Kinetic Isotope Effect Experiments Research Articles

Palladium-Mediated C(sp3)-H Bond Activation of N-Methyl-N-(pyridin-2-yl)benzamide: Direct Arylation/Alkylation and Mechanistic Investigation.

Herein, we present a facile synthetic methodology to produce a range of N-(CH2-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides via palladium-mediated C(sp3)-H bond activation. The N-methyl-N-(pyridin-2-yl)benzamide precursor was first reacted with palladium(II) acetate in a stoichiometric manner to obtain the key dinuclear palladacycle intermediates, whose structures were elucidated by mass spectrometric, NMR spectroscopic, and X-ray crystallographic studies in detail. The subsequent C(sp3)-H bond functionalizations on the N-methyl group of the starting substrate show facile productions of the corresponding N-(CH2-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides with good functional group tolerance. A plausible mechanism was proposed based on density functional theory calculations in conjunction with kinetic isotope effect experiments. Finally, the synthetic transformation from the prepared N-(CH2-aryl)-N-(pyridin-2-yl)benzamides through debenzoylation to N-(CH2-aryl)-2-aminopyridine was successfully demonstrated.

Computational Study on Ni−Al Bimetal‐Catalyzed Twofold C−H Annulation Reaction: Mechanism, Origin of Selectivity, and Role of SPO Ligand

AbstractDensity functional theory calculations have been performed to probe the reaction mechanism of the recently reported twofold C−H annulation of arylformamides with alkynes by Ni−Al bimetallic catalysis. The main pathway consisted of the formyl C−H activation, phenyl C−H activation, migratory insertion, and reductive elimination steps in succession. The desired annulation product was found to be the equilibrium‐controlled product, while the minor hydrocarbamoylation product was the kinetics‐controlled product. Calculated free‐energy profiles indicated that the annulation and hydrocarbamoylation pathways could intersect with each other and merge into a mechanism unity, in which the reaction system entered the hydrocarbamoylation pathway in early stage and switched to the annulation pathway afterwards via suitable transition states. The modified mechanism picture demonstrated the phenyl C−H activation step to be the rate‐determining step with a free‐energy barrier of 27.7 kcal/mol. However, this barrier was only 19.4 kcal/mol on the basis of a single annulation pathway, which could not satisfactorily agree with the experimental conditions and kinetic isotope effects. In addition, the crucial role of the bifunctional secondary phosphine oxide ligand in promoting the reactivity and selectivity was elucidated in detail.

Site‐Selective C−H Alkenylation of N‐Heteroarenes by Ligand‐Directed Co/Al and Co/Mg Cooperative Catalysis**

AbstractWe report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site‐selective C−H alkenylation of nitrogen‐containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2‐a]pyridine derivatives at the C−H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt‐diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C−H activation step, the relevance of which to the rate‐limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand‐to‐ligand hydrogen transfer is the common mechanism of C−H activation.

Site-Selective C-H Alkenylation of N-Heteroarenes by Ligand-Directed Co/Al and Co/Mg Cooperative Catalysis.

We report herein the design and development of Co/Al and Co/Mg bimetallic catalysts, supported by a phosphine/secondary phosphine oxide (PSPO) bifunctional ligand, for the site-selective C-H alkenylation of nitrogen-containing heteroarenes with alkynes. These catalysts enable the alkenylation of pyridine, pyridone, and imidazo[1,2-a]pyridine derivatives at the C-H site proximal to the Lewis basic nitrogen or oxygen atom, which represents a selectivity profile distinct from that of the previously developed cobalt-diphosphine/aluminum catalyst. The alkenylated products were obtained in moderate to good yields using various heterocycles and differently substituted internal alkynes. Kinetic isotope effect experiments suggest the irreversibility of the C-H activation step, the relevance of which to the rate-limiting step depends on the reaction conditions. Density functional theory calculations indicate that ligand-to-ligand hydrogen transfer is the common mechanism of C-H activation.

