Kinetic Isotope Effect Experiments Research Articles

Enantioselective Si‐H Insertion of Arylvinyldiazoesters Promoted by Rhodium(I)/Diene Complexes

An enantioselective Si‐H insertion between α‐arylvinyldiazoesters and silanes has been developed by employing rhodium(I)/chiral diene complex as the catalyst. This protocol offers a catalytic asymmetric approach to various chiral silanes containing α‐arylvinyl functionality at room temperature with good enantiocontrol (89‐99% ee). Transformation of the resulting enantiomerically enriched α‐arylvinyl‐α‐silyl esters led to a variety of other valuable building blocks. The deuterium kinetic isotope effect experiments revealed a rhodium(I)‐mediated concerted Si‐H insertion mechanism.

Divergent Synthesis of Multi-Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare-Earth-Catalyzed Benzylic C(sp3 )-H Activation.

The search for efficient and selective methods for the divergent synthesis of multi-substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2-methyl aromatic aldimines with alkenes via benzylic C(sp3 )-H activation by half-sandwich rare-earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi-substituted 1-aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5 Me4 SiMe3 or C5 Me5 exclusively afforded the trans-selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis-selective annulation products. This protocol features 100 % atom-efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Divergent Synthesis of Multi‐Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare‐Earth‐Catalyzed Benzylic C(sp3)−H Activation

AbstractThe search for efficient and selective methods for the divergent synthesis of multi‐substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2‐methyl aromatic aldimines with alkenes via benzylic C(sp3)−H activation by half‐sandwich rare‐earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi‐substituted 1‐aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5Me4SiMe3 or C5Me5 exclusively afforded the trans‐selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis‐selective annulation products. This protocol features 100 % atom‐efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.

Understanding the Interplay of the Brønsted Acidity of Catalyst Ancillary Groups and the Solution Components in Iron-porphyrin-Mediated Carbon Dioxide Reduction.

The rapid and efficient conversion of carbon dioxide (CO2) to carbon monoxide (CO) is an ongoing challenge. Catalysts based on iron-porphyrin cores have emerged as excellent electrochemical mediators of the two proton + two electron reduction of CO2 to CO, and many of the design features that promote function are known. Of those design features, the incorporation of Brønsted acids in the second coordination sphere of the iron ion has a significant impact on catalyst turnover kinetics. The Brønsted acids are often in the form of hydroxyphenyl groups. Herein, we explore how the acidity of an ancillary 2-hydroxyphenyl group affects the performance of CO2 reduction electrocatalysts. A series of meso-5,10,15,20-tetraaryl porphyrins were prepared where only the functional group at the 5-meso position has an ionizable proton. A series of cyclic voltammetry (CV) experiments reveal that the complex with -OMe positioned para to the ionizable -OH shows the largest CO2 reduction rate constants in acetonitrile solvent. This is the least acidic -OH of the compounds surveyed. The turnover frequency of the -OMe derivative can be further improved with the addition of 4-trifluoromethylphenol to the solution. In contrast, the iron-porphyrin complex with -CF3 positioned opposite the ionizable -OH shows the smallest CO2 reduction rate constants, and its turnover frequency is less enhanced with the addition of phenols to the reaction solutions. The origin of this effect is rationalized based on kinetic isotope effect experiments and density functional calculations. We conclude that catalysts with weaker internal acids coupled with stronger external acid additives provide superior CO2 reduction kinetics.

Radical-Smiles Rearrangement by a Vitamin B2-Derived Photocatalyst in Water.

Herein, we report a catalytic radical-Smiles rearrangement system of arene migration from ether to carboxylic acid with riboflavin tetraacetate (RFT), a readily available ester of natural vitamin B2, as the photocatalyst and water as a green solvent, being free of external oxidant, base, metal, inert gas protection, and lengthy reaction time. Not only the known substituted 2-phenyloxybenzoic acids substrates but also a group of naphthalene- and heterocycle-based analogues was converted to the corresponding aryl salicylates for the first time. Mechanistic studies, especially a couple of kinetic isotope effect (KIE) experiments, suggested a sequential electron transfer-proton transfer processes enabled by the bifunctional flavin photocatalyst.

Ligand Combination Approach in Pd‐Catalyzed Direct C−H Arylation of Benzothiazole and Benzoxazole

AbstractThe use of ligand combination approach in the Pd catalyzed direct C−H arylation of benzothiazole and benzoxazole is demonstrated. The combination of two ligands phenanthroline monohydrate (Phen.H2O) and triphenylphosphine (PPh3) with Pd‐catalyst have offered the best catalytic properties in the C−H bond arylation reactions than either of two ligands employed separately in the reactions. This protocol exhibits a wide substrate scope affording C2 arylated benzothiazole and benzoxazole derivatives in excellent yields. Mechanistic studies including kinetic isotope effect (KIE), H/D exchange and competition experiments show that the C−H bond activation of benzothiazole and benzoxazole is very facile under reaction conditions. Furthermore, different mechanistic pathways originated from the catalytically active species such as Pd(η2‐dba)L (dba=dibenzylidene acetone) and Pd(PPh3)2 were investigated using computational methods. The computational results suggest that the generation of catalytically active species from Pd2(dba)3 is endoergic process and Phen ligand can initiate this step. On the other hand, the presence of PPh3 could be crucial for the oxidative addition of aryl halides which is the rate‐determining step of the reaction.

