Transition State For Hydride Transfer Research Articles

Enhancing Hydride Transfer in Catalytic Hydrogenation via σ-Electron-Induced Polarization of Imines.

In this study, we investigated the role of aluminum cations in facilitating hydride transfer during the hydrogenation of imines within the context of Noyori-type metal-ligand cooperative catalysis. We propose a novel model involving aluminum cations directly coordinated with imines to induce activation from the lone pair electron site, a phenomenon termed σ-induced activation. The aluminum metal-hydride amidate complex ("HMn-NAl") exhibits a higher ability of hydride transfer in the hydrogenation of imines compared to its lithium counterpart ("HMn-NLi"). Density functional theory (DFT) calculations uncover that the aluminum cation efficiently polarizes unsaturated bonds through σ-electron-induced activation in the transition state of hydride transfer, thereby enhancing substrate electrophilicity more efficiently. Additionally, upon substrate coordination, aluminum's coordination saturation improves the hydride nucleophilicity of the HMn-NAl complex via the breakage of the Al-H coordination bond.

Group 14 metallocene catalysts for carbonyl hydroboration and cyanosilylation

A series of six Group 14 metallocene compounds (M = Ge, Sn, Pb) were studied as catalysts for carbonyl hydroboration and cyanosilylation reactions at room temperature. Both bis(pentamethylcyclopentadienyl) and tetramethyldisiloxa[3]metallocenophane compounds were compared. The tin and lead metallocenophanes exhibited the highest reactivity in hydroboration and cyanosilylation reactions. Hammett analysis of aldehyde hydroboration provided a ρ value of 0.73, suggesting a buildup of negative charge during the turnover-limiting step, consistent with the transition state for hydride transfer to the carbonyl center. NMR studies of Lewis acidity indicate that the Ge, Sn, and Pb tetramethyldisiloxa[3]metallocenophane compounds are weak Lewis acids.

Concerted Hydrosilylation Catalysis by Silica-Immobilized Cyclic Carbonates and Surface Silanols.

Developing a method for creating a novel catalysis of organic molecules is essential because of the growing interest in organocatalysis. In this study, we found that cyclic carbonates immobilized on a nonporous or mesoporous silica support showed catalytic activity for hydrosilylation, which was not observed for the free cyclic carbonates, silica supports, or their physical mixture. Analysis of the effects of linker lengths and pore sizes on the catalytic activity and carbonate C=O stretching frequency revealed that the proximity of carbonates and surface silanols was crucial for synergistic hydrosilylation catalysis. A carbonate and silanol concertedly activated the silane and aldehyde for efficient hydride transfer. Density functional theory calculations on a model reaction system demonstrated that both the carbonate and silanol contributed to the stabilization of the transition state of hydride transfer, which resulted in a reasonable barrier height of 16.8 kcal mol-1. Furthermore, SiO2/carbonate(C4) enabled the hydrosilylation of an aldehyde with an amino group without catalyst poisoning, owing to the weak acidity of surface silanols, in sharp contrast to previously developed acid catalysts. This study demonstrates that immobilization on a solid support can convert inactive organic molecules into active and heterogeneous organocatalysts.

Employing deuterium kinetic isotope effects to uncover the mechanism of (R)-3-hydroxybutyrate dehydrogenase.

Short-chain dehydrogenases/reductases (SDR) form a large enzyme superfamily playing important roles in health and disease. Furthermore, they are useful tools in biocatalysis. Unveiling the nature of the transition state for hydride transfer is a crucial undertaking toward defining the physicochemical underpinnings of catalysis by SDR enzymes, including possible contributions from quantum mechanical tunneling. Primary deuterium kinetic isotope effects can uncover the contribution from chemistry to the rate-limiting step and potentially provide detailed information on the hydride-transfer transition state in SDR-catalyzed reactions. For the latter, however, one needs to determine the intrinsic isotope effect: that which would be measured if hydride transfer were rate determining. Alas, as is the case for many other enzymatic reactions, those catalyzed by SDRs are often limited by the rate of isotope-insensitive steps, such as product release and conformational changes, which masks the expression of the intrinsic isotope effect. This can be overcome by the powerful yet underexplored method of Palfey and Fagan via which intrinsic kinetic isotope effects can be extracted from pre-steady-state kinetics data. SDRs are ideal systems to which this method can be applied. We have employed this approach to elucidate the transition states for hydride transfer catalyzed by NADH-dependent cold- and warm-adapted (R)-3-hydroxybutyrate dehydrogenase. Experimental conditions which simplify the analysis are discussed.

