Establishing the Thermodynamic Cards of Dipine Models' Oxidative Metabolism on 21 Potential Elementary Steps

Dipines are a type of important antihypertensive drug as L-calcium channel blockers, whose core skeleton is the 1,4-dihydropyridine structure. Since the dihydropyridine ring is a key structural factor for biological activity, the thermodynamics of the aromatization dihydropyridine ring is a significant feature parameter for understanding the mechanism and pathways of dipine metabolism in vivo. Herein, 4-substituted-phenyl-2,6-dimethyl-3,5-diethyl-formate-1,4-dihydropyridines are refined as the structurally closest dipine models to investigate the thermodynamic potential of dipine oxidative metabolism. In this work, the thermodynamic cards of dipine models' aromatization on 21 potential elementary steps in acetonitrile have been established. Based on the thermodynamic cards, the thermodynamic properties of dipine models and related intermediates acting as electrons, hydrides, hydrogen atoms, protons, and two hydrogen ions (atoms) donors are discussed. Moreover, the thermodynamic cards are applied to evaluate the redox properties, and judge or reveal the possible oxidative mechanism of dipine models.

Study of the Effects of Remote Heavy Group Vibrations on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD+ Model Reactions

It has recently been observed that the temperature(T)-dependence of KIEs in H-tunneling reactions, characterized by isotopic activation energy difference (ΔEa = EaD - EaH), is correlated to the rigidity of the tunneling ready states (TRSs) in enzymes. A more rigid system with narrowly distributed H-donor-acceptor distances (DADs) at the TRSs gives rise to a weaker T-dependence of KIEs (i.e., a smaller ΔEa). Theoreticians have attempted to develop new H-tunneling models to explain this, but none has been universally accepted. In order to further understand the observations in enzymes and provide useful data to build new theoretical models, we have studied the electronic and solvent effects on ΔEa's for the hydride-tunneling reactions of NADH/NAD+ analogues. We found that a tighter charge-transfer (CT) complex system gives rises to a smaller ΔEa, consistent with the enzyme observations. In this paper, we use the remote heavy group (R) vibrational effects to mediate the system rigidity to study the rigidity-ΔEa relationship. The specific hypothesis is that slower vibrations of a heavier remote group would broaden the DAD distributions and increase the ΔEa value. Four NADH/NAD+ systems were studied in acetonitrile but most of such heavy group vibrations do not appear to significantly increase the ΔEa. The remote heavy group vibrations in these systems may have not affected the CT complexation rigidity to a degree that can significantly increase the DADs, and further, the ΔEa values.

Structural Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD+ Model Reactions in Acetonitrile: Charge-Transfer Complex Tightness Is a Key

It has recently frequently been found that the kinetic isotope effect (KIE) is independent of temperature (T) in H-tunneling reactions in enzymes but becomes dependent on T in their mutants. Many enzymologists found that the trend is related to different donor-acceptor distances (DADs) at tunneling-ready states (TRSs), which could be sampled by protein dynamics. That is, a more rigid system of densely populated short DADs gives rise to a weaker T dependence of KIEs. Theoreticians have attempted to develop H-tunneling theories to explain the observations, but none have been universally accepted. It is reasonable to assume that the DAD sampling concept, if it exists, applies to the H-transfer reactions in solution, as well. In this work, we designed NADH/NAD+ model reactions to investigate their structural effects on the T dependence of hydride KIEs in acetonitrile. Hammett correlations together with N-CH3/CD3 secondary KIEs were used to provide the electronic structure of the TRSs and thus the rigidity of their charge-transfer complexation vibrations. In all three pairs of reactions, a weaker T dependence of KIEs always corresponds to a steeper Hammett slope on the substituted hydride acceptors. It was found that a tighter/rigid charge-transfer complexation system corresponds with a weaker T dependence of KIEs, consistent with the observations in enzymes.