Dehydrogenation of Aqueous Formic Acid by Iridium and Ruthenium Complexes with Nitrogen‐rich Diamino‐bis(1H‐1,2,4‐triazole)

AbstractThe development of efficient catalysts for the dehydrogenation of formic acid (FA) through molecular level control and precise regulation remains challenging. Herein, we report the newly developed complexes of iridium and ruthenium with nitrogen‐rich ligand for aqueous FA dehydrogenation. The Cp*Ir complex 1 (Cp*=cyclopentadienyl) and Ru(p‐cym) complex 2 (p‐cym=p‐Cymene) were synthesized and fully characterized. The single crystal structures of these complexes have a typical piano‐stool structure. A turnover frequency of 12468 h−1 was achieved for FA dehydrogenation at 90 °C by using complex 1 bearing 3,3’‐diamino‐5,5’‐bis(1H‐1,2,4‐triazole) (DABT). The effects of pH, temperature, concentration of formic acid and amount of catalyst on the reaction kinetics were studied in detail. Kinetic isotope effect (KIE) experiments and 1H NMR detection of reaction revealed that a plausible mechanism involves the decarboxylation of formate and the formation of H2. More importantly, the influence of H2O and H+ was involved in the rate‐determining step. This study demonstrated the effectiveness of nitrogen‐rich ligand for FA dehydrogenation, which would contribute to the development of new and effective hydrogen release systems.

Visible-light-induced electron density enrichment of the active sites in the core-satellite structured CuWO4@NiO for fast hydrogen generation from ammonia borane methanolysis

Catalytic dehydrogenation of ammonia borane (AB) is an integrated technology for producing hydrogen with promising applications in mobile fuel cells. However, its commercial prospect is significantly hampered by the high expenditure of precious metal catalysts. Herein, a low-cost and efficient Z-schemed core-satellite structured CuWO4@NiO nanocomposite catalyst was developed for the methanolysis of AB for fast hydrogen production. The experimental and theoretical calculation results revealed charge transfer between CuWO4 (core) and NiO (satellite), thus creating active Ni and Cu sites for the adsorption and activation of methanol and AB, respectively. When the CuWO4@NiO catalyst was irradiated with visible light, numerous photogenerated electrons were accumulated in the conduction band of NiO of CuWO4@NiO by virtue of the special Z-scheme heterojunction, resulting in the electron density enrichment of Ni sites. The electron-enriched Ni sites effectively activated the adsorbed methanol, thus making the O–H bond cleavage easier, which was identified as the rate-determining step in AB methanolysis using kinetic isotope effect experiments. Based on the special electronic and energy band structures, our CuWO4@NiO catalyst exhibited outstanding catalytic activity with a turnover frequency of 26.5 min−1 in AB methanolysis, surpassing most noble-metal-free catalysts reported in the literature. Our findings show that visible light can be applied as a useful tool to optimize the electronic states of both active sites of catalysts and adsorbed reactants. This study provides insight into the design of more efficient catalytic systems toward AB methanolysis for fast hydrogen evolution.

Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations.

(Cyclopentadienone)iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4'-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

Catalytic Mechanism of Nonglycosidic C-N Bond Formation by the Pseudoglycosyltransferase Enzyme VldE.

The pseudoglycosyltransferase (PsGT) enzyme VldE is a homologue of the retaining glycosyltransferase (GT) trehalose 6-phosphate synthase (OtsA) that catalyzes a coupling reaction between two pseudo-sugar units, GDP-valienol and validamine 7-phosphate, to give a product with α,α-N-pseudo-glycosidic linkage. Despite its biological importance and unique catalytic function, the molecular bases for its substrate specificity and reaction mechanism are still obscure. Here, we report a comparative mechanistic study of VldE and OtsA using various engineered chimeric proteins and point mutants of the enzymes, X-ray crystallography, docking studies, and kinetic isotope effects. We found that the distinct substrate specificities between VldE and OtsA are most likely due to topological differences within the hot spot amino acid regions of their N-terminal domains. We also found that the Asp158 and His182 residues, which are in the active site, play a significant role in the PsGT function of VldE. They do not seem to be directly involved in the catalysis but may be important for substrate recognition or contribute to the overall architecture of the active site pocket. Moreover, results of the kinetic isotope effect experiments suggest that VldE catalyzes a C-N bond formation between GDP-valienol and validamine 7-phosphate via an SNi-like mechanism. The study provides new insights into the substrate specificity and catalytic mechanism of a member of the growing family of PsGT enzymes, which may be used as a basis for developing new PsGTs from GTs.

Atomically Dispersed Cobalt/Copper Dual-Metal Catalysts for Synergistically Boosting Hydrogen Generation from Formic Acid.

The development of practical materials for (de)hydrogenation reactions is a prerequisite for the launch of a sustainable hydrogen economy. Herein, we present the design and construction of an atomically dispersed dual-metal site Co/Cu-N-C catalyst allowing significantly improved dehydrogenation of formic acid, which is available from carbon dioxide and green hydrogen. The active catalyst centers consist of specific CoCuN6 moieties with double-N-bridged adjacent metal-N4 clusters decorated on a nitrogen-doped carbon support. At optimal conditions the dehydrogenation performance of the nanostructured material (mass activity 77.7 L ⋅ gmetal -1  ⋅ h-1 ) is up to 40 times higher compared to commercial 5 % Pd/C. In situ spectroscopic and kinetic isotope effect experiments indicate that Co/Cu-N-C promoted formic acid dehydrogenation follows the so-called formate pathway with the C-H dissociation of HCOO* as the rate-determining step. Theoretical calculations reveal that Cu in the CoCuN6 moiety synergistically contributes to the adsorption of intermediate HCOO* and raises the d-band center of Co to favor HCOO* activation and thereby lower the reaction energy barrier.