Glycerol-3-Phosphate Dehydrogenase: The K120 and K204 Side Chains Define an Oxyanion Hole at the Enzyme Active Site.

The cationic K120 and K204 side chains lie close to the C-2 carbonyl group of substrate dihydroxyacetone phosphate (DHAP) at the active site of glycerol-3-phosphate dehydrogenase (GPDH), and the K120 side chain is also positioned to form a hydrogen bond to the C-1 hydroxyl of DHAP. The kinetic parameters for unactivated and phosphite dianion-activated GPDH-catalyzed reduction of glycolaldehyde and acetaldehyde (AcA) show that the transition state for the former reaction is stabilized by ca 5 kcal/mole by interactions of the C-1 hydroxyl group with the protein catalyst. The K120A and K204A substitutions at wild-type GPDH result in similar decreases in kcat, but Km is only affected by the K120A substitution. These results are consistent with 3 kcal/mol stabilizing interactions between the K120 or K204 side chains and a negative charge at the C-2 oxygen at the transition state for hydride transfer from NADH to DHAP. This stabilization resembles that observed at oxyanion holes for other enzymes. There is no detectable rescue of the K204A variant by ethylammonium cation (EtNH3+), compared with the efficient rescue of the K120A variant. This is consistent with a difference in the accessibility of the variant enzyme active sites to exogenous EtNH3+. The K120A/K204A substitutions cause a (6 × 106)-fold increase in the promiscuity of wild-type hlGPDH for catalysis of the reduction of AcA compared to DHAP. This may reflect conservation of the active site for an ancestral alcohol dehydrogenase, whose relative activity for catalysis of reduction of AcA increases with substitutions that reduce the activity for reduction of the specific substrate DHAP.

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements.

The change in sign of the interaction force constant between element-hydrogen stretching modes of trans-dihydrides of the d block and p block elements is analyzed for the first time. As the transition metal M approaches group 12, the higher energy symmetric trans-H-M-H vibration νsym approaches the energy of the antisymmetric vibration νasym. Crossing to group 13 elements E, the trans-H-E-H vibration νsym increasingly drops below νasym. This reversal is attributed to the d orbital that participates in the H-M-H bonding but is nonbonding in the H-E-H compounds. DFT calculations are used to probe the energetics of isoelectronic triatomic [H-M-H]n+ and [H-E-H]n- to reveal this trend and also to demonstrate that the magnitude of these interactions (νgap) increases down groups 11, 12, and 14 but remains fairly constant for group 13. They are also used to show that this reversal is seen in the transition state for hydride transfer to CO2 from the model compounds trans-NiH2(porphyrin) and trans-EH2(porphyrin), E = Si and Ge in their singlet states.

Microsecond timescale MD simulations at the transition state of PmHMGR predict remote allosteric residues.

Understanding the mechanisms of enzymatic catalysis requires a detailed understanding of the complex interplay of structure and dynamics of large systems that is a challenge for both experimental and computational approaches. More importantly, the computational demands of QM/MM simulations mean that the dynamics of the reaction can only be considered on a timescale of nanoseconds even though the conformational changes needed to reach the catalytically active state happen on a much slower timescale. Here we demonstrate an alternative approach that uses transition state force fields (TSFFs) derived by the quantum-guided molecular mechanics (Q2MM) method that provides a consistent treatment of the entire system at the classical molecular mechanics level and allows simulations at the microsecond timescale. Application of this approach to the second hydride transfer transition state of HMG-CoA reductase from Pseudomonas mevalonii (PmHMGR) identified three remote residues, R396, E399 and L407, (15–27 Å away from the active site) that have a remote dynamic effect on enzyme activity. The predictions were subsequently validated experimentally via site-directed mutagenesis. These results show that microsecond timescale MD simulations of transition states are possible and can predict rather than just rationalize remote allosteric residues.

