Hydride Transfer Mechanism Research Articles

Oxoammonium-Catalyzed Oxidation of N-Substituted Amines.

We report the development of oxoammonium-catalyzed oxidation of N-substituted amines via a hydride transfer mechanism. Steric and electronic tuning of catalyst led to complementary sets of conditions that can oxidize a broad scope of carbamates, sulfonamides, ureas, and amides into the corresponding imides. The reaction was further demonstrated on a 100-g scale using a continuous flow setup.

Single nickel atoms anchored on porous N-doped nanocarbon with dual reaction sites for highly-efficient transfer hydrogenation of furfural to furfuryl alcohol

The non-precious metal single-atom catalysts (SACs) supported on carbon materials for the catalytic transfer hydrogenation (CTH) of α,β-unsaturated aldehydes (UAL) still suffer from the problems of harsh reaction conditions and low activity. In this work, nickel SACs anchored on porous N-doped nanocarbon (Ni1/NC900) with dual reaction sites were synthesized using the high internal-phase emulsion (HIPE) template method combined with impregnation. The Ni1/NC900 catalyst exhibited remarkable catalytic activity for the CTH of furfural (FF) to furfuryl alcohol (FFA), achieving > 99.0 % conversion and selectivity at 100 °C for 60 min, with a turnover frequency (TOF) of 2908.1 h−1, surpassing other reported nickel-based catalysts by 1–3 orders of magnitude. The excellent catalytic performance of Ni1/NC900 was attributed to the high mass transfer efficiency (HMTE) of the support and the dual reaction sites of Ni–N4 and pyridinic N, where Ni–N4 as Lewis acidic site for adsorption of FF and isopropanol (i-PrOH), while the adjacent pyridinic N as Lewis basic site for the deprotonation of i-PrOH. The isotope labeling experiments revealed that the hydrogenation of FF was accomplished by proton transfer with i-PrOH, in agreement with the intermolecular hydride transfer mechanism. Furthermore, Ni1/NC900 demonstrated excellent stability and well general applicability, demonstrating its potential for industrial applications. This work provides a reference for the application of carbon-based non-precious metal SACs in the CTH reaction of UAL.

Deciphering the Two-Step Hydride Mechanism of Monoamine Oxidase Flavoenzymes.

The complete two-step hydride transfer mechanism of amine oxidation involved in the metabolism of monoamine neurotransmitters was scrutinized by DFT calculations. In living organisms, this process is catalyzed by monoamine oxidase enzymes. Herein, we focus on some intriguing aspects of the reaction that may have been previously noticed but have not been clarified to date. The first step of the reaction includes the C-H bond cleavage on the methylene group vicinal to the amino group of the monoamine substrate and the subsequent transfer of hydrogen to the N5 atom of the flavin prosthetic group of the enzyme. We confirmed the nature of this step to be hydride transfer by evaluation of the pertinent HOMO-LUMO gap together with analysis of orbital contours alongside the intrinsic reaction coordinate profile. Next, we investigated the rather peculiar intermediate adduct that may form between the amine substrate and the flavin molecule, featuring an unusually long C-N bond of ∼1.62 Å. Although this bond is quite stable in the gas phase, the presence of just a few explicit water molecules facilitates its dissociation almost without energy input so that the amine-flavin intermediate can form an ionic pair instead. We attribute the existence of the unusual C-N bond to a fragile balance between opposing electronic structure effects, as evaluated by the natural bond orbital analysis. In line with this, the intermediate in the solution or in the enzyme active site can exist in two energetically almost equivalent forms, namely, as a covalently bound complex or as an ion pair, as suggested by previous studies. Finally, we characterized the transformation of the intermediate to the fully reduced flavin and imine products via proton transfer from the amino group to the flavin N1 atom, completing the reductive part of the catalytic cycle. Although we found that explicit solvation substantially boosts the kinetics of this step, the corresponding barrier is significantly lower than that in the hydride transfer step, confirming hydrogen abstraction as the rate-limiting step of amine oxidation and validating the two-step hydride transfer mechanism of monoamine oxidases.