Elucidating the alloying effect of PdAg/CNT catalysts on formic acid dehydrogenation with kinetic isotope effect

Formic acid has emerged as a promising liquid hydrogen carrier for addressing the transport and storage issues of hydrogen, owing to its high hydrogen density (4.4 wt.%) and easy-handling liquid phase. Therefore, the conversion of formic acid into hydrogen and vice versa is a significant technique. Despite the development of various metal alloy catalysts for efficient production of hydrogen from formic acid, it remains unclear how the metal alloy affects formic acid dehydrogenation from the mechanistic perspective. This study addresses this gap by investigating the alloy effect of Pd and Ag on formic acid dehydrogenation, using PdAg alloy nanoparticles supported on carbon nanotube catalysts. The PdAg alloy, analyzed using multiple characterization methods, demonstrated enhanced catalytic activity, which was maximized at a Pd:Ag molar ratio of 7:3. More importantly, temperature-dependent and kinetic isotope effect experiments demonstrated that the electronic modification of Pd by Ag improved its activity by promoting the cleavage of the CH bond of formic acid. The results suggest that the kinetic isotope effect would be a potent technique for elucidating the alloying effect, as well as providing a deeper understanding of the mechanism.

Rattle-structured CuO/Co3O4@C microspheres, a potent bifunctional catalyst for hydrogen production from ammonia borane hydrolysis and methanolysis

Engineering and fabricating cost-effective and high-performing bifunctional catalysts for ammonia borane (AB) hydrolysis and methanolysis is paramount for their commercial availability. A series of rattle-structured CuO/Co3O4@C microsphere bifunctional catalysts for AB hydrolysis and methanolysis were produced using a simple hard template etching approach. The CuO/Co3O4@C-4 (4 refers to designed the mole ratio of Cu/Co) exhibits the best activity with TOF of 18.8 and 26.8 molH2·molcat-1·min−1 for AB hydrolysis and methanolysis, respectively. It is discovered that the excellent performance is mainly attributed to its unique rattle structure and the synergetic effect between Co3O4 and CuO. Kinetic isotope effect (KIE) experiments and infrared spectroscopy were applied to investigate AB hydrolysis and methanolysis reaction mechanisms. A suggested mechanism for AB methanolysis on rattle-structured CuO/Co3O4@C-4 is proposed based on the experiment results. This work could provide useful guidance for the construction of high-performance bifunctional catalysts for AB hydrolysis and methanolysis for hydrogen production.