Iridium-Catalyzed Asymmetric Hydrogenation of α-Fluoro Ketones via a Dynamic Kinetic Resolution Strategy.

The discrimination of a fluorine atom from a hydrogen atom has been challenging in asymmetric catalysis. We herein report iridium-catalyzed hydrogenation of α-fluoro ketones using a strategy of dynamic kinetic resolution. Both enantiomeric and diastereomeric selectivities were satisfactory in the preparation of β-fluoro alcohols. The DFT calculation revealed a C-F···Na charge-dipole interaction in the transition state of hydride transfer. This noncovalent interaction would be responsible for the diastereomeric control.

Relaxation Dispersion NMR to Reveal Fast Dynamics in Brønsted Acid Catalysis: Influence of Sterics and H-Bond Strength on Conformations and Substrate Hopping.

NMR provides both structural and dynamic information, which is key to connecting intermediates and to understanding reaction pathways. However, fast exchanging catalytic intermediates are often inaccessible by conventional NMR due its limited time resolution. Here, we show the combined application of the 1H off-resonance R1ρ NMR method and low temperature (185–175 K) to resolve intermediates exchanging on a μs time scale (ns at room temperature). The potential of the approach is demonstrated on chiral phosphoric acid (CPA) catalysts in their complexes with imines. The otherwise inaccessible exchange kinetics of the E-I ⇌ E-II imine conformations and thermodynamic E-I:E-II imine ratios inside the catalyst pocket are experimentally determined and corroborated by calculations. The E-I ⇌ E-II exchange rate constants (kex185 K) for different catalyst–substrate binary complexes varied between 2500 and 19 000 s–1 (τex = 500–50 μs). Theoretical analysis of these exchange rate constants revealed the involvement of an intermediary tilted conformation E-III, which structurally resembles the hydride transfer transition state. The main E-I and E-II exchange pathway is a hydrogen bond strength dependent tilting–switching–tilting mechanism via a bifurcated hydrogen bond as a transition state. The reduction in the sterics of the catalyst showed an accelerated switching process by at least an order of magnitude and enabled an additional rotational pathway. Hence, the exchange process is mainly a function of the intrinsic properties of the 3,3′-substituents of the catalyst. Overall, we believe that the present study opens a new dimension in catalysis via experimental access to structures, populations, and kinetics of catalyst–substrate complexes on the μs time scale by the 1H off-resonance R1ρ method.

Dynamically Bifurcating Hydride Transfer Mechanism and Origin of Inverse Isotope Effect for Heterodinuclear AgRu-Catalyzed Alkyne Semihydrogenation

The mechanism and heterodinuclear cooperative effects for AgRu-catalyzed alkyne semihydrogenation were analyzed with density-functional theory (DFT) and experiment. This combined effort revealed the following: (1) AgRu-catalyzed diphenylacetylene hydrogenation initially gives a kinetic mixture of cis-stilbene and trans-stilbene by an ionic Ag–H hydride transfer transition state and post-transition state bifurcation, which was identified by quasiclassical direct dynamics simulations. (2) The hydrogenation reaction exhibits an unexpected inverse kinetic isotope effect (KIE < 1) resulting from an inverse equilibrium isotope effect (EIE) for heterodinuclear H2/D2 activation. (3) The Ag–Ru heterodinuclear cooperative effective is critical for both H2 activation and vinylsilver protonolysis reaction steps. (4) Rate studies and computational analysis show that electron-donating groups accelerate catalysis.

