consistent with the presence of the positively charged alcohol moiety in the transition state (TS) for cleavage of the alpha-C-H bond, the delocalization of the positive charge over the alpha-C-OH group, and the stepwise hydride and proton transfer processes. Comparison of the activation parameters for the reactions in the two solvent systems as well as those in the i-PrOH/AN (1:1 v/v) reported earlier suggests that the AN medium promotes the reaction by activating the ground-state alcohol reactant through weak interactions with the electron pairs on alcohol O, while water and parent alcohol media facilitate the reaction by H-bonding stabilization of the alcohol moiety of the TS. Results suggest that in the alcohol dehydrogenases without a Zn(II) cofactor in the active sites alcohols would be oxidized via hydride transfer to NAD(+) coenzyme followed by the rapid deprotonation to the nearby basic species in the active site of the enzymes.

Effect of a conservative mutation of an active site residue involved in substrate binding on the hydride tunneling reaction catalyzed by choline oxidase

The reaction of alcohol oxidation catalyzed by choline oxidase was previously shown to occur through the tunneling of a hydride ion from the α-carbon of an activated alkoxide species to the N(5) atom of FAD within a highly preorganized enzyme-substrate complex [G. Gadda, Biochemistry 47 (2008) 13745–13753]. In the present study, a glutamate residue (312) that participates in substrate binding has been replaced with aspartate by site-directed mutagenesis, and the effect of the substitution on the kinetic parameters of the enzyme and the mechanism of hydride ion transfer were investigated using stopped-flow spectrophotometry. The thermodynamic parameters associated with the enzymatic reaction catalyzed by the mutant enzyme and the temperature dependence of the kinetic isotope effects are consistent with a hydride transfer reaction occurring either through environmentally assisted tunneling or a classical over-the-barrier transition state, but not within a preorganized enzyme-substrate complex. In contrast, less than threefold changes in the K d values for choline were observed in the mutant enzyme with respect to the wild-type enzyme. Thus, the conservative substitution of an active site residue that is involved in substrate binding, but not directly in catalysis, affects primarily the k red value that reports on catalysis and minimally the K d value that reports on binding, thereby providing an example of the interplay that these kinetic parameters may have in enzymatic reactions.

Fundamental Differences of Substrate Hydroxylation by High-Valent Iron(IV)-Oxo Models of Cytochrome P450

An Iron(IV)-oxo heme(+*) complex (Compound I, Cpd I) is the proposed active species of heme enzymes such as the cytochromes P450 and is elusive; therefore, biomimetic studies on active site mimics give valuable insight into the fundamental properties of heme active species. In this work we present density functional theory (DFT) calculations on substrate hydroxylation by a Compound I mimic [Fe(IV)=O(Por(+*))Cl] and its one-electron reduced form [Fe(IV)=O(Por)Cl](-). Thus, recent experimental studies showed that [Fe(IV)=O(Por)Cl](-) is able to react with substrates via hydride transfer reactions [Jeong, Y. J.; Kang, Y.; Han, A.-R.; Lee, Y.-M.; Kotani, H.; Fukuzumi, S.; Nam, W. Angew. Chem., Int. Ed. 2008, 47, 7321-7324]. By contrast, theoretical studies on camphor hydroxylation by these two oxidants concluded that the one-electron reduced form of Compound I is a sluggish oxidant of hydroxylation reactions [Altun, A.; Shaik, S.; Thiel, W. J. Am. Chem. Soc. 2007, 129, 8978-8987]. To resolve the question why the one-electron reduced Compound I is an oxidant in one case and a sluggish oxidant in other cases, we have performed a DFT study on 10-methyl-9,10-dihydro acridine (AcrH(2)) hydroxylation by [Fe(IV)=O(Por(+*))Cl] and [Fe(IV)=O(Por)Cl](-). The calculations presented in this work show that both [Fe(IV)=O(Por(+*))Cl] and [Fe(IV)=O(Por)Cl](-) are plausible oxidants, but [Fe(IV)=O(Por(+*))Cl] reacts via much lower reaction barriers. Moreover, [Fe(IV)=O(Por(+*))Cl] reacts via hydride transfer, while [Fe(IV)=O(Por)Cl](-) by hydrogen abstraction. The differences between hydride and hydrogen atom transfer reactions have been rationalized with thermodynamic cycles and shown to be the result of differences in electron abstraction abilities of the two oxidants. Thus, the calculations predict that [Fe(IV)=O(Por)Cl](-) is only able to hydroxylate weak C-H bonds, whereas [Fe(IV)=O(Por(+*))Cl] is more versatile.