Hydride and halide abstraction reactions behind the enhanced basicity of Be and Mg clusters with nitrogen bases.

In this study, we investigate the protonation effects on the structure, relative stability and basicity of complexes formed by the interaction of monomers and dimers of BeX2 and MgX2 (X = H, F) with NH3, CH2NH, HCN, and NC5H5 bases. Calculations were performed using the M06-2X/aug-cc-pVTZ formalism, along with QTAIM, ELF and NCI methods for electron density analysis and MBIE and LMO-EDA energy decomposition analyses for interaction enthalpies. The protonation of the MH2- and M2H4-Base complexes occurs at the negatively charged hydrogen atoms of the MH2 and M2H4 moieties through typical hydride abstraction reactions, while protonation at the N atom of the base is systematically less exothermic. The preference for the hydride transfer mechanism is directly associated with the significant exothermicity of H2 formation through the interaction between H- and H+, and the high hydride donor ability of these complexes. The basicity of both, MH2 and M2H4 compounds increases enormously upon association with the corresponding bases, with the increase exceeding 40 orders of magnitude in terms of ionization constants. Due to the smaller exothermicity of HF formation, the basicity of fluorides is lower than that of hydrides. In Be complexes, the protonation at the N atom of the base dominates over the fluoride abstraction mechanism. However, for the Mg complexes the fluoride abstraction mechanism is energetically the most favorable process, reflecting the greater facility of Mg complexes to lose F-.

Structural and mechanistic insights into Streptococcus pneumoniae NADPH oxidase

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) have a major role in the physiology of eukaryotic cells by mediating reactive oxygen species production. Evolutionarily distant proteins with the NOX catalytic core have been found in bacteria, including Streptococcus pneumoniae NOX (SpNOX), which is proposed as a model for studying NOXs because of its high activity and stability in detergent micelles. We present here cryo-electron microscopy structures of substrate-free and nicotinamide adenine dinucleotide (NADH)-bound SpNOX and of NADPH-bound wild-type and F397A SpNOX under turnover conditions. These high-resolution structures provide insights into the electron-transfer pathway and reveal a hydride-transfer mechanism regulated by the displacement of F397. We conducted structure-guided mutagenesis and biochemical analyses that explain the absence of substrate specificity toward NADPH and suggest the mechanism behind constitutive activity. Our study presents the structural basis underlying SpNOX enzymatic activity and sheds light on its potential in vivo function.

Computational insights on the hydride and proton transfer mechanisms of L-proline dehydrogenase.

L-Proline dehydrogenase (ProDH) is a flavin-dependent oxidoreductase, which catalyzes the oxidation of L-proline to (S)-1-pyrroline-5-carboxylate. Based on the experimental studies, a stepwise proton and hydride transfer mechanism is supported. According to this mechanism, the amino group of L-proline is deprotonated by a nearby Lys residue, which is followed by the hydride transfer process from C5 position of L-proline to N5 position of isoalloxazine ring of FAD. It was concluded that the hydride transfer step is rate limiting in the reductive half-reaction, however, in the overall reaction, the oxidation of FAD is the rate limiting step. In this study, we performed a computational mechanistic investigation based on ONIOM method to elucidate the mechanism of the reductive half-reaction corresponding to the oxidation of L-proline into iminoproline. Our calculations support the stepwise mechanism in which the deprotonation occurs initially as a fast step as result of a proton transfer from L-proline to the Lys residue. Subsequently, a hydride ion transfers from L-proline to FAD with a higher activation barrier. The enzyme-product complex showed a strong interaction between reduced FAD and iminoproline, which might help to explain why a step in the oxidative half-reaction is rate-limiting.