Essential Rule Derived from Thermodynamics and Kinetics Studies of Benzopyran Compounds

Compounds with benzopyran as the core structure play an important role in the total synthesis of antioxidants, drugs, and natural products. Herein, the thermodynamic data of benzopyran compounds and their intermediates were measured and calculated by combining thermodynamics with kinetics. The mechanism of reactions between four benzopyran compounds and organic hydride acceptors was proven to be a one-step hydride transfer. The thermodynamic properties of these compounds and their corresponding intermediates were elucidated. The rationality and accuracy of the electrochemical measurement method were proved. Furthermore, the essential rule of unique structures being present between the C-H bond and para-substituent constants on the benzene ring, as shown in previous studies, was investigated. A simultaneous correlation between thermodynamics and kinetics was found for the hydride transfer reaction, in which the reaction site is connected with the substituent through the benzene ring, a double bond, or a N atom. The likely reason for the correlation between thermodynamic and kinetic is that the benzene ring, double bond, or N atom have the role of transferring the electronic effect. This finding can be applied to the calculation of the activation energy of hydride self-exchange reactions, the prediction of kinetic isotope effects, and explorations of selective reduction processes of hydride transfer in such organic hydride compounds.

Thermodynamic Evaluations of Amines as Hydrides or Two Hydrogen Ions Reductants and Imines as Protons or Two Hydrogen Ions Acceptors, as Well as Their Application in Hydrogenation Reactions

Since the hydrogenation of imines (X) and the dehydrogenation of amines (XH2) generally involve the two hydrogen ions (H- + H+) transfer, the thermodynamic abilities of various amines releasing hydrides or two hydrogen ions as well as various imines accepting protons or two hydrogen ions are important and characteristic physical parameters. In this work, the pKa values of 84 protonated imines (XH+) in acetonitrile were predicted. Combining Gibbs free energy changes of amines releasing hydrides in acetonitrile from our previous work with the pKa(XH+) values, the Gibbs free energy changes of amines releasing two hydrogen ions and imines accepting two hydrogen ions were derived using Hess's law by constructing thermochemical cycles, and the thermodynamic evaluations of amines as hydrides or two hydrogen ions reductants and imines as protons or two hydrogen ions acceptors are well compared and discussed. Eventually, the practical application of thermodynamic data for amines and imines on hydrogenation feasibility, mechanism, and possible elementary steps was shown and discussed in this paper from the point of thermodynamics.

Characteristic Activity Parameters of Electron Donors and Electron Acceptors

It is well-known that for an electron transfer reaction, the electron-donating ability of electron donors and the electron-accepting ability of electron acceptors can be quantitatively described by the oxidation potential of electron donors and the reduction potential of electron acceptors. However, for an electron transfer reaction, the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors cannot be quantitatively described by a characteristic parameter of electron donors and a characteristic parameter of electron acceptors till now. In this paper, a characteristic activity parameter of electron donors and electron acceptors named as their thermo-kinetic parameter is proposed to quantify the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors in electron transfer reactions. At the same time, the thermo-kinetic parameter values of 70 well-known electron donors and the corresponding 70 conjugated electron acceptors in acetonitrile at 298 K are determined. The activation free energies of 4900 typical electron transfer reactions in acetonitrile at 298 K are estimated according to the thermo-kinetic parameter values of 70 electron donors and 70 conjugated electron acceptors, and the estimated results have received good verification of the corresponding independent experimental measurements. The physical meaning of the thermo-kinetic parameter is examined. The relationship of the thermo-kinetic parameter with the corresponding redox potential as well as the relationship of the activation free energy with the corresponding thermodynamic driving force of electron transfer reactions is examined. The results show that the observed relationships between the thermo-kinetic parameters and the redox potentials as well as the observed relationships between the activation free energy and the thermodynamic driving force depend on the choice of electron donors and electron acceptors as well as the electron transfer reactions. The greatest contribution of this paper is to realize the symmetry and unification of kinetic equations and the corresponding thermodynamic equations of electron transfer reactions.

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

Over the years, Ru^IV(bpy)₂(py)(O)²⁺([Ru^IVO]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, [Ru^IVO]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 [Ru^IVO]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 [Ru^IVO]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 [Ru^IVO]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.