Parallel Pathways and Free‐Energy Landscapes for Enzymatic Hydride Transfer Probed by Hydrostatic Pressure

Mutation of an active-site residue in morphinone reductase leads to a conformationally rich landscape that enhances the rate of hydride transfer from NADH to FMN at standard pressure (1 bar). Increasing the pressure causes interconversion between different conformational substates in the mutant enzyme. While high pressure reduces the donor-acceptor distance in the wild-type enzyme, increased conformational freedom "dampens" its effect in the mutant.We show that hydride transfer from NADH to FMN catalysed by the N189A mutant of morphinone reductase occurs along parallel "chemical" pathways in a conformationally rich free-energy landscape. We have developed experimental kinetic and spectroscopic tools by using hydrostatic pressure to explore this free-energy landscape. The crystal structure of the N189A mutant enzyme in complex with the unreactive coenzyme analogue NADH(4) indicates that the nicotinamide moiety of the analogue is conformationally less restrained than the corresponding structure of the wild-type NADH(4) complex. This increased degree of conformational freedom in the N189A enzyme gives rise to the concept of multiple reactive configurations (MRCs), and we show that the relative population of these states across the free-energy landscape can be perturbed experimentally as a function of pressure. Specifically, the amplitudes of individual kinetic phases that were observed in stopped-flow studies of the hydride transfer reaction are sensitive to pressure; this indicates that pressure drives an altered distribution across the energy landscape. We show by absorbance spectroscopy that the loss of charge-transfer character of the enzyme-coenzyme complex is attributed to the altered population of MRCs on the landscape. The existence of a conformationally rich landscape in the N189A mutant is supported by molecular dynamics simulations at low and high pressure. The work provides firm experimental and computational support for the existence of parallel pathways arising from multiple conformational states of the enzyme-coenzyme complex. Hydrostatic pressure is a powerful and general probe of multidimensional energy landscapes that can be used to analyse experimentally parallel pathways for enzyme-catalysed reactions. We suggest that this is especially the case following directed mutation of a protein, which can lead to increased population of reactant states that are essentially inaccessible in the free-energy landscape of wild-type enzyme.

Computational Approach for Ranking Mutant Enzymes According to Catalytic Reaction Rates

A computationally efficient approach for ranking mutant enzymes according to the catalytic reaction rates is presented. This procedure requires the generation and equilibration of the mutant structures, followed by the calculation of partial free energy curves using an empirical valence bond potential in conjunction with biased molecular dynamics simulations and umbrella integration. The individual steps are automated and optimized for computational efficiency. This approach is used to rank a series of 15 dihydrofolate reductase mutants according to the hydride transfer reaction rate. The agreement between the calculated and experimental changes in the free energy barrier upon mutation is encouraging. The computational approach predicts the correct direction of the change in free energy barrier for all mutants, and the correlation coefficient between the calculated and experimental data is 0.82. This general approach for ranking protein designs has implications for protein engineering and drug design.

Hydride‐Donor Abilities of 1,4‐Dihydropyridines: A Comparison with π Nucleophiles and Borohydride Anions

How are dihydropyridines like indoles? Both groups of compounds have similar nucleophilicity parameters N and are therefore suitable substrates for iminium-catalyzed reactions of alpha,beta-unsaturated aldehydes. The N parameters of 1,4-dihydropyridines were derived from the rates of hydride transfer reactions to benzhydrylium ions (see scheme).

Nuclear Quantum Tunneling in the Light-activated Enzyme Protochlorophyllide Oxidoreductase