Highly active, ultra-low loading single-atom iron catalysts for catalytic transfer hydrogenation

Highly effective and selective noble metal-free catalysts attract significant attention. Here, a single-atom iron catalyst is fabricated by saturated adsorption of trace iron onto zeolitic imidazolate framework-8 (ZIF-8) followed by pyrolysis. Its performance toward catalytic transfer hydrogenation of furfural is comparable to state-of-the-art catalysts and up to four orders higher than other Fe catalysts. Isotopic labeling experiments demonstrate an intermolecular hydride transfer mechanism. First principles simulations, spectroscopic calculations and experiments, and kinetic correlations reveal that the synthesis creates pyrrolic Fe(II)-plN3 as the active center whose flexibility manifested by being pulled out of the plane, enabled by defects, is crucial for collocating the reagents and allowing the chemistry to proceed. The catalyst catalyzes chemoselectively several substrates and possesses a unique trait whereby the chemistry is hindered for more acidic substrates than the hydrogen donors. This work paves the way toward noble-metal free single-atom catalysts for important chemical reactions.

Computational Insights on the Hydride and Proton Transfer Mechanisms of D-Arginine Dehydrogenase.

D-Arginine dehydrogenase from Pseudomonas aeruginosa (PaDADH) is an amine oxidase which catalyzes the conversion of D-arginine into iminoarginine. It contains a non-covalent FAD cofactor that is involved in the oxidation mechanism. Based on substrate, solvent, and multiple kinetic isotope effects studies, a stepwise hydride transfer mechanism is proposed. It was shown that D-arginine binds to the active site of enzyme as α-amino group protonated, and it is deprotonated before a hydride ion is transferred from its α-C to FAD. Based on a mutagenesis study, it was concluded that a water molecule is the most likely catalytic base responsible from the deprotonation of α-amino group. In this study, we formulated computational models based on ONIOM method to elucidate the oxidation mechanism of D-arginine into iminoarginine using the crystal structure of enzyme complexed with iminoarginine. The calculations showed that Arg222, Arg305, Tyr249, Glu87, His 48, and two active site water molecules play key roles in binding and catalysis. Model systems showed that the deprotonation step occurs prior to hydride transfer step, and active site water molecule(s) may have participated in the deprotonation process.

Computational Mechanistic Study of l-Aspartate Oxidase by ONIOM Method.

l-Aspartate oxidase (Laspo) is responsible for the oxidation of l-aspartate into iminoaspartate using flavin as a cofactor. During this process flavin is reduced, and it can be reoxidized by either molecular oxygen or fumarate. The overall fold and the catalytic residues of Laspo are similar to succinate dehydrogenase and fumarate reductase. On the basis of deuterium kinetic isotope effects as well as other kinetic and structural data, it is proposed that the enzyme can catalyze the oxidation of l-aspartate through a mechanism similar to amino acid oxidases. It is suggested that a proton is removed from the α-amino group, while a hydride is transferred from C2 to flavin. It is also suggested that the hydride transfer is a rate-limiting step. However, there is still an ambiguity about the stepwise or concerted mechanism of hydride- and proton-transfer steps. In this study, we formulated some computational models to study the hydride-transfer mechanism using the crystal structure of Escherichia colil-aspartate oxidase in complexes with succinate. The calculations involved our own N-layered integrated molecular orbital and molecular mechanics method, and we evaluated the geometry and energetics of the hydride/proton-transfer processes while probing the roles of active site residues. Based on the calculations, it is concluded that proton- and hydride-transfer steps are decoupled, and a stepwise mechanism might be operative as opposed to the concerted one.

Further evidence in favour of a carbanion mechanism for glycolate oxidase.