Superscalar Phase Boundaries Derived Multiple Active Sites in SnO2/Cu6Sn5/CuO for Tandem Electroreduction of CO2 to Formic Acid

AbstractThe electrocatalytic CO2 reduction reaction (CO2RR) to fuels driven by electrocatalysts is a viable strategy for efficient utilization of emitted CO2. CO2RR involves multiple‐steps, including adsorption, activation, hydrogenation, etc. At present, copper‐tin alloy catalysts have shown the capability to reduce CO2 to formic acid or formate. However, their poor adsorption and activation capacities for CO2 molecules, as well as the sluggish kinetics in *H supply restrict the proton‐coupled electron transfer processes in the electrocatalytic CO2RR to produce formic acid. In order to solve the above problems, the ultra‐small SnO2/Cu6Sn5/CuO nanocatalysts with superscalar phase boundaries are fabricated by laser sputtering. The introduction of SnO2 enhances the adsorption and activation of CO2, while CuO promotes H2O decomposition and provides abundant *H intermediates, resulting in tandem catalytic sites on the SnO2/Cu6Sn5/CuO composite catalysts and thus leading to excellent CO2RR activity and high selectivity to formic acid. The Faradic efficiency of formic acid (FEHCOOH) at the SnO2/Cu6Sn5/CuO electrode reaches 90.13% along with a high current density of 25.2 mA cm−2 at −0.95 V versus reversible hydrogen electrode. The role of the multiphase boundaries constructed by introduction of oxides is confirmed by in situ infrared spectroscopy and kinetic isotope effects experiments, which is consistent with the design concept.

Stepwise photoassisted decomposition of carbohydrates to H2

Stepwise photoassisted decomposition of carbohydrates to H2

Catalytic Access to Chiral δ‐Lactams via Nucleophilic Dearomatization of Pyridine Derivatives

AbstractNitrogen‐bearing rings are common features in the molecular structures of modern drugs, with chiral δ‐lactams being an important subclass due to their known pharmacological properties. Catalytic dearomatization of preactivated pyridinium ion derivatives emerged as a powerful method for the rapid construction of chiral N‐heterocycles. However, direct catalytic dearomatization of simple pyridine derivatives are scarce and methodologies yielding chiral δ‐lactams are yet to be developed. Herein, we describe an enantioselective C4‐dearomatization of methoxypyridine derivatives for the preparation of functionalised enantioenriched δ‐lactams using chiral copper catalysis. Experimental 13C kinetic isotope effects and density functional theory calculations shed light on the reaction mechanism and the origin of enantioselectivity.

Structure, Mutagenesis, and QM:MM Modeling of 3-Ketosteroid Δ1-Dehydrogenase from Sterolibacterium denitrificans─The Role of a New Putative Membrane-Associated Domain and Proton-Relay System in Catalysis.

3-Ketosteroid Δ1-dehydrogenases (KstD) are important microbial flavin enzymes that initiate the metabolism of steroid ring A and find application in the synthesis of steroid drugs. We present a structure of the KstD from Sterolibacterium denitrificans (AcmB), which contains a previously uncharacterized putative membrane-associated domain and extended proton-relay system. The experimental and theoretical studies show that the steroid Δ1-dehydrogenation proceeds according to the Ping-Pong bi-bi kinetics and a two-step base-assisted elimination (E2cB) mechanism. The mechanism is validated by evaluating the experimental and theoretical kinetic isotope effect for deuterium-substituted substrates. The role of the active-site residues is quantitatively assessed by point mutations, experimental activity assays, and QM/MM MD modeling of the reductive half-reaction (RHR). The pre-steady-state kinetics also reveals that the low pH (6.5) optimum of AcmB is dictated by the oxidative half-reaction (OHR), while the RHR exhibits a slight optimum at the pH usual for the KstD family of 8.5. The modeling confirms the origin of the enantioselectivity of C2-H activation and substrate specificity for Δ4-3-ketosteroids. Finally, the cholest-4-en-3-one turns out to be the best substrate of AcmB in terms of ΔG of binding and predicted rate of dehydrogenation.

Oxidation of Aldehydes into Carboxylic Acids by a Mononuclear Manganese(III) Iodosylbenzene Complex through Electrophilic C-H Bond Activation.