Quantitative evaluation of the actual hydrogen atom donating activities of O-H bonds in phenols: structure-activity relationship

The H-donating activity of phenol and the H-abstraction activity of phenol radicals have been extensively studied. In this article, the second-order rate constants of 25 hydrogen atom transfer (HAT) reactions between phenols and PINO and DPPH radicals in acetonitrile at 298 K were studied. Thermo-kinetic parameters ΔG ^≠o(XH) were obtained using a kinetic equation [ΔG ^≠ _XH/Y = ΔG ^≠o(XH) + ΔG ^≠o(Y)]. Bond dissociation free energies ΔG ^o(XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies ΔG ^≠ _XH/X and ΔG ^≠o(X) were determined as ΔG ^≠o(XH) and ΔG ^o(XH) were available. ΔG ^o(XH), ΔG ^≠ _XH/X, ΔG ^≠o(XH) and ΔG ^≠o(X) were used to assess the H-donating abilities of the studied phenols and the H-abstraction abilities of phenol radicals in thermodynamics, kinetics and actual HAT reactions. The effect of structures on these four parameters was discussed. The reliabilities of ΔG ^≠o(XH) and ΔG ^≠o(X) were examined. The difference between the method of determining ΔG ^≠ _XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only ΔG ^o(XH), ΔG ^≠ _XH/X, ΔG ^≠o(XH) and ΔG ^≠o(X) of phenols could be quantitatively evaluated, but also the structure-activity relationship of phenols is clearly demonstrated. Moreover, it lays the foundation for designing and synthesizing more antioxidants and radicals.

Comparison of the Hydride-Donating Ability and Activity of Five- and Six-Membered Benzoheterocyclic Compounds in Acetonitrile

In this work, we compared the hydride-donating ability of five-membered benzoheterocyclic compounds (FMB) and six-membered benzoheterocyclic compounds (SMB), isomers of DMBI and DMIZ and of DMPZ and DMPX, using detailed thermodynamic driving forces [ΔG^o (XH)], kinetic intrinsic barriers (ΔG^≠_XH/X), and thermo-kinetic parameters [ΔG^≠° (XH)]. For DMBI and DMIZ, the values of ΔG^o (XH), ΔG^≠_XH/X, and ΔG^≠° (XH) are 49.2 and 53.7 kcal/mol, 35.88 and 42.04 kcal/mol, and 42.54 and 47.87 kcal/mol, respectively. For DMPZ and DMPX, the values of ΔG^o (XH), ΔG^≠_XH/X, and ΔG^≠° (XH) are 73.2 and 79.5 kcal/mol, 35.34 and 25.02 kcal/mol, and 54.27 and 52.26 kcal/mol, respectively. It is easy to see that the FMB isomers are thermodynamically dominant and that the SMB isomers are kinetically dominant. Moreover, according to the analysis of ΔG^≠° (XH), compared to the SMB isomers, the FMB isomers have a stronger hydride-donating ability in actual chemical reactions.

Solvent Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects: Correlation to the Donor-Acceptor Distances

Protein structural effects on the temperature (T) dependence of kinetic isotope effects (KIEs) in H-tunneling reactions have recently been used to discuss about the role of enzyme thermal motions in catalysis. Frequently observed nearly T-independent KIEs in the wild-type enzymes and T-dependent KIEs in variants suggest that H-tunneling in the former is assisted by the naturally evolved protein constructive vibrations that help sample short donor-acceptor distances (DADs) needed. This explanation that correlates the T-dependence of KIEs with DAD sampling has been highly debated as simulations following other H-tunneling models sometimes gave alternative explanations. In this paper, solvent effects on the T-dependence of KIEs of two hydride tunneling reactions of NADH/NAD⁺ analogues (represented by ΔE_a = E_aD - E_aH) were determined in attempts to replicate the observations in enzymes and test the protein vibration-assisted DAD sampling concept. Effects of selected aprotic solvents on the DAD_PRC's of the productive reactant complexes (PRCs) and the DAD_TRS's of the activated tunneling ready states (TRSs) were obtained through computations and analyses of the kinetic data, including 2° KIEs, respectively. A weaker T-dependence of KIEs (i.e., smaller ΔE_a) was found in a more polar aprotic solvent in which the system has a shorter average DAD_PRC and DAD_TRS. Further results show that a charge-transfer (CT) complexation made of a stronger donor/acceptor gives rise to a smaller ΔE_a. Overall, the shorter and less broadly distributed DADs resulting from the stronger CT complexation vibrations give rise to a smaller ΔE_a. Our results appear to support the explanation that links the T-dependence of KIEs to the donor-acceptor rigidity in enzymes.