Primary Deuterium Kinetic Isotope Effects: A Probe for the Origin of the Rate Acceleration for Hydride Transfer Catalyzed by Glycerol-3-Phosphate Dehydrogenase.

Large primary deuterium kinetic isotope effects (1° DKIEs) on enzyme-catalyzed hydride transfer may be observed when the transferred hydride tunnels through the energy barrier. The following 1° DKIEs on kcat/Km and relative reaction driving force are reported for wild-type and mutant glycerol-3-phosphate dehydrogenase (GPDH)-catalyzed reactions of NADL (L = H, D): wild-type GPDH, ΔΔG⧧ = 0 kcal/mol, 1° DKIE = 1.5; N270A, 5.6 kcal/mol, 3.1; R269A, 9.1 kcal/mol, 2.8; R269A + 1.0 M guanidine, 2.4 kcal/mol, 2.7; R269A/N270A, 11.5 kcal/mol, 2.4. Similar 1° DKIEs were observed on kcat. The narrow range of 1° DKIEs (2.4–3.1) observed for a 9.1 kcal/mol change in reaction driving force provides strong evidence that these are intrinsic 1° DKIEs on rate-determining hydride transfer. Evidence is presented that the intrinsic DKIE on wild-type GPDH-catalyzed reduction of DHAP lies in this range. A similar range of 1° DKIEs (2.4–2.9) on (kcat/KGA, M–1 s–1) was reported for dianion-activated hydride transfer from NADL to glycolaldehyde (GA) [Reyes, A. C.; Amyes, T. L.; Richard, J. P. J. Am. Chem. Soc.2016, 138, 14526–14529]. These 1° DKIEs are much smaller than those observed for enzyme-catalyzed hydrogen transfer that occurs mainly by quantum mechanical tunneling. These results support the conclusion that the rate acceleration for GPDH-catalyzed reactions is due to the stabilization of the transition state for hydride transfer by interactions with the protein catalyst. The small 1° DKIEs reported for mutant GPDH-catalyzed and for wild-type dianion-activated reactions are inconsistent with a model where the dianion binding energy is utilized in the stabilization of a tunneling ready state.

Structurally Flexible Oxocarbenium/Borohydride Ion Pair: Dynamics of Hydride Transfer on the Background of Conformational Roaming.

We apply Born-Oppenheimer molecular dynamics to the practically significant [dioxane-H(+)-acetone][(C6F5)3B-H(-)] and [Et2O-H(+)-OCPr2][(C6F5)3B-H(-)] ion pair intermediates. Dynamics of hydride transfer in cation/anion ion pair takes place on the background of large-amplitude configurational changes. Geometry of oxocarbenium/borohydride ion pairs is flexible, meaning that we uncover significant actual structural disorder at a finite temperature. Therefore, although the starting structure can be fairly close to the configurational area of the hydride transfer transition state (TS) and despite a low potential energy barrier (ca. 1.5 kcal/mol, according to the literature), already at T ≈ 325 K the system can remain ignorant of the TS region and move round and about ("roam") in the configurational space for a period of time in the range between 10 and 100 ps. This indicates structural flexibility of oxocarbenium/borohydride ion pair on apparently a flat potential energy "landscape" of cation/anion interaction, and this has not been taken into consideration by the free energy estimations in static considerations made thus far. The difference between the dynamics-based representation of the system versus the static representation amounts to the difference between quasi-bimolecular versus unimolecular descriptions of the hydride transfer step.

Bacterial versus human thymidylate synthase: Kinetics and functionality.

