Catalytic Mode Research Articles

Nickel/Photo-Cocatalyzed Regioselective Ring Opening of N-Tosyl Styrenyl Aziridines with Aldehydes.

Aldehydes and aziridines are both intrinsic electrophilic reagents, and thus the coupling reaction between these two compounds is highly challenging. In this protocol, the merger of nickel and hydrogen-atom-transfer photocatalysis successfully enables the ring opening of N-tosyl styrenyl aziridines with aldehydes, providing a novel and atom-economical access to a variety of β-amino ketones with complete regiocontrol. The preliminary mechanistic studies reveal that the ring opening reaction proceeds with a cooperative catalytic mode: aldehydes are converted into acyl radicals by tetrabutylammonium decatungstate under irradiation, whereas the nickel catalyst is engaged in the ring opening of aziridines and the following carbon-carbon bond-forming step.

Catalysis with Supramolecular Carbon‐Bonding Interactions

AbstractHerein, we describe a new catalysis platform, supramolecular carbon‐bonding catalysis, which exploits the highly directional weak interactions between carbon centers of catalysts and electron donors to drive chemical reactions. To demonstrate this catalysis approach, we discovered a class of cyclopropane derivatives incorporated with carbonyl, ester and cyano groups as catalysts which showed general catalysis capability in different types of benchmark reactions. Among these typical examples, a challenging tail‐to‐head terpene cyclization can be achieved by supramolecular carbon‐bonding catalysis. The co‐crystal structures of catalyst and electron donors, comparison experiments, and titrations support a catalysis mode of carbon‐bonding activation of Lewis basic reactants.

Catalysis with Supramolecular Carbon‐Bonding Interactions

Herein, we describe a new catalysis platform, supramolecular carbon-bonding catalysis, which exploits the highly directional weak interactions between carbon centers of catalysts and electron donors to drive chemical reactions. To demonstrate this catalysis approach, we discovered a class of cyclopropane derivatives incorporated with carbonyl, ester and cyano groups as catalysts which showed general catalysis capability in different types of benchmark reactions. Among these typical examples, a challenging tail-to-head terpene cyclization can be achieved by supramolecular carbon-bonding catalysis. The co-crystal structures of catalyst and electron donors, comparison experiments, and titrations support a catalysis mode of carbon-bonding activation of Lewis basic reactants.

In-situ catalytic effect of potassium on petroleum coke gasification with a Mn ore-based oxygen carrier

Potassium was used as a catalyst for Chemical Looping Gasification (CLG) process in order to accelerate petroleum coke gasification. Two catalytic modes were applied, one was to use potassium to modify coke and another one was to a Mn ore based oxygen carrier. The in-situ catalytic effect of potassium during CLG process was experimentally evaluated in a fluidized bed. The influences of reaction temperature, oxygen carrier to fuel ratio, potassium introduction modes and cycles on syngas yields and distribution, carbon conversion and potassium evolution were investigated. Results indicated that both catalytic modes can accelerate coke conversion. Beneficial coke conversion was brought into play after several cycles’ activation for the potassium-modified Mn ore oxygen carrier applied for CLG process. Its activation with cycles led to an increase of the average carbon conversion rate, obtaining a stable value of 4.39%/min, higher than the one of 3.91%/min for the condition without potassium addition. Part of potassium on the surface of oxygen carrier was partially lost in the initial cycles and then fixed on the oxygen carrier particles. The remaining potassium existed in the form of K2Si4O9. The catalytic model of using the potassium to modify coke for CLG process accelerated coke conversion remarkably, obtaining an average carbon conversion rate of 7.8%/min, which is 1.78 times higher than that of using potassium to modify the Mn ore. Besides, the potassium into the reaction process had little effect on the oxygen carrier’s reactivity, but accelerated the water gas shift reaction, producing more H2 and less CO in the syngas.

Dissection of Alkylpyridinium Structures to Understand Deamination Reactions.

Via conversion to Katritzky pyridinium salts, alkyl amines can now be used as alkyl radical precursors for a range of deaminative functionalization reactions. The key step of all these methods is single electron reduction of the pyridinium ring, which triggers C-N bond cleavage. However, little has been done to understand how the precise nature of the pyridinium influences these events. Using a combination of synthesis, computation, and electrochemistry, this study delineates the steric and electronic effects that substituents have on the canonical steps and the overall process. Depending on the approach taken, consideration of both the reduction and the subsequent radical dissociation may be necessary. Whereas the electronic effects on these steps work in opposition to each other, the steric effects are synergistic, with larger substituents favoring both steps. This understanding provides a framework for future design of pyridinium salts to match the mode of catalysis or activation.