Comparison between 1,2-Dihydropyridine and 1,4-Dihydropyridine on Hydride-Donating Ability and Activity

In this paper, detailed comparisons of the driving force in thermodynamics and intrinsic force in the kinetics of 1,2-dihydropyridine and 1,4-dihydropyridine isomers of PNAH, HEH, and PYH in hydride transfer reactions are made. For 1,2-PNAH and 1,4-PNAH, the values of the thermodynamic driving forces, kinetic intrinsic barriers, and thermo-kinetic parameters are 60.50 and 61.90 kcal/mol, 27.92 and 26.34 kcal/mol, and 44.21 and 44.12 kcal/mol, respectively. For 1,2-HEH and 1,4-HEH, the values of the thermodynamic driving forces, kinetic intrinsic barriers, and thermo-kinetic parameters are 63.40 and 65.00 kcal/mol, 31.68 and 34.96 kcal/mol, and 47.54 and 49.98 kcal/mol, respectively. For 1,2-PYH and 1,4-PYH, the order of thermodynamic driving forces, kinetic intrinsic barriers, and thermo-kinetic parameters are 69.90 and 72.60 kcal/mol, 33.06 and 25.74 kcal/mol, and 51.48 and 49.17 kcal/mol, respectively. It is not difficult to find that thermodynamically favorable structures are not necessarily kinetically favorable. In addition, according to the analysis of thermo-kinetic parameters, 1,4-PNAH, 1,2-HEH, and 1,4-PYH have a strong hydride-donating ability in actual chemical reactions.

Kinetic Studies of Hantzsch Ester and Dihydrogen Donors Releasing Two Hydrogen Atoms in Acetonitrile

In this work, kinetic studies on HEH₂, 2-benzylmalononitrile, 2-benzyl-1H-indene-1,3(2H)-dione, 5-benzyl-2,2-dimethyl-1,3-dioxane-4,6-dione, 5-benzyl-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione, 2-(9H-fluoren-9-yl)malononitrile, ethyl 2-cyano-2-(9H-fluoren-9-yl)acetate, diethyl 2-(9H-fluoren-9-yl)malonate, and the derivatives (28 XH₂) releasing two hydrogen atoms were carried out. The thermokinetic parameters ΔG ^⧧° of 28 dihydrogen donors (XH₂) and the corresponding hydrogen atom acceptors (XH^•) in acetonitrile at 298 K were determined. The abilities of releasing two hydrogen atoms for these organic dihydrogen donors were researched using their thermokinetic parameters ΔG ^⧧°(XH₂), which can be used not only to compare the H-donating ability of different XH₂ qualitatively and quantitatively but also to predict the rates of HAT reactions. Predictions of rate constants for 12 HAT reactions using thermokinetic parameters were determined, and the reliabilities of the predicted results were also examined.

New Insights into the Actual H-Abstraction Activities of Important Oxygen and Nitrogen Free Radicals: Thermodynamics and Kinetics in Acetonitrile

The H-abstraction activity of a free radical is a research hotspot and has been extensively studied. In this article, the second-order rate constants of 21 HAT reactions in acetonitrile at 298 K were chosen from several published literature. A kinetic study on the H-abstraction reaction from TEMPOH by a DPPH^• radical was carried out. This reaction was researched as an insertion point. By combining this reaction with the 21 HAT reactions in this paper, the thermokinetic parameters of 28 free radicals X and their corresponding antioxidants XH were obtained by the cross-HAT reaction method. The scales of the H-abstraction activities of these 28 oxygen and nitrogen free radicals were determined by using the thermokinetic parameters ΔG ^≠o(X). Applications of the thermokinetic parameter ΔG ^≠o(X) in assessing the actual H-abstraction activity of a free radical quantitatively and selecting a suitable free radical in scientific research and chemical production were discussed. Predictions of the rate constants by using thermokinetic parameters of reactants were researched, and the reliabilities of the predicted activation free energies of XH/Y reactions were also examined.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Evaluation and comparison of antioxidant abilities of five bioactive molecules with C–H and O–H bonds in thermodynamics and kinetics