The oxidation of aldehyde is one of the fundamental reactions in the biological system. Various synthetic procedures and catalysts have been developed to convert aldehydes into corresponding carboxylic acids efficiently under ambient conditions. In this work, we report the oxidation of aldehydes by a mononuclear manganese(III) iodosylbenzene complex, [MnIII(TBDAP)(OIPh)(OH)]2+ (1), with kinetic and mechanistic studies in detail. The reaction of 1 with aldehydes resulted in the formation of corresponding carboxylic acids via a pre-equilibrium state. Hammett plot and reaction rates of 1 with 1°-, 2°-, and 3°-aldehydes revealed the electrophilicity of 1 in the aldehyde oxidation. A kinetic isotope effect experiment and reactivity of 1 toward cyclohexanecarboxaldehyde (CCA) analogues indicate that the reaction of 1 with aldehyde occurs through the rate-determining C-H bond activation at the formyl group. The reaction rate of 1 with CCA is correlated to the bond dissociation energy of the formyl group plotting a linear correlation with other aliphatic C-H bonds. Density functional theory calculations found that 1 electrostatically interacts with CCA at the pre-equilibrium state in which the C-H bond activation of the formyl group is performed as the most feasible pathway. Surprisingly, the rate-determining step is characterized as hydride transfer from CCA to 1, affording an (oxo)methylium intermediate. At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction followed by a fast electron transfer. Catalytic reactions of aldehydes by 1 are also presented with a broad substrate scope. This novel mechanistic study gives better insights into the metal oxygen chemistry and would be prominently valuable for development of transition metal catalysts.

Gold-Catalyzed Cyclization/Hydroboration of 1,6-Enynes: Synthesis of Bicyclo[3.1.0]hexane Boranes

The gold-catalyzed cyclization/hydroboration of 1,6-enynes offers facile, versatile, and atom-economical one-step access to bicyclo[3.1.0]hexane boranes. This new protocol proceeds in moderate to good yields under mild conditions. Different from bicyclo[3.1.0]hexane borates, these products are stable in air and during chromatography. Moreover, the borane moiety of the products can readily undergo a diverse array of transformations. The kinetic isotope effect experiment indicates that the hydrogen-transfer step is a fast process, which is not involved in the rate-limiting step.

Solvent Deuterium Isotope Effects of Substrate Reduction by Nitrogenase from Azotobacter vinelandii.

The mechanism of nitrogenase, the enzyme responsible for biological nitrogen fixation, has been of great interest for understanding the catalytic strategy utilized to reduce dinitrogen to ammonia under ambient temperatures and pressures. The reduction mechanism of nitrogenase is generally envisioned as involving multiple cycles of electron and proton transfers, with the known substrates requiring at least two cycles. Solvent kinetic isotope effect experiments, in which changes of reaction rates or product distribution are measured upon enrichment of solvent with heavy atom isotopes, have been valuable for deciphering the mechanism of complex enzymatic reactions involving proton or hydrogen transfer. We report the distribution of ethylene, dihydrogen, and methane isotopologue products measured from nitrogenase-catalyzed reductions of acetylene, protons, and cyanide, respectively, performed in varying levels of deuterium enrichment of the solvent. As has been noted previously, the total rate of product formation by nitrogenase is largely insensitive to the presence of D2O in the solvent. Nevertheless, the incorporation of H/D into products can be measured for these substrates that reflect solvent isotope effects on hydrogen atom transfers that are faster than the overall rate-determining step for nitrogenase. From these data, a minimal isotope effect is observed for acetylene reduction (1.4 ± 0.05), while the isotope effects for hydrogen and methane evolution are significantly higher at 4.2 ± 0.1 and 4.4 ± 0.1, respectively. These results indicate that there are pronounced differences in the sensitivity to isotopic substitution of the hydrogen atom transfer steps associated with the reduction of these substrates by nitrogenase.

Carboxylic Acid Directed γ-Lactonization of Unactivated Primary C-H Bonds Catalyzed by Mn Complexes: Application to Stereoselective Natural Product Diversification.