The flavoenzyme glycolate oxidase oxidizes glycolic acid to glyoxylate and the latter, more slowly, to oxalate. It is a member of an FMN-dependent enzyme family that oxidizes l-2-hydroxy acids to keto acids. There has been a controversy concerning the chemical mechanism of substrate oxidation by these enzymes. Do they proceed by hydride transfer, as observed for NAD-dependent enzymes, or by initial formation of a carbanion that transfers the electrons to the flavin? The present work describes a comparison of the reactivity of glycolate, lactate and trifluorolactate with recombinant human glycolate oxidase, by means of rapid-kinetics experiments in anaerobiosis. We show that trifluorolactate is a substrate for glycolate oxidase, whereas it is known as an inhibitor for NAD-dependent enzymes, as is trifluoroethanol for NAD-dependent alcohol dehydrogenases. Unexpectedly, it was observed that, once reduced, a flavin transfers an electron to an oxidized flavin, so that the end species is a flavin semiquinone, whatever the substrate. This phenomenon has not previously been described for a glycolate oxidase. Altogether, considering that another member of this flavoenzyme family (flavocytochrome b2 , a lactate dehydrogenase) has also been shown to oxidize trifluorolactate (Lederer F etal. (2016) Biochim Biophys Acta 1864, 1215-21), this work provides another important piece of evidence which is hardly compatible with a hydride transfer mechanism for this flavoenzyme family.

Reactions of 3d transition metal hydride cations with CO2

Quantum chemical calculations were conducted to study reactions between cationic 3d transition metal hydride and carbon dioxide. The calculated results reveal that there are mainly-two reaction channels, which are to form HCOO− (Path A) and [M(CO)(OH)]+ (Path B), respectively. Path A is always kinetically more favorable than Path B except for Zn. However, only Path A of Sc-Cr could occur under ambient conditions in the gas phase. According to the analyses of charges and spin populations along the reaction pathways, Path A was found to be a hydride transfer mechanism, and Path B is a hydrogen atom transfer mechanism except for Zn, which should be a proton-coupled electron transfer mechanism.

Two-Electron- and One-Electron-Transfer Pathways for TEMPO-Catalyzed Greener Electrochemical Dimerization of 3-Substituted-2-Oxindoles

Dimerized 3-substituted 2-oxindoles are essential building blocks for the total synthesis of alkaloids. Herein, three different types of 3-substituted-2-oxindoles were dimerized with a total of 41 examples employing TEMPO as a redox mediator and TEMPO+ as an in situ generated electrocatalyst using control potential electrolysis at an applied potential of only 0.8 V versus a Ag/Ag+ nonaqueous reference electrode. These reactions did not yield any product formation in the absence of TEMPO, keeping other experimental conditions the same. Two-electron- and one-electron-transfer pathways were operational depending on the oxidation potentials of 2-oxindoles. 3-Carboxylate-2-oxindoles having a lower oxidative energy barrier for a two-electron-transfer pathway followed a hydride-transfer mechanism to generate a carbocation, which thereafter reacted with the enol form of oxindole to produce dimerized 2-oxindoles. On the other hand, 3-alkyl-2-oxindoles and 3-alkylcarboxylate-2-oxindoles had significantly higher oxidative energy barrier compared to 3-carboxylate-2-oxindoles and followed the one-electron-transfer pathway to generate a radical species for dimerization. The two-electron transfer pathway was significantly faster compared to the one-electron transfer pathway, evident from reaction time (15 min vs 1 h 20 min or longer), which was due to a faster second-order rate constant for the two-electron transfer pathway (1.2 M–1 s–1) compared to the one-electron transfer pathway (0.29 M–1 s–1). Furthermore, four aspects of transforming greener electro-organic synthesis have been demonstrated. They are as follows: (a) electron-transfer rate measurement as an alternative method for synthesis optimization instead of lengthy product isolation, (b) prevention of electrode passivation employing electrocatalysis routes, (c) recovery of the electrolyte, and (d) mild reaction conditions employing control potential electrolysis. Finally, an efficient electrochemical oxidation strategy has been demonstrated for the total synthesis of a dimeric hexahydropyrrolo[2,3-b]indole alkaloids, (±)-chimonanthine (I) and (±)-folicanthine (II).