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 carbohydrate decomposition realizes H2 production and storage•Photocatalysis over Ta-CeO2 splits carbohydrates into C1 LHCs•Heating promotes selective C–C bond breaking powered by visible light•Concentrated solar light splits glucose to C1 LHCs in a flow apparatus Hydrogen (H2), as an indispensable clean energy vector, has been well demonstrated to be produced via biomass photoreforming powered by solar light. For future biomass refining, biomass photoreforming deserves a high decomposition extent of biomass to maximize H2 production, as the greenhouse gas emissions from biomass acquisition and pretreatment will minimize per mass of H2. The main obstacle to high H2 yield is the far insufficient C–C bond breaking to convert biomass carbons into CO2 with maximization of H2 production. Here, we emphasize C–C bond breaking instead of direct H2 production. Such a “C–C bond first” strategy realizes conversion of carbohydrates into C1 liquid hydrogen carriers (LHCs, consisting of HCOOH and HCHO) over Ta-CeO2 photocatalyst and is demonstrated in a flow apparatus powered solely by solar energy. The LHCs can release H2 on-site where needed by either photocatalysis or thermocatalysis. This work provides a new perspective for H2 production by photocatalysis. Biomass reforming by harvesting solar energy can provide green hydrogen. Current biomass photoreforming provides H2 erratically and in limited yield although efficiently, owing to intermittent features of solar light and incomplete degradation of biomass C–C bonds. Here, we detour the flaws by prioritizing conversion of carbohydrates to liquid hydrogen carriers (LHCs, consisting of HCOOH and HCHO), appropriate for transportation. Subsequently, the LHCs are fully decomposed, releasing only H2 and CO2. This stepwise process enables complete scission of carbohydrate C–C bonds, affording 44 g of H2 per kg of glucose thereof. Intermittent solar light provides the photoenergy and heat to split glucose carbons to produce LHCs (2.5 mmol h−1) in a flow apparatus. This work demonstrates hydrogen production and storage by emphasizing the complete scission of biomass C–C bonds. Biomass reforming by harvesting solar energy can provide green hydrogen. Current biomass photoreforming provides H2 erratically and in limited yield although efficiently, owing to intermittent features of solar light and incomplete degradation of biomass C–C bonds. Here, we detour the flaws by prioritizing conversion of carbohydrates to liquid hydrogen carriers (LHCs, consisting of HCOOH and HCHO), appropriate for transportation. Subsequently, the LHCs are fully decomposed, releasing only H2 and CO2. This stepwise process enables complete scission of carbohydrate C–C bonds, affording 44 g of H2 per kg of glucose thereof. Intermittent solar light provides the photoenergy and heat to split glucose carbons to produce LHCs (2.5 mmol h−1) in a flow apparatus. This work demonstrates hydrogen production and storage by emphasizing the complete scission of biomass C–C bonds. Hydrogen (H2) has emerged as a sustainable and indispensable energy vector that can be produced from renewable energy resources.1Turner J.A. Sustainable hydrogen production.Science. 2004; 305: 972-974https://doi.org/10.1126/science.1103197Crossref PubMed Scopus (4261) Google Scholar,2Liu W. Cui Y. Du X. Zhang Z. Chao Z.S. Deng Y.L. High efficiency hydrogen evolution from native biomass electrolysis.Energy Environ. Sci. 2016; 9: 467-472https://doi.org/10.1039/C5EE03019FCrossref Google Scholar Storage and transportation of H2 remain challenging in the aspect of its worldwide applications.3Chen Z. Li P. Anderson R. Wang X. Zhang X. Robison L. Redfern L.R. Moribe S. Islamoglu T. Gómez-Gualdrón D.A. et al.Balancing volumetric and gravimetric uptake in highly porous materials for clean energy.Science. 2020; 368: 297-303https://doi.org/10.1126/science.aaz8881Crossref PubMed Scopus (268) Google Scholar In this context, formic acid (HCOOH) has been identified as a promising liquid hydrogen carrier (LHC), thermally stable up to 200°C,4Fein D.E. Wachs I.E. Quantitative determination of the catalytic activity of bulk metal oxides for formic acid oxidation.J. Catal. 2002; 210: 241-254https://doi.org/10.1006/jcat.2002.3683Crossref Scopus (32) Google Scholar,5Zhou J. Mullins D.R. Adsorption and reaction of formaldehyde on thin-film cerium oxide.Surf. Sci. 2006; 600: 1540-1546https://doi.org/10.1016/j.susc.2006.02.009Crossref Scopus (80) Google Scholar easy to transport, and be back decomposed with H2 release by thermo-, photo-, or electrocatalysis.6Kumar A. Daw P. Milstein D. Homogeneous catalysis for sustainable energy: hydrogen and methanol economies, fuels from biomass, and related topics.Chem. Rev. 2022; 122: 385-441https://doi.org/10.1021/acs.chemrev.1c00412Crossref PubMed Scopus (63) Google Scholar,7Boddien A. Mellmann D. Gärtner F. Jackstell R. Junge H. Dyson P.J. Laurenczy G. Ludwig R. Beller M. Efficient dehydrogenation of formic acid using an iron catalyst.Science. 2011; 333: 1733-1736https://doi.org/10.1126/science.1206613Crossref PubMed Scopus (641) Google Scholar,8Wei D. Sang R. Sponholz P. Junge H. Beller M. Reversible hydrogenation of carbon dioxide to formic acid using a Mn-pincer complex in the presence of lysine.Nat. Energy. 2022; 7: 438-447https://doi.org/10.1038/s41560-022-01019-4Crossref Scopus (21) Google Scholar Therefore, a very promising and challenging way of exploiting the hydrogen economy is based on the use of HCOOH, which must be produced from renewable resources.9Zhang P. Guo Y.-J. Chen J. Zhao Y.-R. Chang J. Junge H. Beller M. Li Y. Streamlined hydrogen production from biomass.Nat. Catal. 2018; 1: 332-338https://doi.org/10.1038/s41929-018-0062-0Crossref Scopus (73) Google Scholar,10Han X. Sheng H. Yu C. Walker T.W. Huber G.W. Qiu J. Jin S. Electrocatalytic oxidation of glycerol to formic acid by CuCo2O4 spinel oxide nanostructure catalysts.ACS Catal. 