In this work, the antioxidant abilities of NADH coenzyme analogue BNAH, F420 reduction prototype analogue F420H, vitamin C analogue iAscH−, caffeic acid, and (+)-catechin in acetonitrile in chemical reactions were studied and discussed.

Thermodynamics of the elementary steps of organic hydride chemistry determined in acetonitrile and their applications

This review focuses on the thermodynamics of the elementary step of 421 organic hydrides and unsaturated compounds releasing or accepting hydride or hydrogen determined in acetonitrile as well as their potential applications.

Quantitative Estimation of the Hydrogen-Atom-Donating Ability of 4-Substituted Hantzsch Ester Radical Cations

The purpose of this study is to investigate thermodynamic and kinetic properties on the hydrogen-atom-donating ability of 4-substituted Hantzsch ester radical cations (XRH•+), which are excellent NADH coenzyme models. Gibbs free energy changes and activation free energies of 17 XRH•+ releasing H• [denoted as ΔG HD o(XRH•+) and ΔG HD ≠(XRH•+)] were calculated using density functional theory (DFT) and compared with that of Hantzsch ester (HEH2) and NADH. ΔG HD o(XRH•+) range from 19.35 to 31.25 kcal/mol, significantly lower than that of common antioxidants (such as ascorbic acid, BHT, the NADH coenzyme, and so forth). ΔG HD ≠(XRH•+) range from 29.81 to 39.00 kcal/mol, indicating that XRH•+ spontaneously releasing H• are extremely slow unless catalysts or active intermediate radicals exist. According to the computed data, it can be inferred that the Gibbs free energies and activation free energies of the core 1,4-dihydropyridine radical cation structure (DPH•+) releasing H• [ΔG HD o(DPH•+) and ΔG HD ≠(DPH•+)] should be 19-32 kcal/mol and 29-39 kcal/mol in acetonitrile, respectively. The correlations between the thermodynamic driving force [ΔG HD o(XRH•+)] and the activation free energy [ΔG HD ≠(XRH•+)] are also explored. Gibbs free energy is the important and decisive parameter, and ΔG HD ≠(XRH•+) increases in company with the increase of ΔG HD o(XRH•+), but no simple linear correlations are found. Even though all XRH•+ are judged as excellent antioxidants from the thermodynamic view, the computed data indicate that whether XRH•+ is an excellent antioxidant in reaction is decided by the R substituents in 4-position. XRH•+ with nonaromatic substituents tend to release R• instead of H• to quench radicals. XRH•+ with aromatic substituents tend to release H• and be used as antioxidants, but not all aromatic substituted Hantzsch esters are excellent antioxidants.

Direct Correlation between Donor-Acceptor Distance and Temperature Dependence of Kinetic Isotope Effects in Hydride-Tunneling Reactions of NADH/NAD+ Analogues

Recent study of structural effects on primary kinetic isotope effects (1° KIEs) of H-transfer reactions in enzymes and solution revealed that a more rigid reaction system gave rise to a weaker temperature dependence of 1° KIEs, i.e., a smaller isotopic activation energy difference (ΔE_a = E_aD - E_aH). This has been explained within the contemporary vibrationally assisted activated H-tunneling (VA-AHT) model in which rigidity is defined according to the density of donor-acceptor distance (DAD_TRS) populations at the tunneling ready state (TRS) sampled by heavy atom motions. To test the relationship between DAD_TRS and ΔE_a in the model, we developed a computational method to obtain the TRS structures for H-transfer reactions. The method was applied to three hydride transfer reactions of NADH/NAD⁺ analogues for which the ΔE_a's as well as secondary (2°) KIEs have been reported. The 2° KIEs computed from each TRS structure were fitted to the observed values to obtain the optimal TRSs/DAD_TRS's. It was found that a shorter DAD_TRS does correspond with a smaller ΔE_a. This appears to support the VA-AHT model. Moreover, an analysis of hybridizations at the bent TRS structures shows that rehybridizations at the donor-acceptor centers are much more advanced than predicted from the classical mechanism. This implies that more orbital preparations are required for the nonclassical H-tunneling to take place.