Exploration of Ligand-Centered Hydride Transfer in La/Y-Catalyzed Deoxygenative Reduction of Tertiary Amides with Pinacolborane

A number of rare-earth metals and actinides have proven to be active in a wide variety of atom-efficient transformations. As compared to the related organometallic catalysts, the detailed mechanisms for the rare-earth metal-catalyzed reactions remain largely unexplored. Herein, the detailed catalyst activation process and reaction mechanisms of deoxygenative reduction of amides with pinacolborane (HBpin) catalyzed by Y[N(TMS)2]3 and La[N(TMS)2]3 complexes as well as a La4(O)acac10 cluster are investigated by density functional theory calculations. The M(III)-hemiaminal complex is disclosed to be the active catalyst for both the complexes and the cluster. During catalyst activation for both the Y and La complexes, the H-B bond polarity results in the formation of a transient M(III)-hydride intermediate, which is converted into an on-cycle M(III)-hemiaminal complex via facile migratory insertion. However, this kind of La(III)-hydride species cannot be formed for the La cluster. Starting from the M(III)-hemiaminal complex, the reaction proceeds via the ligand-centered hydride transfer mechanism that involves B-O bond formation, hydride transfer to B, C-O cleavage within the hemiaminal borane, hydride transfer to C, and σ-bond metathesis. The additional HBpin molecule is vital for the first hydride transfer that leads to the formation of [H2Bpin]- species. Our calculations reveal several important cooperative effects of the HBpin component during the hydride transfer processes. The improved mechanistic insights will be helpful for further development of selective C═O reduction.

Irreversible inactivation of lactate racemase by sodium borohydride reveals reactivity of the nickel-pincer nucleotide cofactor.

The nickel-pincer nucleotide (NPN) cofactor discovered in lactate racemase from Lactiplantibacillus plantarum (LarALp) is essential for the activities of racemases/epimerases in the highly diverse LarA superfamily. Prior mechanistic studies have established a proton-coupled hydride-transfer mechanism for LarALp, but direct evidence showing that hydride attacks the C4 atom in the pyridinium ring of NPN has been lacking. Here, we show that sodium borohydride (NaBH4) irreversibly inactivates LarALp accompanied by a rapid color change of the enzyme. The altered ultraviolet-visible spectra during NaBH4 titration supported hydride transfer to C4 of NPN, and the concomitant Ni loss unraveled by mass spectrometry experiments accounted for the irreversible inactivation. High resolution structures of LarALp revealed a substantially weakened C-Ni bond in the metastable sulfite-NPN adduct where the NPN cofactor is in the reduced state. These findings allowed us to propose a mechanism of LarALp inactivation by NaBH4 that provides key insights into the enzyme-catalyzed reaction and sheds light on the reactivity of small molecule NPN mimetics.

Structural Properties of Monoamine Oxidase B Complexed with Adrenaline in the Deamination Reaction: A Theoretical Study

Monoamine Oxidase B (MAO B) is a flavoenzyme that regulates and oxidizes monoamine neurotransmitters such as adrenaline, noradrenaline and dopamine. The oxidation reaction also produces hydrogen peroxide that can be harmful to dopaminergic neurons leading to neurological disorders such as Alzheimer’s disease and Parkinson’s disease. MAO B inhibitor is consequently used for preventing those neurological disorders. The structural details insight into the oxidation reaction mechanism of MAO B are vital to design specific inhibitors. A theoretical study is used to explain structural details of theoretical states in molecular level. In this work, the deamination reaction of adrenaline by MAO B was carried out to gain molecular-level details via ONIOM technique at M062X/6–31+G(d,p):PM6 level of theory. According to ONIOM calculation results, there are 4 key MAO B active site residues which are Leu171, Phe343, Tyr398 and Tyr435. Residues that bind adrenaline via hydrophobic interactions are Leu171, Phe343 and Tyr398 while Tyr435 binds via hydrogen bond. Additionally, NBO analysis suggests that the mechanism of this reaction is a hybrid between hydride transfer and polar nucleophilic characters. HIGHLIGHTS The structural properties along the adrenaline deamination reaction by MAO B have been revealed by ONIOM technique Key MAO B active site residues for designing more specific MAO B inhibitors have been identified in this study The mechanism of adrenaline deamination reaction by MAO B is a mixture between hydride transfer mechanism and polar nucleophilic mechanism GRAPHICAL ABSTRACT