The surprising structural and mechanistic dichotomy of membrane-associated phosphoglycosyl transferases.

Phosphoglycosyl transferases (PGTs) play a pivotal role at the inception of complex glycoconjugate biosynthesis pathways across all domains of life. PGTs promote the first membrane-committed step in the en bloc biosynthetic strategy by catalyzing the transfer of a phospho-sugar from a nucleoside diphospho-sugar to a membrane-resident polyprenol phosphate. Studies on the PGTs have been hampered because they are integral membrane proteins, and often prove to be recalcitrant to expression, purification and analysis. However, in recent years exciting new information has been derived on the structures and the mechanisms of PGTs, revealing the existence of two unique superfamilies of PGT enzymes that enact catalysis at the membrane interface. Genome neighborhood analysis shows that these superfamilies, the polytopic PGT (polyPGT) and monotopic PGT (monoPGT), may initiate different pathways within the same organism. Moreover, the same fundamental two-substrate reaction is enacted through two different chemical mechanisms with distinct modes of catalysis. This review highlights the structural and mechanistic divergence between the PGT enzyme superfamilies and how this is reflected in differences in regulation in their varied glycoconjugate biosynthesis pathways.

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H’ elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Asymmetric Organocatalysis of Activated Alkynes and Enynes

AbstractEnantioselective organocatalytic protocols for the functionalization of electron‐deficient internal alkynes and enynes are reviewed. This review focuses on the applicability of activated conjugated π‐systems in asymmetric synthesis for providing diversified chiral entities under metal‐free reaction conditions. Various organocatalytic strategies with different catalytic modes are demonstrated.

Recent advancement in copper-catalyzed asymmetric reactions of alkynes

Alkynes are among the most diverse functional groups with a great synthetic profile. In the presence of transition metal catalysts, an alkyne could be transformed into various functionalities such as alkanes, alkenes, carbonyls, allenes, carbocycles, and heterocycles, etc. Copper salts are inexpensive, stable, and extensively used catalysts for alkyne activation that happens either via sigma-bond or pi-bond complex formation. Sigma-bond complexation leads to nucleophilic alkyne species, whereas pi-bond complexation results in electrophilic alkyne species. Different domains of the copper-catalyzed reactions of alkynes have separately been reviewed by several research groups. This review covers the recent reports (2011–2020) on copper-catalyzed asymmetric reactions of alkynes, provides insight into the mode of catalysis, and discusses the evolution of alkyne-based organic transformations such as addition to carbonyls and imines, A3 coupling, allylic alkynylation, hydrofunctionalization, cycloaddition, click chemistry, Kinugasa reaction, and allene formation.

Catalyst Control over Twofold and Higher-Order Stereogenicity by Atroposelective Arene Formation.

Contradictory to the first intuitive impression that forging putatively flat aromatic rings evades stereoisomerism, a striking variety of atropisomeric compounds are conceivable by the formation of arenes, offering captivating avenues for catalyst-controlled stereoselective strategies. Since the assembled atropisomeric products that contain one or several rotationally restricted single bonds are characterized by especially well defined molecular architectures, they are distinctly suitable for numerous pertinent applications. In view of the fascinating arene-forming aldol condensation pathways taking place in polyketide biosynthesis (cyclases/aromatases (CYC/ARO)), the versatile small-molecule-catalyzed aldol reaction appeared as an exceptionally appealing synthetic means to prepare various unexplored atropisomeric compounds in our efforts presented herein. In our initial studies, the use of secondary amine organocatalysts provided excellent selectivities in stereoselective arene-forming aldol condensations for a broad range of atropisomeric products, such as biaryls and rotationally restricted aromatic amides. In further analogy to polyketide biosynthesis, it was also conceivable that several aromatic rings are formed in catalytic cascade reactions. The use of small-molecule catalysts thereby enabled us to transfer this concept to the conversion of unnatural and noncanonical polyketide substrates, thus giving access to atropisomers with particular value for synthetic applications. The versatility of the stereoselective aldol reactions with numerous catalytic activation modes further provided a strategy to individually control several stereogenic axes, similar to the various methodologies developed for controlling stereocenter configurations. By the use of iterative building block additions combined with catalyst-controlled aldol reactions to form the aromatic rings, stereodivergent pathways for catalyst-substrate-matched and -mismatched products were obtained. Besides secondary amines, cinchona-alkaloid-based quaternary ammonium salts also proved to be highly efficient in overcoming severe substrate bias. The obtained atropisomeric multiaxis systems, with all of the biaryl bonds suitably restricted in rotation even at high temperatures, are spatially distinctly defined. The helical secondary structure is therefore excellently suited for several captivating applications.While previous catalyst-controlled stereoselective methods distinguish two stereoisomers for each stereogenic unit, catalyst control beyond the realms of this dualistic stereoisomerism remained unexplored. By the selective preparation of O̅ki atropisomers characterized by their sixfold stereogenicity in Rh-catalyzed [2 + 2 + 2] cyclotrimerizations, one out of the six possible stereoisomers resulting from the restricted rotation of a single bond was shown to be catalytically addressable. Catalyst control over higher-order stereogenicity therefore further interconnects conformational analysis and stereoselective catalysis and offers captivating avenues to explore uncharted stereochemical space for creating a broad range of unprecedented molecular motifs.