Reactions that enable selective functionalization of strong aliphatic C-H bonds open new synthetic paths to rapidly increase molecular complexity and expand chemical space. Particularly valuable are reactions where site-selectivity can be directed toward a specific C-H bond by catalyst control. Herein we describe the catalytic site- and stereoselective γ-lactonization of unactivated primary C-H bonds in carboxylic acid substrates. The system relies on a chiral Mn catalyst that activates aqueous hydrogen peroxide to promote intramolecular lactonization under mild conditions, via carboxylate binding to the metal center. The system exhibits high site-selectivity and enables the oxidation of unactivated primary γ-C-H bonds even in the presence of intrinsically weaker and a priori more reactive secondary and tertiary ones at α- and β-carbons. With substrates bearing nonequivalent γ-C-H bonds, the factors governing site-selectivity have been uncovered. Most remarkably, by manipulating the absolute chirality of the catalyst, γ-lactonization at methyl groups in gem-dimethyl structural units of rigid cyclic and bicyclic carboxylic acids can be achieved with unprecedented levels of diastereoselectivity. Such control has been successfully exploited in the late-stage lactonization of natural products such as camphoric, camphanic, ketopinic, and isoketopinic acids. DFT analysis points toward a rebound type mechanism initiated by intramolecular 1,7-HAT from a primary γ-C-H bond of the bound substrate to a highly reactive MnIV-oxyl intermediate, to deliver a carbon radical that rapidly lactonizes through carboxylate transfer. Intramolecular kinetic deuterium isotope effect and 18O labeling experiments provide strong support to this mechanistic picture.

Probing the Free Energy Landscape of Organophotoredox-Catalyzed Anti-Markovnikov Hydrofunctionalization of Alkenes.

Experimental 13C kinetic isotope effects (KIEs) provide unprecedented mechanistic insight into three intermolecular anti-Markovnikov alkene hydrofunctionalization reactions─hydroesterification, hydroamination, and hydroetherification─enabled by organophotoredox catalysis. All three reactions are found to proceed via initial oxidation of the model alkenes to form a radical cation intermediate, followed by sequential nucleophilic attack and hydrogen-atom transfer to deliver the hydrofunctionalized product. A normal 13C KIE on the olefinic carbon that undergoes nucleophilic attack provides qualitative evidence for rate-limiting nucleophilic attack in all three reactions. Comparison to predicted 13C KIE values obtained from density functional theory (DFT) calculations for this step reveals that alkene oxidation has partial rate-limiting influence in hydroesterification and hydroamination, while the nucleophilic attack is solely rate-limiting in the hydroetherification reaction. The basic additive (2,6-lutidine) activates the nucleophile via deprotonation and is an integral part of the transition state for nucleophilic attack on the radical cation, providing an important design principle for the development of asymmetric versions of these reactions. A more electron-rich pyridine base (2,6-dimethoxypyridine) exhibits considerable rate enhancements in both inter- and intramolecular hydrofunctionalization reactions.

Mechanism and Origin of Remote Stereocontrol in the Organocatalytic Enantioselective Formal C(sp2)-H Alkylation Using Nitroalkanes as Alkylating Agents.

Experimental 13C kinetic isotope effects (KIEs) and density functional theory (DFT) calculations are used to evaluate the mechanism and origin of enantioselectivity in the formal C(sp2)-H alkylative desymmetrization of cyclopentene-1,3-diones using nitroalkanes as the alkylating agent. An unusual combination of an inverse (∼0.980) and a normal (∼1.033) KIE is observed on the bond-forming carbon atoms of the cyclopentene-1,3-dione and nitroalkane, respectively. These data provide strong support for a mechanism involving reversible carbon-carbon bond formation followed by rate- and enantioselectivity-determining nitro group elimination. The theoretical free-energy profile and the predicted KIEs indicate that this elimination event occurs via an E1cB pathway. The origin of remote stereocontrol is evaluated by distortion-interaction and SAPT0 analyses of the E1cB transition states leading to both enantiomers.