Theoretical investigation on the nature of 4-substituted Hantzsch esters as alkylation agents

Recently, a variety of 4-substituted Hantzsch esters (XRH) with different structures have been widely researched as alkylation reagents in chemical reactions, and the key step of the chemical process is the elementary step of XRH˙⁺ releasing R˙. The purpose of this work is to investigate the essential factors which determine whether or not an XRH is a great alkylation reagent using density functional theory (DFT). This study shows that the ability of an XRH acting as an alkylation reagent can be reasonably estimated by its ΔG^≠_RD(XRH˙⁺) value, which can be conveniently obtained through DFT computations. Moreover, the data also show that ΔG^≠_RD(XRH˙⁺) has no simple correlation with the structural features of XRH, including the electronegativity of the R substituent group and the magnitude of steric resistance; therefore, it is difficult to judge whether an XRH can provide R˙ solely by experience. Thus, these results are helpful for chemists to design 4-substituted Hantzsch esters (XRH) with novel structures and to guide the application of XRH as a free radical precursor in organic synthesis.

This work presents a convenient computation method to estimate whether a 4-substituted Hantzsch ester can be a good alkyl radical donor.

Competition between N and O: use of diazine N-oxides as a test case for the Marcus theory rationale for ambident reactivity

The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the 1H-15N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG ‡ 0) and Δr G° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG ‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of Δr G° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG ‡ 0) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.

Substituent Effects on Temperature Dependence of Kinetic Isotope Effects in Hydride-Transfer Reactions of NADH/NAD+ Analogues in Solution: Reaction Center Rigidity Is the Key

Substituent effects on the temperature dependence of primary kinetic isotope effects, characterized by ΔE_a = E_aD - E_aH, for two series of the title reactions in acetonitrile were studied. The change from ΔE_a ≈ 0 for a highly rigid system to ΔE_a > 0 for systems with reduced rigidities was observed. The rigidities were controlled by the electronic and steric effects. This work replicates the observations in enzymes and opens a new research direction that studies structure-ΔE_a relationship.

Replication of the Enzymatic Temperature Dependency of the Primary Hydride Kinetic Isotope Effects in Solution: Caused by the Protein-Controlled Rigidity of the Donor-Acceptor Centers?

The change from the temperature independence of the primary (1°) H/D kinetic isotope effects (KIEs) in wild-type enzyme-catalyzed H-transfer reactions (ΔE_a = E_aD - E_aH ∼ 0) to a strong temperature dependence with the mutated enzymes (ΔE_a ≫ 0) has recently been frequently observed. This has prompted some enzymologists to develop new H-tunneling models to correlate ΔE_a with the donor-acceptor distance (DAD) at the tunneling-ready state (TRS) as well as the protein thermal motions/dynamics that sample the short DAD_TRS's for H-tunneling to occur. While extensive evidence supporting or disproving the thermally activated DAD sampling concept has emerged, a comparable study of the simpler bimolecular H-tunneling reactions in solution has not been carried out. In particular, small ΔE_a's (∼0) have not been found. In this paper, we report a study of the hydride-transfer reactions from four NADH models to the same hydride acceptor in acetonitrile. The ΔE_a's were determined: 0.37 (small), 0.60, 0.99, and 1.53 kcal/mol (large). The α-secondary (2°) KIEs on the acceptor that serve as a ruler for the rigidity of reaction centers were previously reported or determined. All possible productive reactant complex (PRC) configurations were computed to provide insight into the structures of the TRS's. Relationships among structures, 2° KIEs, DAD_PRC's, and ΔE_a's were discussed. The more rigid system with more suppressed 2° C-H vibrations at the TRS and more narrowly distributed DAD_PRC's in PRCs gave a smaller ΔE_a. The results replicated the trend observed in enzymes versus mutated enzymes and appeared to support the concepts of different thermally activated DAD_TRS sampling processes in response to the rigid versus flexible donor-acceptor centers.