Understanding the chemoselectivity switch in CO2 reduction catalyzed by Co and Fe complexes bearing a pentadentate N5 ligand

The Co and Fe complexes bearing a pentadentate N5 ligand could efficiently catalyze the photochemical CO2 reduction with excellent chemoselectivity favoring CO (Co catalysis) and formate (Fe catalysis). Density functional calculations were carried out to gain mechanistic insights and uncover the origin of the selectivity of these reactions. The pentadentate N5 ligand was delineated to be redox noninnocent that can accept electrons and protons, and the oxidation state of Co and Fe remained at +2 during the reduction. The theoretically established catalytic mechanisms suggest that, for the Co (1) catalysis, a nucleophilic attack on CO2 takes place by the double-reduced catalyst (formally Co0) to generate a CoII-carboxylate complex. Then, a proton is transferred from Et3NH+ to CoII-carboxylate to generate a CoII-COOH intermediate, which was further protonated on its hydroxyl group to cleave the C-O bond and release the CO molecule. For the Fe (2) catalysis, the nucleophilic attack on CO2 by a protonated and double-reduced species (formally Fe0-N-H) generates HCOO-. In this step, two electrons and one proton are transferred to the CO2 moiety from the N5 ligand. Such an orthogonal hydride (electron/proton) transfer mechanism is similar to the formate dehydrogenase metalloenzyme catalysis. The CO formation pathway is favored over the formic acid formation pathway in the Co-catalysis. However, the chemoselectivity reverses when changing the metal of the catalyst from Co to Fe, which promotes the formation of formic acid over CO by the orthogonal hydride transfer mechanism.

NTO Degradation by Nitroreductase: A DFT Study.

NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment during manufacturing, transportation, storage, training, and disposal. A detailed investigation of the possible mechanism for all steps of reduction of NTO by oxygen-insensitive nitroreductase, as one of the pathways for NTO environmental degradation, was performed by computational study at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. Obtained results reveal an overall sequence for NTO transformation into ATO (5-amino-1,2,4-triazol-3-one) with the flavin mononucleotide (FMN) cofactor of nitroreductase. Reduction of the nitro group to the nitroso group and the nitroso group to the hydroxylamino group follow a similar mechanism that consists of the sequential electron and proton transfer from the flavin cofactor. The hydride transfer mechanism may contribute to reduction of the nitroso group by the anionic form of the reduced flavin cofactor. Reduction of 5-(hydroxylamino)-1,2,4-triazol-3-one by the neutral form of the reduced flavin is impossible, whereas reduction of the hydroxylamino group to the amino group occurs with the anionic form of the reduced cofactor by a mechanism involving an initial proton transfer from the hydroxonium ion followed by two electrons and one proton transfers from the flavin cofactor. Small activation energies and high exothermicity support the significant contribution of oxygen-insensitive nitroreductase and other enzymes, containing FMN as a cofactor, to NTO degradation in the environment.

A Practical Oxidation Experiment for Undergraduate Students: Bobbitt’s Salt as a “Green” Alternative