Atroposelective Synthesis of Axially Chiral 4-Aryl α-Carbolines via N-Heterocyclic Carbene Catalysis

The first catalytic asymmetric construction of axially chiral 4-aryl α-carboline skeletons has been accomplished through an N-heterocyclic carbene (NHC)-catalyzed atroposelective formal [3 + 3] annulation of 4-nitrophenyl 3-arylpropiolates with 2-sulfonamidoindolines. The synthetic utility of the title compounds has been demonstrated by the diverse late-stage structural modifications. Density functional theory calculations were also conducted to illuminate the key factors for controlling the origin of the enantioselectivity. This strategy not only provides an efficient pathway to access axially chiral α-carboline atropisomers but also offers a novel catalytic enantioselective mode for the construction of axially chiral heterobiaryls by using NHC-bound alkynyl acylazoliums.

Strategies of In Situ Generated Magnesium Catalysis in Asymmetric Reactions

AbstractMagnesium (Mg) is a cheap, non-toxic, and recyclable alkaline earth metal that constitutes about 2% weight in the Earth’s crust. The use of magnesium catalysts to forge chiral moieties in molecules is highly attractive. Based on our work in recent years, we describe the current progress in the development of in situ generated magnesium catalysts and their application in asymmetric synthesis. In this perspective, a critically concise classification of in situ generated magnesium catalytic modes, with relevant examples, is presented, and representative mechanisms of each category are discussed. Building on the established diverse strategies, one can foresee that more innovative and structurally creative magnesium catalysts that are generated in situ will be developed to overcome more formidable challenges of catalytic enantioselective reactions.1 Introduction2 Magnesium Catalysts Generated in Situ from Chiral Ligands Containing Dual Reactive Hydrogens3 Magnesium Catalysts Generated in Situ from Monoanionic Chiral Ligands4 Bimetallic and Polymetallic Magnesium Catalysts Assembled in Situ5 Summary and Outlook

Catalysis by Bidentate Iodine(III)-Based Halogen Donors: Surpassing the Activity of Strong Lewis Acids.

The poorly understood mode of activation and catalysis of bidentate iodine(III)-based halogen donors have been quantitatively explored in detail by means of state-of-the-art computational methods. To this end, the uncatalyzed Diels–Alder cycloaddition reaction between cyclohexadiene and methyl vinyl ketone is compared to the analogous process mediated by a bidentate iodine(III)-organocatalyst and by related, highly active iodine(I) species. It is found that the bidentate iodine(III)-catalyst accelerates the cycloaddition by lowering the reaction barrier up to 10 kcal mol–1 compared to the parent uncatalyzed reaction. Our quantitative analyses reveal that the origin of the catalysis is found in a significant reduction of the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a significant polarization of the π-system of the dienophile away from the incoming diene. Notably, the activity of the iodine(III)-catalyst can be further enhanced by increasing the electrophilic nature of the system. Thus, novel systems are designed whose activity actually surpasses that of strong Lewis acids such as BF3.

Direct α-Acylation of Alkenes via N-Heterocyclic Carbene, Sulfinate, and Photoredox Cooperative Triple Catalysis.

N-Heterocyclic carbene (NHC) catalysis has emerged as a versatile tool in modern synthetic chemistry. Further increasing the complexity, several processes have been introduced that proceed via dual catalysis, where the NHC organocatalyst operates in concert with a second catalytic moiety, significantly enlarging the reaction scope. In biological transformations, multiple catalysis is generally used to access complex natural products. Guided by that strategy, triple catalysis has been studied recently, where three different catalytic modes are merged in a single process. In this Communication, direct α-C–H acylation of various alkenes with aroyl fluorides using NHC, sulfinate, and photoredox cooperative triple catalysis is reported. The method allows the preparation of α-substituted vinyl ketones in moderate to high yields with excellent functional group tolerance. Mechanistic studies reveal that these cascades proceed through a sequential radical addition/coupling/elimination process. In contrast to known triple catalysis processes that operate via two sets of interwoven catalysis cycles, in the introduced process, all three cycles are interwoven.