2020; 10: 6741-6752https://doi.org/10.1021/acscatal.0c01498Crossref Scopus (112) Google Scholar Broadly available next-generation biomasses containing both carbon and hydrogen are such kinds of renewable resources.11Huber G.W. Shabaker J.W. Dumesic J.A. Raney Ni-Sn catalyst for H2 production from biomass-derived hydrocarbons.Science. 2003; 300: 2075-2077https://doi.org/10.1126/science.1085597Crossref PubMed Scopus (847) Google Scholar,12Liao Y. Koelewijn S.F. Van den Bossche G. Van Aelst J. Van den Bosch S. Renders T. Navare K. Nicolaï T. Van Aelst K. Maesen M. et al.A sustainable wood biorefinery for low-carbon footprint chemicals production.Science. 2020; 367: 1385-1390https://doi.org/10.1126/science.aau1567Crossref PubMed Scopus (404) Google Scholar Both hydrogen production and storage can be achieved if biomass is converted to HCOOH. Cellulose, hemicelluloses, and their hydrolyzates can generate HCOOH by thermocatalysis,9Zhang P. Guo Y.-J. Chen J. Zhao Y.-R. Chang J. Junge H. Beller M. Li Y. Streamlined hydrogen production from biomass.Nat. Catal. 2018; 1: 332-338https://doi.org/10.1038/s41929-018-0062-0Crossref Scopus (73) Google Scholar processes that require stoichiometric oxidants with strong oxidation potential to break the C–C bonds. However, these carbohydrates suffer over-oxidation to CO2 on catalysts.13Zhang Z. Huber G.W. Catalytic oxidation of carbohydrates into organic acids and furan chemicals.Chem. Soc. Rev. 2018; 47: 1351-1390https://doi.org/10.1039/C7CS00213KCrossref PubMed Google Scholar,14Hou Y. Niu M. Wu W. Catalytic oxidation of biomass to formic acid using O2 as an oxidant.Ind. Eng. Chem. Res. 2020; 59: 16899-16910https://doi.org/10.1021/acs.iecr.0c01088Crossref Scopus (20) Google Scholar Producing strong oxidants to cleave C–C bonds represents the advantage of photocatalysis.15Nosaka Y. Nosaka A.Y. Generation and detection of reactive oxygen species in photocatalysis.Chem. Rev. 2017; 117: 11302-11336https://doi.org/10.1021/acs.chemrev.7b00161Crossref PubMed Scopus (1840) Google Scholar,16Uekert T. Pichler C.M. Schubert T. Reisner E. Solar-driven reforming of solid waste for a sustainable future.Nat. Sustain. 2020; 4: 383-391https://doi.org/10.1038/s41893-020-00650-xCrossref Scopus (70) Google Scholar Carbohydrates are easily over-oxidized with less HCOOH generation because of the relatively long carbon chains; nevertheless, more H2 is produced from carbohydrates concomitantly due to the charge balance of photocatalysts,17Wakerley D.W. Kuehnel M.F. Orchard K.L. Ly K.H. Rosser T.E. Reisner E. Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst.Nat. Energy. 2017; 2: 17021https://doi.org/10.1038/nenergy.2017.21Crossref Scopus (302) Google Scholar which is the current consensus of exploiting photogenerated oxidants for H2 generation from biomass (Figure S1 path A).18Kuehnel M.F. Reisner E. Solar hydrogen generation from lignocellulose.Angew. Chem. Int. Ed. Engl. 2018; 57: 3290-3296https://doi.org/10.1002/anie.201710133Crossref PubMed Scopus (137) Google Scholar The principle is to generate holes or ⋅OH, which breaks the O–H bonds of biomass, mainly the carbohydrate components, generating oxygen-centered radicals that experience β-scission to cleave the C–C bonds.19Zhang K. Chang L. An Q. Wang X. Zuo Z. Dehydroxymethylation of alcohols enabled by cerium photocatalysis.J. Am. Chem. Soc. 2019; 141: 10556-10564https://doi.org/10.1021/jacs.9b05932Crossref PubMed Scopus (85) Google Scholar,20Nguyen S.T. Murray P.R.D. Knowles R.R. Light-driven depolymerization of native lignin enabled by proton-coupled electron transfer.ACS Catal. 2020; 10: 800-805https://doi.org/10.1021/acscatal.9b04813Crossref Scopus (51) Google Scholar However, the co-generated carbon-centered radicals and those derived from direct oxidation of the C–H bond by photogenerated oxidants tend to form C–C bonds when external or concurrently generated oxidants are absent (Figure S1 path B).21Kisch H. Semiconductor photocatalysis for chemoselective radical coupling reactions.Acc. Chem. Res. 2017; 50: 1002-1010https://doi.org/10.1021/acs.accounts.7b00023Crossref PubMed Scopus (123) Google Scholar,22Luo N. Nie W. Mu J. Liu S. Li M. Zhang J. Gao Z. Fan F. Wang F. Low-work function metals boost selective and fast scission of methanol C–H bonds.ACS Catal. 2022; 12: 6375-6384https://doi.org/10.1021/acscatal.1c06005Crossref Scopus (5) Google Scholar This mechanism of photochemical recondensation is adverse to C–C bond breaking, as biomass is converted to recalcitrant architecture like humin and wasted.23Huang Z. Luo N. Zhang C. Wang F. Radical generation and fate control for photocatalytic biomass conversion.Nat. Rev. Chem. 2022; 6: 197-214https://doi.org/10.1038/s41570-022-00359-9Crossref Scopus (18) Google Scholar Neglecting the principle for C–C bond scission limits the amount of H2 per mass of biomass.18Kuehnel M.F. Reisner E. Solar hydrogen generation from lignocellulose.Angew. Chem. Int. Ed. Engl. 2018; 57: 3290-3296https://doi.org/10.1002/anie.201710133Crossref PubMed Scopus (137) Google Scholar Besides, the intermittent feature of solar light provides H2 erratically. Tackling these challenges requires prioritized biomass conversion to HCOOH or equivalent C1 LHCs, appropriate for on-site hydrogen production independent of solar light irradiation.24Cao S. Chen Y. Wang H. Chen J. Shi X. Li H. Cheng P. Liu X. Liu M. Piao L. Ultrasmall CoP nanoparticles as efficient cocatalysts for photocatalytic formic acid dehydrogenation.Joule. 2018; 2: 549-557https://doi.org/10.1016/j.joule.2018.01.007Abstract Full Text Full Text PDF Scopus (96) Google Scholar,25Kar S. Rauch M. Leitus G. Ben-David Y. Milstein D. Highly efficient additive-free dehydrogenation of neat formic acid.Nat. Catal. 