Oxidation reactions are critical components of the synthetic toolbox taught to undergraduate students during introductory organic chemistry courses. However, the oxidation reactions discussed in the undergraduate curriculum are often outdated as many organic chemistry textbooks emphasize chromium-based oxidants that are no longer in regular use by practitioners, which may limit an instructor’s time to allocate to discussion of other oxidants. Further, laboratory courses have since either removed oxidation experiments or replaced them with oxidative processes not discussed in lectures, thus leading to a disconnect between the two learning settings. As part of an effort to bridge this divide and modernize the oxidation reactions discussed in our curricula, we have developed a new laboratory experiment that uses a commercially available oxoammonium salt (Bobbitt’s salt) to cleanly oxidize cinnamyl alcohol to cinnamaldehyde. In addition to being a safe, convenient, colorimetric, and “green oxidant” suitable for use in the undergraduate teaching laboratory, the hydride-transfer mechanism allows for overlap with key course concepts presented in both introductory and advanced lecture courses. The procedure is well-suited for small and large organic I, II, or advanced laboratory sections alike and can be completed within a standard 3–4 h laboratory period. Aside from exposing students to a modern green oxidation protocol, the experiment contains expanded opportunities for interpretation of 1H NMR, 13C NMR, and IR spectra. An optional addendum for advanced students involving Hammett correlations was also developed.

Hydride Transfer Mechanism of Enzymatic Sugar Nucleotide C2 Epimerization Probed with a Loose-Fit CDP-Glucose Substrate.

Transient oxidation–reduction through hydride transfer with tightly bound NAD coenzyme is used by a large class of sugar nucleotide epimerases to promote configurational inversion of carbon stereocenters in carbohydrate substrates. A requirement for the epimerases to coordinate hydride abstraction and re-addition with substrate rotation in the binding pocket poses a challenge for dynamical protein conformational selection linked to enzyme catalysis. Here, we studied the thermophilic C2 epimerase from Thermodesulfatator atlanticus (TaCPa2E) in combination with a slow CDP-glucose substrate (kcat ≈ 1.0 min–1; 60 °C) to explore the sensitivity of the enzymatic hydride transfer toward environmental fluctuations affected by temperature (20–80 °C). We determined noncompetitive primary kinetic isotope effects (KIE) due to 2H at the glucose C2 and showed that a normal KIE on the kcat (Dkcat) reflects isotope sensitivity of the hydrogen abstraction to enzyme-NAD+ in a rate-limiting transient oxidation. The Dkcat peaked at 40 °C was 6.1 and decreased to 2.1 at low (20 °C) and 3.3 at high temperature (80 °C). The temperature profiles for kcat with the 1H and 2H substrate showed a decrease in the rate below a dynamically important breakpoint (∼40 °C), suggesting an equilibrium shift to an impaired conformational landscape relevant for catalysis in the low-temperature region. Full Marcus-like model fits of the rate and KIE profiles provided evidence for a high-temperature reaction via low-frequency conformational sampling associated with a broad distribution of hydride donor–acceptor distances (long-distance population centered at 3.31 ± 0.02 Å), only poorly suitable for quantum mechanical tunneling. Collectively, dynamical characteristics of TaCPa2E-catalyzed hydride transfer during transient oxidation of CDP-glucose reveal important analogies to mechanistically simpler enzymes such as alcohol dehydrogenase and dihydrofolate reductase. A loose-fit substrate (in TaCPa2E) resembles structural variants of these enzymes by extensive dynamical sampling to balance conformational flexibility and catalytic efficiency.

Mechanistic Insight into Hydroboration of Imines from Combined Computational and Experimental Studies.

Boron Lewis acid-catalyzed and catalyst-free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N-(α-methylbenzylidene)aniline catalyzed by tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF 3 ), our calculations show that the reaction proceeds through a boron Lewis acid-promoted hydride transfer mechanism rather than the classical Lewis acid activation mechanism. For the catalyst- and solvent-free hydroboration reaction of imine, N-benzylideneaniline, our calculations and experimental studies indicate that this reaction is difficult to occur under the reaction conditions reported previously. With a combination of computational and experimental studies, we have established that the commercially available BH3 ⋅ SMe2 can serve as an efficient catalyst for the hydroboration reactions of N-benzylideneaniline and similar imines. The hydroboration reactions catalyzed by BH3 ⋅ SMe2 are most likely to proceed through a hydroboration/B-H/B-N σ-bond metathesis pathway, which is very different from that of the reaction catalyzed by BArF 3 .