In chlorophyll biosynthesis, the light-activated enzyme protochlorophyllide oxidoreductase catalyzes trans addition of hydrogen across the C-17-C-18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide). This unique light-driven reaction plays a key role in the assembly of the photosynthetic apparatus, but despite its biological importance, the mechanism of light-activated catalysis is unknown. In this study, we show that Pchlide reduction occurs by dynamically coupled nuclear quantum tunneling of a hydride anion followed by a proton on the microsecond time scale in the Pchlide excited and ground states, respectively. We demonstrate the need for fast dynamic searches to form degenerate "tunneling-ready" configurations within the lifetime of the Pchlide excited state from which hydride transfer occurs. Moreover, we have found a breakpoint at -27 degrees C in the temperature dependence of the hydride transfer rate, which suggests that motions/vibrations that are important for promoting light-activated hydride tunneling are quenched below -27 degrees C. We observed no such breakpoint for the proton-tunneling reaction, indicating a reliance on different promoting modes for this reaction in the enzyme-substrate complex. Our studies indicate that the overall photoreduction of Pchlide is endothermic and that rapid dynamic searches are required to form distinct tunneling-ready configurations within the lifetime of the photoexcited state. Consequently, we have established the first important link between photochemical and nuclear quantum tunneling reactions, linked to protein dynamics, in a biologically significant system.

High-valent manganese(v)–oxo porphyrin complexes in hydride transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues to trans-dioxomanganese(v) porphyrin complexes proceeds via proton-coupled electron transfer, followed by rapid electron transfer.

Conformational changes in an ultrafast light-driven enzyme determine catalytic activity

The role of conformational changes in explaining the huge catalytic power of enzymes is currently one of the most challenging questions in biology. Although it is now widely regarded that enzymes modulate reaction rates by means of short- and long-range protein motions, it is almost impossible to distinguish between conformational changes and catalysis. We have solved this problem using the chlorophyll biosynthetic enzyme NADPH:protochlorophyllide (Pchlide) oxidoreductase, which catalyses a unique light-driven reaction involving hydride and proton transfers. Here we report that prior excitation of the enzyme-substrate complex with a laser pulse induces a more favourable conformation of the active site, enabling the coupled hydride and proton transfer reactions to occur. This effect, which is triggered during the Pchlide excited-state lifetime and persists on a long timescale, switches the enzyme into an active state characterized by a high rate and quantum yield of formation of a catalytic intermediate. The corresponding spectral changes in the mid-infrared following the absorption of one photon reveal significant conformational changes in the enzyme, illustrating the importance of flexibility and dynamics in the structure of enzymes for their function.

Solvent Effects on Environmentally Coupled Hydrogen Tunnelling During Catalysis by Dihydrofolate Reductase from Thermotoga maritima

Protein motions may be perturbed by altering the properties of the reaction medium. Here we show that dielectric constant, but not viscosity, affects the rate of the hydride-transfer reaction catalysed by dihydrofolate reductase from Thermotoga maritima (TmDHFR), in which quantum-mechanical tunnelling has previously been shown to be driven by protein motions. Neither dielectric constant nor viscosity directly alters the kinetic isotope effect of the reaction or the mechanism of coupling of protein motions to tunnelling. Glycerol and sucrose cause a significant increase in the rate of hydride transfer, but lead to a reduction in the magnitude of the kinetic isotope effect as well as an extension of the temperature range over which "passive" protein dynamics (rather than "active" gating motions) dominate the reaction. Our results are in agreement with the proposal that non-equilibrium dynamical processes (promoting motions) drive the hydride-transfer reaction in TmDHFR.

A study on the reactions of NADH models with electron-deficient alkenes. A probe for the extreme of concerted electron-hydrogen atom transfer mechanism

The reactions of 9-fluorenylidenemalononitrile (FDCN) and 1,1-diphenyl-2,2-dicyanoethylene (DPCN) with Hantzsch ester (HEH), N-methyl Hantzsch ester (Me-HEH), and 1-benzyl-1,4-dihydronicotinamide (BNAH) in oxygen-saturated acetonitrile have been studied. The aerobic reactions with HEH give solely reduction products. However, reactions with Me-HEH and BNAH not only result in reduction products, but also give varying amounts of oxidation products. The amount of oxidation product appears to be related to the electronic character and bulkiness of reactants. We propose that all these reactions follow a general mechanism of concerted electron-hydrogen atom transfer mechanism. If the electron-transfer complex is very tight, only ‘concerted hydride transfer reaction’ occurs. However, if the electron-transfer complex is not so tight, oxygen can capture the radicaloid intermediate to result in oxidation products.