2021; 4: 193-201https://doi.org/10.1038/s41929-021-00575-4Crossref Scopus (49) Google Scholar However, fully breaking the C–C bonds to produce these active C1 LHCs is challenging, as the C1 LHCs are readily decomposed into side products either on photocatalysts or with the catalysis of acids generated concurrently.26Wang M. Liu M. Lu J. Wang F. Photo splitting of bio-polyols and sugars to methanol and syngas.Nat. Commun. 2020; 11: 1083https://doi.org/10.1038/s41467-020-14915-8Crossref PubMed Scopus (46) Google Scholar In an approach orthogonal to direct H2 production from biomass, here we devise the strategy of “C–C bond first” that carbohydrates from hydrolysis of lignocellulose are first converted to LHCs with nearly 100% yield of C1 products (Figure 1). Subsequent degradation of the LHCs provides H2 and CO2 exclusively. The photocatalytic oxidation of carbohydrates in the first step targets splitting carbohydrate carbons into C1 LHCs, distinct from those reported for oxidative functionalization of carbohydrates. This oxidation step was conducted over strongly distorted Ta-doped CeO2 (Ta-CeO2), and the kinetics of C–C bond scission, C–C bond coupling, and conversion of recalcitrant carbonyl intermediates were controlled to realize a high LHCs selectivity. The obtained aqueous solution of LHCs releases gaseous H2 where needed by either photocatalysis or thermocatalysis. Solar-light-driven catalytic oxidation of glucose in a flow apparatus demonstrates the allure of the biomass conversion to LHCs. The Ta content of Ta-CeO2 was determined to be 0.78 mol % by inductively coupled plasma atomic emission spectroscopy (ICP-AES). X-ray diffraction (XRD) patterns (Figure S2A) of CeO2, the contrast catalyst, and Ta-CeO2 show fluorite structure. The diffraction peaks of Ta-CeO2 negligibly shift (Figure S2B), in agreement with the low amount of Ta dopant. Representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the Ta-CeO2 shows the morphology of a rectangular structure (Figure 2A) with an average size of about 7.7 nm (Figure S3). The lattice fringes with spacings of 0.27 and 0.29 nm (Figure 2A) are attributed to the (200) and (020) interplanar distances of fluorite CeO2, respectively. The HAADF-STEM image and the elemental mappings by energy-dispersive X-ray spectra (EDS) show the uniform dispersion of Ta and Ce elements (Figure 2B), implying that Ta is homogeneously incorporated into the matrix of CeO2 (Figure 2C). The inclusion of Ta in the CeO2 matrix is confirmed by X-ray absorption fine structure (XAFS) analysis of the Ta-CeO2 sample at the Ta L3 edge. The X-ray absorption near edge structure (XANES) spectrum of the Ta-CeO2 (Figure 2D) shows that Ta is in the +5 oxidation state as in Ta2O5. The fitting of the k1-weighted extended X-ray absorption fine structure (EXAFS) signal (Figure 2E) reveals that the average local structure around Ta atoms can be reconstructed, including ∼8 oxygen atoms at two different Ta–O distances and ∼2 Ce atoms in the second shell (results are summarized in Table S2). The possibility of observing a Ta–Ce distance in the second shell is a clear indication of the substitution of Ta5+ for Ce4+ in the CeO2 matrix. Despite this, the local environment around Ta ions is strongly distorted with respect to the cubic symmetry of the site hosting the cations in the fluorite structure of CeO2, in agreement with HAADF-STEM results. Moreover, the Ta–O and Ta–Ce distances are significantly shorter than those expected for the fluorite structure of CeO2 (Ce–O distance of 0.234 nm and Ce–Ce distance of 0.382 nm).27Shahin A.M. Grandjean F. Long G.J. Schuman T.P. Cerium LIII-edge XAS investigation of the structure of crystalline and amorphous cerium oxides.Chem. Mater. 2005; 17: 315-321https://doi.org/10.1021/cm0492437Crossref Scopus (75) Google Scholar The strong lattice distortion of the local structure around Ta can be easily justified considering the ionic radii of Ta5+ and Ce4+ in the cubic coordination: 0.074 and 0.097 nm, respectively.28Shannon R.D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides.Acta Cryst. A. 1976; 32: 751-767https://doi.org/10.1107/S0567739476001551Crossref Scopus (53685) Google Scholar Considering the smaller ionic radius of Ta5+, the local structure around Ta ions must be contracted with the oxygen anions moving toward the dopant, with consecutive relaxation of the successive shell of Ce ions. This distortion can be partially compensated by the formation of one Ce3+ expected in the proximity of each Ta5+ dopant as a result of charge compensation and that Ce3+ is larger than Ce4+ (ionic radii of 0.114 versus 0.097 nm),28Shannon R.D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides.Acta Cryst. A. 1976; 32: 751-767https://doi.org/10.1107/S0567739476001551Crossref Scopus (53685) Google Scholar in agreement with the results of Raman spectra (Figure S4). Compared with the as-prepared CeO2, which has an indirect band gap of 2.60 eV (see Note S1), the band gap of Ta-CeO2 is narrower (2.37 eV) owing to the lattice distortion and increased Ce3+ concentration (Figure S5). In direct photoreforming of biomass, the biomass is partially oxidized, H2 is thus produced in a limited yield, and biomass degrades to by-products that require tedious separations or disposal.18Kuehnel M.F. Reisner E. Solar hydrogen generation from lignocellulose.Angew. Chem. Int. Ed. Engl. 2018; 57: 3290-3296https://doi.org/10.1002/anie.201710133Crossref PubMed Scopus (137) Google Scholar The underutilization of biomass originates from incomplete scission of biomass C–C bond and radical coupling in an environment lacking oxidative oxygen radicals, such as O2⋅− and ⋅OH. Glycerol was used as a model molecule at the beginning of the research, although we realize that oxidative splitting of carbohydrate carbons is much more arduous than splitting glycerol carbons because of the longer carbon chains (Table S1). However, the simple and representative polyol structure is beneficial for intermediate detection and mechanism study. Photocatalytic oxidation of glycerol by O2 (5 bar) was evaluated at near room temperature (36°C) over Ta-CeO2 using a homemade photoreactor (Figures 3A and S6). The conversion of glycerol was 26%, and the yield of LHCs was 6% after photoirradiation for 10 h by blue light-emitting diodes (LEDs, 452 ± 10 nm, 200 mW cm−2), indicating that C–C bonds were broken incompletely as widely reported.29Montini T. Gombac V. Sordelli L. Delgado J.J. Chen X. Adami G. Fornasiero P. Nanostructured Cu/TiO2 photocatalysts for H2 production from ethanol and glycerol aqueous solutions.ChemCatChem. 