Suppressing catalyst deactivation on Pd/CeO2 for selective oxidation of glucose into gluconic acid

Catalytic oxidizing of glucose into gluconic acid (GLUC) provides an attractive and feasible pathway for the high value-added utilization of glucose, in which Pd-based catalysts deliver outstanding catalytic performance. However, the surface poisoning of Pd caused by the strong adsorption of GLUC becomes the bottleneck for their further applications. Herein, oxygen-vacancy-riched ceria supported Pd catalyst (Pd/CeO2) is employed, which effectively alleviates the catalyst deactivation, and realizes high catalytic activity and stability with 100 % GLUC selectivity for glucose oxidation at room temperature. Kinetic analysis confirms the strong anti-poisoning ability of Pd/CeO2. The superior catalytic performance is attributed to the modulation of the electronic structure of Pd nanoparticles by the strong metal-support interaction between Pd and CeO2. The valence band photoemission spectra reveal that the downshift of the d-band center of Pd leads to the weak adsorption of GLUC on Pd/CeO2. Meanwhile, kinetic isotope effect experiments indicate that the OHads generated from H2O dissociation on the CeO2 surface affords another path for glucose oxidation, thus improves the catalytic activity.

Copper-Catalyzed Asymmetric Conjugate Addition of Alkene-Derived Nucleophiles to Alkenyl-Substituted Heteroarenes

We herein report the copper-catalyzed asymmetric conjugate addition of β-substituted alkenyl heteroarenes, the one of most challenging Michael acceptors, with alkenes as the latent nucleophiles. Diverse chiral heteroarenes bearing two vicinal stereocenters were obtained in good to excellent yields, generally excellent enantioselectivity, and a good level of diastereoselectivity. The products of the Michael addition can be readily transformed into other valuable acyclic enantioenriched structures bearing three contiguous stereocenters via chiral amine catalysis. Mechanistic studies, including an isotope labeling experiment, a nonlinear effect study, kinetic isotope effect experiments, and initial-rate kinetics studies, were implemented, and the experimental results indicated that the hydrocupration step might be the turnover-limiting step. Moreover, the origin of preferential alkene hydrocupration in the presence of β-substituted alkenyl heteroarenes and the Ph-BPE-ligated CuH catalyst was also elucidated via some control experiments.

Mechanistic Studies of a Skatole-Forming Glycyl Radical Enzyme Suggest Reaction Initiation via Hydrogen Atom Transfer.

Gut microbial decarboxylation of amino acid-derived arylacetates is a chemically challenging enzymatic transformation which generates small molecules that impact host physiology. The glycyl radical enzyme (GRE) indoleacetate decarboxylase from Olsenella uli (Ou IAD) performs the non-oxidative radical decarboxylation of indole-3-acetate (I3A) to yield skatole, a disease-associated metabolite produced in the guts of swine and ruminants. Despite the importance of IAD, our understanding of its mechanism is limited. Here, we characterize the mechanism of Ou IAD, evaluating previously proposed hypotheses of: (1) a Kolbe-type decarboxylation reaction involving an initial 1-e– oxidation of the carboxylate of I3A or (2) a hydrogen atom abstraction from the α-carbon of I3A to generate an initial carbon-centered radical. Site-directed mutagenesis, kinetic isotope effect experiments, analysis of reactions performed in D2O, and computational modeling are consistent with a mechanism involving initial hydrogen atom transfer. This finding expands the types of radical mechanisms employed by GRE decarboxylases and non-oxidative decarboxylases, more broadly. Elucidating the mechanism of IAD decarboxylation enhances our understanding of radical enzymes and may inform downstream efforts to modulate this disease-associated metabolism.

Acidic Ionic Liquids as Metal-Free and Recyclable Catalysts for Direct Reduction of Aromatic Allylic Alcohol in Dimethyl Carbonate via Hydrogen Transfer