Thymidylate Synthase (TSase) is a highly conserved enzyme that catalyzes the production of the DNA building block thymidylate. Structurally, functionally and mechanistically, bacterial and mammalian TSases share remarkable similarities. Because of this closeness, bacterial enzymes have long been used as model systems for human TSase. Furthermore, while TSase inhibitors have long served as chemotherapeutic drugs, no TSase inhibitor serves as an antibiotic. Despite their high resemblance, the mammalian TSases are distinct in a few known aspects, such as having a N-terminal tail and two insertions in the primary sequence and active/inactive conformations. Here, we aim to comprehensively characterize human (hs) TSase and delineate its contrasts and the similarities to the well-studied Escherichia coli (ec) TSase. We found that, in contrast to ecTSase, Mg2+ does not enhance reaction rates for hsTSase. The temperature dependence of intrinsic kinetic isotope effects (KIEs), on the other hand, suggests that Mg2+ has little or no impact on the transition state of hydride transfer in either enzyme, and that the transition state for the hydride transfer in hsTSase is looser than in ecTSase. Additionally, the substrates’ binding order is strictly ordered for ecTSase but slightly less ordered for hsTSase. The observed kinetic and functional differences between bacterial and human enzymes may aid in the development of antibiotic drugs with reduced toxicity.

Revisiting the Stereodetermining Step in Enantioselective Iridium-Catalyzed Imine Hydrogenation

The mechanism for the iridium-catalyzed asymmetric hydrogenation of prochiral imines has been investigated for an experimentally relevant ligand–substrate combination using DFT calculations. The possible stereoisomers of the stereodetermining hydride transfer transition state were considered for four possible hydrogenation mechanisms starting from the recently disclosed active catalyst consisting of iridium phosphine-oxazoline with cyclometalated imine substrate. The hydrogenation was found to proceed via an outer-sphere pathway. The transition state accurately describes the experimental observations of the active catalyst and provides a structural rationale for the high stereoinduction despite the lack of direct interaction points in the outer-sphere mechanism. The predicted enantioselectivity was consistent with experimental observations. Experimental studies support the hypothesis that the iridacycle forms spontaneously and functions as the active catalyst in the hydrogenation.

Alkanal transfer hydrogenation catalyzed by solid Brønsted acid sites

Catalytic pathway and requirements for transfer hydrogenation of n-alkanals (CnH2nO, n=3–6) on Brønsted acid sites (H+) immobilized in microporous MFI and FAU crystalline structures or dispersed on H4SiW12O40 polyoxometalate clusters are established by isolating its rates from those of the various concomitant catalytic cycles. Transfer hydrogenation of alkanals involves a kinetically-relevant, inter-molecular hydride transfer step from substituted tetralins or cyclohexadienes produced from the parallel alkanal coupling and ring closure reactions as the hydride donor (R′H2) to protonated alkanals (RCH2CHOH+) as the hydride acceptor, via a bi-molecular transition state with a shared hydride ion, (RCH2CHOH+--H−–R′H+)‡. The rate constants for the inter-molecular hydride transfer step correlate directly to the hydride ion affinity difference between the carbenium ions of the H-donors (R′H+) and the protonated alkanals (RCH2CHOH+). As a result, smaller alkanals with higher hydride ion affinities are more effective in abstracting hydride ions and in transfer hydrogenation (C4>C5>C6). Propanal is an exception, as it is less effective in transfer hydrogenation than butanal. The deviation of propanal from the reactivity trend is apparently caused by its smaller transition state for hydride transfer, which is solvated to a lesser extent in FAU cages. The transfer hydrogenation occurs much more effectively on partially confined H+ sites in FAU structures than in smaller pore MFI or unconfined H4SiW12O40 polyoxometalate clusters, an indication that FAU solvates and stabilizes the bulky transition state of hydride transfer via van der Waals interactions. These effects of local site structures and the thermochemical properties of reactant determine the reactivity of alkanal transfer hydrogenation and thus selectivity ratio of alkenes, dienes, aromatics, and larger oxygenates during deoxygenation catalysis.