Sequential Electron-Transfer and Proton-Transfer Pathways in Hydride-Transfer Reactions from Dihydronicotinamide Adenine Dinucleotide Analogues to Non-heme Oxoiron(IV) Complexes and p-Chloranil. Detection of Radical Cations of NADH Analogues in Acid-Promoted Hydride-Transfer Reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues, such as 10-methyl-9,10-dihydroacridine (AcrH 2) and its derivatives, 1-benzyl-1,4-dihydronicotinamide (BNAH), and their deuterated compounds, to non-heme oxoiron(IV) complexes such as [(L)Fe (IV)(O)] (2+) (L = N4Py, Bn-TPEN, and TMC) occurs to yield the corresponding NAD (+) analogues and non-heme iron(II) complexes in acetonitrile. Hydride transfer from the NADH analogues to p-chloranil (Cl 4Q) also occurs to produce the corresponding NAD (+) analogues and the hydroquinone anion (Cl 4QH (-)). The logarithms of the observed second-order rate constants (log k H) of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are linearly correlated with those of hydride transfer from the same series of NADH analogues to Cl 4Q, including similar kinetic deuterium isotope effects. The log k H values of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are also linearly correlated with those of deprotonation of the radical cations of NADH analogues. Such linear correlations indicate that overall hydride-transfer reactions of NADH analogues to both non-heme oxoiron(IV) complexes and Cl 4Q occur via electron transfer from NADH analogues to the oxoiron(IV) complexes, followed by rate-limiting deprotonation from the radical cations of NADH analogues and subsequent rapid electron transfer from the deprotonated radicals to the Fe(III) complexes to yield the corresponding NAD (+) analogues and the Fe(II) complexes. The electron-transfer pathway was accelerated by the presence of perchloric acid, and the resulting radical cations of NADH analogues were detected by electron spin resonance spectroscopy and UV-vis spectrophotometry in the acid-promoted hydride-transfer reactions from NADH analogues to non-heme oxoiron(IV) complexes. This result provides the first direct evidence that a hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes proceeds via an electron-transfer pathway.

Implementation of umbrella integration within the framework of the empirical valence bond approach.

The umbrella integration method for calculating the potential of mean force (PMF) for a chemical reaction is implemented within the empirical valence bond (EVB) framework. In this implementation, the PMF is generated along the energy gap reaction coordinate, and the biasing potential is the difference between the mapping potential, which is defined to be a linear combination of the valence bond state energies, and the EVB ground state energy. The umbrella integration method is based on the derivative of the PMF with respect to the reaction coordinate. An analytical expression for this derivative applicable to certain types of EVB potentials is presented. The advantages of the umbrella integration method are illustrated by the application of both umbrella integration and the weighted histogram analysis method to the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase. This application demonstrates that the umbrella integration method reduces the statistical errors, converges efficiently, and does not require significantly overlapping windows. A modified version of the weighted histogram analysis method that shares these advantages is also proposed and implemented.

NADH類縁体からo- 及びp-キノン類縁体へのヒドリド移動反応におけるプロトン共役電子移動

NADH類縁体からo- 及びp-キノン類縁体へのヒドリド移動反応におけるプロトン共役電子移動

Water poolに閉じ込めたロイコメチレンブルーと2,6-ジブチルベンゾキノンとの間のヒドリド移動反応

water poolに閉じ込めたロイコメチレンブルーと2,6-ジブチルベンゾキノンとの間のヒドリド移動反応

Hydrogen Atom Abstraction and Hydride Transfer Reactions by Iron(IV)–Oxo Porphyrins

Wahre Identität enthüllt: Die C-H-Aktivierung von Alkylarenen durch synthetische Eisen(IV)-Oxoporphyrin-Spezies und der Hydridtransfer von NADH-Analoga auf diese Spezies verläuft über H-Atom-Abstraktion bzw. protonengekoppelten Elektronentransfer. Mechanistische Studien ergaben, dass Eisen(IV)-Oxoporphyrine, nicht Eisen(IV)-Oxoporphyrin-π-Radikalkationen die eigentlichen Oxidationsmittel sind.

Hydrogen Atom Abstraction and Hydride Transfer Reactions by Iron(IV)–Oxo Porphyrins

True identity revealed: The CH bond activation of alkyl aromatics by synthetic iron(IV)–oxo porphyrin species and the hydride transfer of NADH analogues to them occur through H-atom abstraction and proton-coupled electron-transfer mechanisms, respectively. Mechanistic studies revealed that iron(IV)–oxo porphyrins, not iron(IV)–oxo porphyrin π-radical cations, are the true oxidant.