2011; 3: 574-577https://doi.org/10.1002/cctc.201000289Crossref Scopus (158) Google Scholar The yield of LHCs was improved to 74% at 120°C, with glycerol conversion of 93% (43% apparent quantum efficiency). The photocatalytic glycerol oxidation can be conducted in air (25 bar), affording LHCs in 58% yield with glycerol conversion of 75% in a similar O2 partial pressure (5 bar, Figure S8). The selectivity of LHCs (65%, Table S3, entry 1) reduces at a higher reaction temperature (150°C), owing to the prevalent over-oxidation of HCOOH to CO2. This degradation of glycerol to LHCs also relies on photoirradiation, considering glycerol is scarcely converted in the dark even at 120°C (Figure S9A). Glycerol is converted to LHCs with high selectivity (80%) even irradiated with weak light intensity (18 mW cm−2), indicating the synergy of photo and thermal is essential for the splitting of glycerol carbons. When irradiated with a light intensity larger than 130 mW cm−2, the conversion of glycerol and selectivity of LHCs remained unchanged, illustrating that either the light absorption was saturated or the elementary steps involving photogenerated charges were no longer rate-limiting. Under the varied range of light intensities, the carbon balance which is defined as Equation 9 is within 95%–103% (Figure S9B). Glycerol was converted rapidly in the first 2 h under the optimized reaction conditions and was entirely converted after reacting for 20 h, according to the time profiles; the final yield of LHCs reached 86% (Figure S10A). During photocatalytic glycerol oxidation, the carbon balance increased from 72% at 2 h to nearly 100% when glycerol was fully converted (Figure S10B). The Ta-CeO2 photocatalyst shows minor deactivation during the first run and slightly higher deactivation (about 12% according to glycerol conversion) for the subsequent runs (Figure S11). The observed slight deactivation is not amazing and can be explained by the poisoning of Ta-CeO2 by HCOOH or formaldehyde (HCHO) during photocatalysis. LHCs consisting of HCHO and HCOOH could be produced over pristine CeO2 in 58% yield (Table S3, entry 4) after 10 h of reaction at 120°C; the conversion of glycerol was 73%, lower than that (93%) using Ta-CeO2. Of note, Ta-doped other metal oxides, such as TiO2 and Nb2O5, are not selective to C1 LHCs. The promotional role of Ta that is doped into CeO2 could be attributed to the abundantly present Ce3+, deriving from the substitution of Ta5+ for Ce4+ in the CeO2 matrix (see Note S1). The higher concentration of Ce3+ in Ta-CeO2 is expected to adsorb more reactant intermediates,30Campbell C.T. Peden C.H. Chemistry. Oxygen vacancies and catalysis on ceria surfaces.Science. 2005; 309: 713-714https://doi.org/10.1126/science.1113955Crossref PubMed Scopus (978) Google Scholar,31Yang C. Yu X. Pleßow P.N. Heißler S. Weidler P.G. Nefedov A. Studt F. Wang Y. Wöll C. Rendering photoreactivity to ceria: the role of defects.Angew. Chem. Int. Ed. Engl. 2017; 56: 14301-14305https://doi.org/10.1002/anie.201707965Crossref PubMed Scopus (30) Google Scholar thereby breaking the C–C bond scission to produce LHCs in high selectivity. The high concentration of Ce3+ in Ta-CeO2 also promotes electron transition from O 2p to Ce 4f orbits (Figure S12),32Xie S. Wang Z. Cheng F. Zhang P. Mai W. Tong Y. Ceria and ceria-based nanostructured materials for photoenergy applications.Nano Energy. 2017; 34: 313-337https://doi.org/10.1016/j.nanoen.2017.02.029Crossref Scopus (102) Google Scholar generating more photogenerated carriers for glycerol conversion. Doping Ta into CeO2 can weaken the adsorption of HCHO and HCOOH on CeO2 according to the results of temperature-programmed desorption (TPD) experiments (Figure S13; Note S6).33Bu Y.B. Chen Y.F. Jiang G.M. Hou X.M. Li S. Zhang Z.T. Understanding of Au-CeO2 interface and its role in catalytic oxidation of formaldehyde.Appl. Catal. B. 2020; 260: 118138https://doi.org/10.1016/j.apcatb.2019.118138Crossref Scopus (61) Google Scholar We studied the reaction routes of the photocatalytic glycerol oxidation. The radical intermediates were captured by benzyl acrylate (BA) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Quadrupole-time of flight mass spectrometer (Q-TOF MS) was used to analyze the products derived from the reaction of radical intermediates and trapping reagents. The adducts derived from the addition reaction of BA and glycerol were detected with the addition of 1 equivalent of BA (Figure S14A), implying that radicals derived from C–H bond scission were the intermediates. C2 intermediates in the form of glycolaldehyde and glycolic acid were detected according to the m/z of 214 and 75 (Figure S14B), attributed to TEMPO-glycolaldehyde adduct and glycolic acid, respectively.34Luo N. Montini T. Zhang J. Fornasiero P. Fonda E. Hou T. Nie W. Lu J. Liu J. Heggen M. et al.Visible-light-driven coproduction of diesel precursors and hydrogen from lignocellulose-derived methylfurans.Nat. Energy. 