The Complex Role of Asn270 in Dianion Activation of Glycerol 3‐Phosphate Dehydrogenase‐Catalyzed Hydride Transfer

We are working to understand the mechanism for the strong phosphite dianion (HPi) activation of proton transfer reactions catalyzed by triosephosphate isomerase (TIM), decarboxylation reactions catalyzed by orotidine monophosphate decarboxylase (OMPDC), and hydride transfer reactions catalyzed by glycerol 3‐phosphate dehydrogenase (GPDH). The phosphodianion of substrate dihydroxyacetone phosphate (DHAP) for GPDH provides 11 kcal/mol stabilization of the transition state for hydride transfer. Eight kcal/mol of this stabilization is observed as HPi activation of GPDH for catalysis of the reduction of glycolaldehyde (GA). The X‐ray crystal structure of the nonproductive complex between GPDH, DHAP and NAD+ shows interactions of Arg269 and Asn270 with the substrate phosphodianion. The R269A mutation of GPDH results in a large 9.1 kcal/mol destabilization of the transition state for enzyme‐catalyzed reduction of DHAP from elimination of the enzyme‐substrate ion‐pair; and 6.7 kcal/mol of this stabilization is recovered when the reaction is carried out in the presence of 1.0 M guanidinium cation.1 Similar effects were reported for K12G and R235A mutations of TIM2 and OMPDC,3 respectively, consistent with a similar architecture of the dianion activation sites for these enzymes. However, the following effects of N270A and R269A/N270A mutations require an unexpected greater mechanistic complexity for dianion activation of GPDH, compared with OMPDC and TIM. 1.) The 5.6 kcal/mol effect from N270A mutation on transition state stability is more than twice as large as that predicted for the deleted phosphodianion‐amide interaction. 2.) The effects of consecutive R269A and N270A mutations depend strongly on the order of these mutations. 3.) The N270A mutation results in a surprising 40‐fold increase in the second‐order rate constant for GPDH‐catalyzed reduction of GA, and causes HPi to change from a strong activator to a weak inhibitor of this GPDH‐catalyzed reaction. These large and complex changes in the activation barriers for hydride transfer show that the amide side chain of Asn270 functions as a “linchpin” in maintaining the structural integrity of the dianion activation site for GPDH.

Mechanism and Dynamics of Intramolecular C-H Insertion Reactions of 1-Aza-2-azoniaallene Salts.

The 1-aza-2-azoniaallene salts, generated from α-chloroazo compounds by treatment with halophilic Lewis acids, undergo intramolecular C-H amination reactions to form pyrazolines in good to excellent yields. This intramolecular amination occurs readily at both benzylic and tertiary aliphatic positions and proceeds at an enantioenriched chiral center with retention of stereochemistry. Competition experiments show that insertion occurs more readily at an electron-rich benzylic position than it does at an electron-deficient one. The C-H amination reaction occurs only with certain tethers connecting the heteroallene cation and the pendant aryl groups. With a longer tether or when the reaction is intermolecular, electrophilic aromatic substitution occurs instead of C-H amination. The mechanism and origins of stereospecificity and chemoselectivity were explored with density functional theory (B3LYP and M06-2X). The 1-aza-2-azoniaallene cation undergoes C-H amination through a hydride transfer transition state to form the N-H bond, and the subsequent C-N bond formation occurs spontaneously to generate the heterocyclic product. This concerted two-stage mechanism was shown by IRC and quasi-classical molecular dynamics trajectory studies.

DFT Study on Mechanism of N-Alkylation of Amino Derivatives with Primary Alcohols Catalyzed by Copper(II) Acetate

DFT calculations have been carried out to study the mechanism of Cu(AcO)2-catalyzed N-alkylation of amino derivatives with primary alcohols. The calculations indicate that tBuOK is necessary for the generation of the active catalyst from Cu(AcO)2 and that the catalytic cycle involves three sequential steps: (1) Cu-catalyzed alcohol oxidation to give the corresponding aldehyde and copper hydride, (2) aldehyde-amine condensation to generate an imine, (3) imine reduction to yield the expected N-alkylation secondary amine product and to regenerate the active catalyst. Based on the comparison of different reaction pathways, we conclude that the outer-sphere hydrogen transfer in a stepwise manner is the most favorable pathway for both alcohol oxidation and imine reduction. Thermodynamically, alcohol oxidation and imine formation are all uphill, but imine reduction is downhill significantly, which is the driving force for the catalytic transformation. Using the energetic span model, we find that the turnover frequency-determining transition state (TDTS) and the turnover frequency-determining intermediate (TDI) are the hydride transfer transition state for imine reduction and the active catalyst, respectively. The calculated turnover frequency (TOF) roughly agrees with the experimental observation and, therefore, further supports the validity of the proposed hydrogen transfer mechanism.