Direct reduction of alcohols, to avoid tedious reaction steps, has drawn broad attention. However, the transformation was mainly catalyzed by transition metals or Lewis acids. Developing a metal-free and recyclable catalytic system is still a challenging task from the viewpoint of green chemistry. Herein, pyrrolidine-based ionic liquids as metal-free and recyclable catalysts for direct reduction of aromatic allylic alcohol were developed using p-methylbenzyl alcohol as the reducing agent in dimethyl carbonate (DMC). DMC not only acted as the green solvent but also could stabilize the reaction intermediates and improve the reactivities. Aromatic allylic alcohols with electron-donating or -withdrawing groups could provide up to 92% yield. The gram-scale experiment was performed smoothly, and the recycle experiment of the ionic liquids could be achieved through simple steps, which was significant for environmental protection and efficient utilization of resources. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy proved that ionic liquids interacted with the substrates and the reducing agent by forming multiple hydrogen bonding interactions. Kinetic isotope effect (KIE) experiments (kH/kD = 2.27) indicated that the C–H bond cleavage might be a rate-determining step, and deuterium-labeled results demonstrated that the hydrogen of the product was derived from the benzylic H of p-methylbenzyl alcohol. Meanwhile, the experiments of two preprepared ether intermediates as the substrates showed that they may be involved during the reaction, and a possible mechanism containing intermediates, hydrogen transfer, and hydrogen bonding effects was proposed.

Determination of concerted or stepwise mechanism of hydrogen tunneling from isotope effects: Departure between experiment and theory.

Isotope substitution is an important experimental technique that offers deep insight into reaction mechanisms, as the measured kinetic isotope effects (KIEs) can be directly compared with theory. For multiple proton transfer processes, there are two types of mechanisms: stepwise transfer and concerted transfer. The Bell-Limbach model provides a simple theory to determine whether the proton transfer mechanism is stepwise or concerted from KIEs. Recent scanning tunneling microscopy (STM) experiments have studied the proton switching process in water tetramers on NaCl(001). Theoretical studies predict that this process occurs via a concerted mechanism; however, the experimental KIEs resemble the Bell-Limbach model for stepwise tunneling, raising questions on the underlying mechanism or the validity of the model. We study this system using ab initio instanton theory, and in addition to thermal rates, we also considered microcanonical rates, as well as tunneling splittings. The instanton theory predicts a concerted mechanism, and the KIEs for tunneling rates (both thermal and microcanonical) upon deuteration are consistent with the Bell-Limbach model for concerted tunneling but could not explain the experiments. For tunneling splittings, partial and full deuteration change the size of it in a similar fashion to how they change the rates. We further examined the Bell-Limbach model in another system, porphycene, which has both stepwise and concerted tunneling pathways. The KIEs predicted by instanton theory are again consistent with the Bell-Limbach model. This study highlights differences between KIEs in stepwise and concerted tunneling and the discrepancy between theory and recent STM experiments. New theory/experiments are desired to settle this problem.

Isotope Effects Reveal the Catalytic Mechanism of the Archetypical Suzuki-Miyaura Reaction.

Experimental and theoretical 13C kinetic isotope effects (KIEs) are utilized to obtain atomistic insight into the catalytic mechanism of the Pd(PPh3)4-catalyzed Suzuki-Miyaura reaction of aryl halides and aryl boronic acids. Under catalytic conditions, we establish that oxidative addition of aryl bromides occurs to a 12-electron monoligated palladium complex (Pd-(PPh3)). This is based on the congruence of the experimental KIE for the carbon attached to bromine (KIEC-Br = 1.020) and predicted KIEC-Br for the transition state for oxidative addition to the Pd(PPh3) complex (1.021). For aryl iodides, the near-unity KIEC-I of ~1.003 suggests that the first irreversible step in the catalytic cycle precedes oxidative addition and is likely the binding of the iodoarene to Pd(PPh3). Our results suggest that the commonly proposed oxidative addition to the 14-electron Pd(PPh3)2 complex can occur only in the presence of excess added ligand or under stoichiometric conditions; in both cases, experimental KIEC-Br of 1.031 is measured, which is identical to the predicted KIEC-Br for the transition state for oxidative addition to the Pd(PPh3)2 complex (1.031). The transmetalation step, under catalytic conditions, is shown to proceed via a tetracoordinate boronate (8B4) intermediate with a Pd-O-B linkage based on the agreement between an experimental KIE for the carbon atom involved in transmetalation (KIEC-Boron = 1.035) and a predicted KIEC-Boron for the 8B4 transmetalation transition state (1.034).