New Insight into the Mechanism of NADH Model Oxidation by Metal Ions in Nonalkaline Media

For a long time, it has been controversial that the three-step (e-H⁺-e) or two-step (e-H^•) mechanism was used for the oxidation of nicotinamide adenine dinucleotide coenzyme (NADH) and its models by metal ions in nonalkaline media. The latter mechanism has been accepted by the majority of researchers. In this work, 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1-phenyl-l,4-dihydronicotinamide are used as NADH models and ferrocenium (Fc⁺) metal ion as an electron acceptor. The kinetics for oxidation of the NADH models by Fc⁺ in pure acetonitrile was monitored by using UV-vis absorption and a quadratic relationship between k_obs and the concentrations of NADH models was found for the first time. The rate expression of the reactions developed according to the three-step mechanism is quite consistent with the quadratic curves. The rate constants, thermodynamic driving forces, and kinetic isotope effects of each elementary step for the reactions were estimated. All results supported the three-step mechanism. The intrinsic kinetic barriers of the proton transfer from BNAH^+• to BNAH and the hydrogen-atom transfer from BNAH^+• to BNAH^+• were estimated by using Zhu equation; the results showed that the former is 11.8 kcal/mol and the latter is larger than 24.3 kcal/mol. It is the large intrinsic kinetic barrier of the hydrogen-atom transfer that makes the reactions choose the three-step rather than two-step mechanism. Further investigation of the factors affecting the intrinsic kinetic barrier of chemical reactions indicated that the large intrinsic kinetic barrier of the hydrogen-atom transfer originated from the repulsion of positive charges between BNAH^+• and BNAH^+•. The greatest contribution of this work is the discovery of the quadratic dependence of k_obs on the concentrations of the NADH models, which is inconsistent with the conventional viewpoint of the "two-step mechanism" on the oxidation of NADH and its models by metal ions in the nonalkaline media.

Theoretical Prediction of Activation Free Energies of Various Hydride Self-Exchange Reactions in Acetonitrile at 298 K

Hydride transfer reactions are very important chemical reactions in organic chemistry. It has been a chemist's dream to predict the rate constants of hydride transfer reactions by only using the physical parameters of the reactants. To realize this dream, we have developed a kinetic equation (Zhu equation) in our previous papers to predict the activation free energies of various chemical reactions using the activation free energies of the corresponding self-exchange reactions and the related bond dissociation energies or redox potentials of the reactants. Because the activation free energy of the hydride self-exchange reaction is difficult to measure using the experimental method, in this study, the activation free energies of 159 hydride self-exchange reactions in acetonitrile at 298 K were systematically computed using an accurately benchmarked density functional theory method with a precision of 1.1 kcal mol^-1. The results show that the range of the activation free energies of the 159 hydride self-exchange reactions is from 16.1 to 46.6 kcal mol^-1. The activation free energies of 25 122 hydride transfer reactions in acetonitrile at 298 K can be estimated using the activation free energies of the 159 hydride self-exchange reactions and the corresponding heterolytic bond dissociation free energies of the reactants. The effects of the heteroatom, substituent, and aromaticity on the activation free energies of hydride self-exchange reactions were examined. The results show that heteroatoms, substituents at the reaction center, and the aromaticity of reactants, all have remarkable effects on the activation free energy of hydride self-exchange reactions. All kinetic information provided in this work on the hydride self-exchange reactions in acetonitrile at 298 K should be very useful in chemical labs and chemical industry.