Homogenous Pd-catalyzed asymmetric hydrogenation of unprotected indoles: scope and mechanistic studies.

An efficient palladium-catalyzed asymmetric hydrogenation of a variety of unprotected indoles has been developed that gives up to 98% ee using a strong Brønsted acid as the activator. This methodology was applied in the facile synthesis of biologically active products containing a chiral indoline skeleton. The mechanism of Pd-catalyzed asymmetric hydrogenation was investigated as well. Isotope-labeling reactions and ESI-HRMS proved that an iminium salt formed by protonation of the C═C bond of indoles was the significant intermediate in this reaction. The important proposed active catalytic Pd-H species was observed with (1)H NMR spectroscopy. It was found that proton exchange between the Pd-H active species and solvent trifluoroethanol (TFE) did not occur, although this proton exchange had been previously observed between metal hydrides and alcoholic solvents. Density functional theory calculations were also carried out to give further insight into the mechanism of Pd-catalyzed asymmetric hydrogenation of indoles. This combination of experimental and theoretical studies suggests that Pd-catalyzed hydrogenation goes through a stepwise outer-sphere and ionic hydrogenation mechanism. The activation of hydrogen gas is a heterolytic process assisted by trifluoroacetate of Pd complex via a six-membered-ring transition state. The reaction proceeds well in polar solvent TFE owing to its ability to stabilize the ionic intermediates in the Pd-H generation step. The strong Brønsted acid activator can remarkably decrease the energy barrier for both Pd-H generation and hydrogenation. The high enantioselectivity arises from a hydrogen-bonding interaction between N-H of the iminium salt and oxygen of the coordinated trifluoroacetate in the eight-membered-ring transition state for hydride transfer, while the active chiral Pd complex is a typical bifunctional catalyst, effecting both the hydrogenation and hydrogen-bonding interaction between the iminium salt and the coordinated trifluoroacetate of Pd complex. Notably, the Pd-catalyzed asymmetric hydrogenation is relatively tolerant to oxygen, acid, and water.

A Combined QM/MM Study on the Reductive Half-Reaction of Xanthine Oxidase: Substrate Orientation and Mechanism

Quantum mechanical/molecular mechanical (QM/MM) methods were used to investigate the conversion of xanthine to uric acid in xanthine oxidase. Seven mechanistic variants were considered with different tautomeric forms of xanthine, different protonation states of the active-site residues, and different substrate orientations. The most favorable pathway (setup G) has a B3LYP/MM barrier of about 14 kcal mol(-1), consistent with the available experimental data. This multistep mechanism starts with Glu1261 deprotonating the xanthine at the N3 position followed by a proton transfer from the cofactor to the N9 atom of xanthine; the thus activated cofactor and substrate then react to form a tetrahedral intermediate, and a subsequent rate-limiting hydride transfer generates the product. The substrate orientation that has commonly been assumed in the literature leads to the most stable reactant complex, but the opposite orientation ("upside down") is computed to be the most favorable one during the reaction (setup G). In the "upside down" conformation, the Arg880 residue can best stabilize the reactive xanthine species with the negatively charged N3 atom, especially the tetrahedral intermediate and the following transition state for hydride transfer which is generally the highest point on the computed energy profiles. QM-only calculations for a minimal gas-phase model and for larger cluster models are performed for comparison, in particular for establishing intrinsic reactivities and a common energy scale. An analysis of the computational results provides detailed insight into the essential mechanistic role of the active-site residues.