2019; 4: 575-584https://doi.org/10.1038/s41560-019-0403-5Crossref Scopus (156) Google Scholar Photocatalytic scission of the C–C bonds in these C2 intermediates also requires heat (Figure S15). Other intermediates were probed by control experiments. Photocatalytic conversion of hypothesized C3 intermediates (Table S3, entries 6, 7, and 8), including glycerol acid, glyceraldehyde, and 1,3-dihydroxyacetone, produces LHCs with yields of 8%, 55%, and 47%, respectively, suggesting that glyceraldehyde and 1,3-dihydroxyacetone are partially involved in the photocatalytic glycerol oxidation. We also tried to reveal possible C2 intermediates. Photocatalytic glycolaldehyde oxidation (Table S3, entry 9) was very fast, affording LHCs and CO2 with 80% and 15% yields in 3 h, respectively. Photocatalytic oxidation of glycolic acid with prolonged reaction time (6 h) produced LHCs (25%) and CO2 (27%) in low yields (Table S3, entry 10). The above results indicate that glycolaldehyde is the major intermediate and derives from the C–C bond scission of glycerol via the initial scission of the terminal C–H bond. Tentative reaction routes were proposed for the photocatalytic glycerol oxidation (Figure S16), combing with the results that photogenerated holes, electrons, and O2⋅− are essential to the photocatalytic glycerol oxidation (Figure S17). Irradiation of Ta-CeO2 generates holes and electrons. Photogenerated holes oxidize the C–H bond of glycerol, affording carbon-centered radicals mainly at the terminal carbon of glycerol (path I). The carbon-centered radicals react with O2⋅− derived from the reduction of O2 by photogenerated electrons, bringing about C–C bond scission with the generation of glycolaldehyde and HCOOH. The glycolaldehyde then experiences C–C bond scission, producing HCOOH and HCHO in equal amounts. Parallel to this reaction path (path I), the C–C bond scission of carbon-centered radicals at the middle carbon produces HCOOH, HCHO, and CO2 (path II). We then studied the catalytic mechanism of heat in promoting photocatalytic oxidation of polyols by monitoring the selectivity of C2+ products, of which the selectivity could be calculated by subtracting C1 products from all the products. A high proportion of C–C bonds is retained at low reaction temperatures according to the moderate selectivity (76%) of C2+ products for photocatalytic glycerol oxidation at 36°C (Figure 3B). The selectivity of C2+ products decreases when improving the reaction temperature, with a selectivity of 5% at 120°C. The undetected carbons were analyzed by Q-TOF MS (Figure S18). In the high-resolution MS, the peaks with m/z of 89.0245, 191.0737, 211.0830, and 255.2341 are ascribed to C3H6O3 and C6H10O4, C8H6O4, and C10H10O5, respectively. The intensities of MS peaks were stronger at 36°C than those at 120°C, in agreement with the results depicted in Figure 3B. Particularly, C3H6O3, ascribed to glyceraldehyde or 1,3-dihydroxyacetone, is produced from the dehydrogenation of glycerol, indicating that the elimination of H2 molecule from glycerol is more overwhelming than breaking the C–C bonds of glycerol at low temperature (36°C). The formation of C6H10O4, C8H6O4, and C10H10O5 during photocatalytic glycerol oxidation indicates a significant contribution of C–C bond coupling. Generally, the C–C bonds cannot be directly activated,35Liao F. Lo T.W.B. Sexton D. Qu J. Wu C.-T. Tsang S.C.E. PdFe nanoparticles as selective catalysts for C–C cleavage in hydrogenolysis of vicinal diol units in biomass-derived chemicals.Catal. Sci. Technol. 2015; 5: 887-896https://doi.org/10.1039/C4CY01159GCrossref Google Scholar,36Li Z. Yan Y. Xu S.M. Zhou H. Xu M. Ma L. Shao M. Kong X. Wang B. Zheng L. et al.Alcohols electrooxidation coupled with H2 production at high current densities promoted by a cooperative catalyst.Nat. Commun. 2022; 13: 147https://doi.org/10.1038/s41467-021-27806-3Crossref PubMed Scopus (32) Google Scholar and their scission is generally less prone than the dehydrogenation of hydroxyl moieties and C–C bond coupling between two radicals.37Liu M. Wang Y. Kong X. Rashid R.T. Chu S. Li C.-C. Hearne Z. Guo H. Mi Z. Li C.-J. Direct catalytic methanol-to-ethanol photo-conversion via methyl carbene.Chem. 2019; 5: 858-867https://doi.org/10.1016/j.chempr.2019.01.005Abstract Full Text Full Text PDF Scopus (40) Google Scholar Considering the above results, the low selectivity of C2+ products at high temperatures implies that heating may assist C–C bond scission. Therefore, both the products from dehydrogenation and C–C bond coupling are converted, preferentially generating HCOOH and HCHO. To prove the hypothesis, we measured the apparent activation energy (Ea) and the kinetic isotope effect (KIE) in the photocatalytic glycerol oxidation. The Ea measured at low light intensity (10 mW cm−2) represents that of dark conditions, considering glycerol is scarcely converted in the dark. The reaction rate constants, fitted from first-order kinetics (a linear relationship between the logarithm of glycerol concentration [Cglycerol] and the reaction time; Figure S19),38Park H. Yun Y.S. Kim T.Y. Lee K.R. Baek J. Yi J. Kinetics of the dehydration of glycerol over acid catalysts with an investigation of deactivation mechanism by coke.Appl. Catal. B. 2015; 176–177: 1-10https://doi.org/10.1016/j.apcatb.2015.03.046Crossref Scopus (52) Google Scholar were obtained at various temperatures under irradiation with light intensities of 10 and 200 mW cm−2, respectively (Figure 3C). The linear fit between the logarithm of rate constants and the reciprocal of the reaction temperature affords the Ea, which is determined to be 24.5 and 24.9 kJ mol−1 for the reactions with light intensities of 10 and 200 mW cm−2, respectively. This result suggests that the rate-limiting step of photocatalytic glycerol oxidation is independent of light intensity. Because the trajectory of the photocatalytic glycerol oxidation involves C–H and C–C bond scission, the KIE experiments were designed to evaluate which step is of kinetic significance. The conversion rates of glycerol and deuterated glycerol were determined at 200 mW cm−2. The KIE values for scission of the C–H bond at the middle and terminal carbons were measured to be 1.20 ± 0.02 and 1.19 ± 0.04, respectively (Table S4, entries 2 and 3). The secondary KIE values excluded the photocatalytic C–H bond scission as the rate-limiting step. Consequently, the C–C bond scission should be rate-limiting. Co

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.