Reversible Hydride Transfer to N,N'-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H₂ to N⁵,N¹⁰-methenyltetrahydromethanopterin (CH-H₄MPT⁺). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N'-diphenylimidazolinium cation (1⁺) as a structurally related alternative to CH-H₄MPT⁺. In connection with the enzymatic mechanism via heterolytic cleavage of H₂ at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF₄] to N,N'-diphenylimidazolidine (2) was achieved under 1 atm H₂ at ambient temperature in the presence of an equimolar amount of NEt₃ as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1⁺. The reverse reaction was accomplished by treatment of 2 with HNMe₂Ph⁺ as the proton source, where [(η⁵-C₅Me₅)Ir{(p-MeC₆H₄SO₂)NCHPhCHPhNH}] was found to catalyze the formation of 1⁺ and H₂ with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1⁺/H₂ and 2/H⁺ was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1⁺ and 2 bearing one para-substituted N-phenyl group.

Why Are Vinyl Cations Sluggish Electrophiles?

The kinetics of the reactions of the vinyl cations 2 [Ph₂C═C⁺-(4-MeO-C₆H₄)] and 3 [Me₂C═C⁺-(4-MeO-C₆H₄)] (generated by laser flash photolysis) with diverse nucleophiles (e.g., pyrroles, halide ions, and solvents containing variable amounts of water or alcohol) have been determined photometrically. It was found that the reactivity order of the nucleophiles toward these vinyl cations is the same as that toward diarylcarbenium ions (benzhydrylium ions). However, the reaction rates of vinyl cations are affected only half as much by variation of the nucleophiles as those of the benzhydrylium ions. For that reason, the relative reactivities of vinyl cations and benzhydrylium ions depend strongly on the nature of the nucleophiles. It is shown that vinyl cations 2 and 3 react, respectively, 227 and 14 times more slowly with trifluoroethanol than the parent benzhydrylium ion (Ph)₂CH⁺, even though in solvolysis reactions (80% aqueous ethanol at 25 °C) the vinyl bromides leading to 2 and 3 ionize much more slowly (half-lives 1.15 yrs and 33 days) than (Ph)₂CH-Br (half-life 23 s). The origin of this counterintuitive phenomenon was investigated by high-level MO calculations. We report that vinyl cations are not exceptionally high energy intermediates, and that high intrinsic barriers for the sp² ⇌ sp rehybridizations account for the general phenomenon that vinyl cations are formed slowly by solvolytic cleavage of vinyl derivatives, and are also consumed slowly by reactions with nucleophiles.

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.

Slow generation of hydrogen sulfide from sulfane sulfurs and NADH models

Here we report the model studies of the reactions between NADH models (using HEH and BNAH) and sulfane sulfurs (using polysulfides). Such reactions could lead to the oxidation of NADH models and the production of hydrogen sulfide (H₂S). Kinetics of the reaction between BNAH and elemental sulfur S₈ were determined in ethanol and the second-order rate constant was found to be 0.074M^-1min^-1 (at 37°C) suggesting this is a slow process.

C–H functionalization of azines. Anodic dehydroaromatization of 9-(hetero)aryl-9,10-dihydroacridines

A simple and efficient electrochemical method for the oxidative conversion of dihydroacridines into the corresponding 9-(hetero)aryl-N-methylacridinium salts has been developed. Current–voltage characteristics of dihydroacridines are given.

Elemental step thermodynamics of various analogues of indazolium alkaloids to obtaining hydride in acetonitrile

A series of analogues of indazolium alkaloids were designed and synthesized. The thermodynamic driving forces of the 6 elemental steps for the analogues of indazolium alkaloids to obtain hydride in acetonitrile were determined using an isothermal titration calorimeter (ITC) and electrochemical methods, respectively. The effects of molecular structure and substituents on the thermodynamic driving forces of the 6 steps were examined. Meanwhile, the oxidation mechanism of NADH coenzyme by indazolium alkaloids was examined using the chemical mimic method. The result shows that the oxidation of NADH coenzyme by indazolium alkaloids in vivo takes place by one-step concerted hydride transfer mechanism.

Thermodynamics of various F420 coenzyme models as sources of electrons, hydride ions, hydrogen atoms and protons in acetonitrile

32 F420 coenzyme model were designed and synthesized; their thermodynamic driving forces to release electrons, hydride ions, hydrogen atoms and protons in acetonitrile were determined. The difference between F420 coenzyme and NADH coenzyme as sources of electrons, hydride ions, hydrogen atoms and protons was examined.