A Palladium Complex as an Asymmetric π-Lewis Base Catalyst for Activating 1,3-Dienes.

Here we report that palladium(0) complexes can coordinate in a η2 fashion to 1,3-dienes and significantly raise the energy of their highest occupied molecular orbital (HOMO) by donating the electrons from the d-orbitals to the empty antibonding molecular orbitals of double bonds (π*) via back-bonding. Thus, the uncoordinated double bond, as a more reactive partner on the basis of the principle of vinylogy, can directly attack imines, furnishing a formal hydrodienylation reaction enantioselectively. A chemoselective cascade vinylogous addition/allylic alkylation difunctionalization process between 1,3-dienes and imines with a nucleophilic group is also compatible, by trapping in situ formed π-allylpalladium species after initial ene addition. This π-Lewis base catalytic mode, featuring simple η2coordination, vinylogous activation, and compatibility with both conjugated neutral polyenes and electron-deficient polyenes, is elucidated by control experiments and density functional theory (DFT) calculations.

Stereoselective organocascades: from fundamentals to recent developments

Abstract Reaction sequences where more bonds are sequentially formed (cascade reactions) may be started either by a stoichiometric or by a catalytic reagent, and proceed in an enantio- diastereo- or non-stereo- selective manner. A wide variety of such strategies has been developed, including both stoichiometric and catalytic ones. Within the widely developed cascade reactions field, this chapter is not meant to be omni-comprehensive, but to offer an as much as possible complete overview on organocatalytic stereoselective methods. We embrace the more general definitions by Tietze and Denmark, considering as cascade reactions all those one-pot processes that involve two or more bond formations, where each subsequent step is enabled by a structural change caused by the previous one. We will include both two- and multi-component reactions where one or more organocatalysts may be responsible either for all or just some of the occurring transformations. Organocascades will be reported according to the number of involved catalytic cycles. In the following paragraphs, only cascade reactions that are stereoselective by means of a chiral catalyst will be considered. It will be shown that multiple possibilities, relying on different catalysis modes, are available to achieve the same reaction sequence.

Recent Advances in Metal‐Catalyzed Asymmetric Hydroboration of Ketones

AbstractMetal‐catalyzed asymmetric reduction of unsaturated functions is a highly useful and fundamental transformation to give diverse chiral synthons. In particular, the enantioselective reduction of prochiral ketones is of great synthetic interest, since it can provide optically active chiral alcohols which have wide applications in organic synthesis, materials science, and pharmaceutical chemistry. Numerous and diverse metal catalytic systems for asymmetric hydrogenation and hydrosilylation of ketones extensively evolved in terms of activity, selectivity, and practicality, while only limited varieties of metal catalysts for the asymmetrichydroborationhad been documented until 2010.Diverseandnewmetal complexes with a range of multi‐dentate chiral ligands have recently emerged as catalysts for the enantioselective ketone hydroboration, which are highly differentiated from the precedence in several aspects. This Minireview summarizes recent examples of the metal catalyst systems for the asymmetric hydroboration of ketones published from 2015 to 2020. Diverse catalytic working modes involved in a process of enantiodifferentiating hydride transfer, are discussed with a strong emphasis on the steric and electronic effects of chiral ligands.

Mechanistic Understanding of Base‐Catalyzed Aldimine/Ketoamine Condensations: An Old Story and A New Model

AbstractAldimine/ketoamine condensations are basic reactions in chemical synthesis. Herein a density functional theory (DFT) study was performed to characterize the mechanism of the ketoamine condensation of acetophenone with tert‐butanesulfinamide catalyzed by NaOH. The calculated results showed that the main N‐tert‐butanesulfinimines products own E configuration, which are due to both thermodynamic and kinetic effects. Meanwhile, the key roles of the substrate cage and the synergistic catalytic mode formed by metal cation and hydroxyl anion were pointed out, which imply the influence of noncovalent interactions on reaction activity. This theoretical study revealed the origin of E sulfinimines achieved in aldimine/ketoamine condensations catalyzed by base, which could provide new insights into the nature for the condensation reactions.

Operando Fourier-transform infrared-mass spectrometry reactor cell setup for heterogeneous catalysis with glovebox transfer process to surface-chemical characterization.

We describe a new type of operando Fourier transform infrared (FTIR)-mass spectrometry setup for surface-chemical and reactivity characterization of heterogeneous catalysts. On the basis of a sophisticated all-quartz FTIR reactor cell, capable of operating between room temperature and 1000 °C in reactive gas atmospheres, the setup offers a unique opportunity to simultaneously collect and accordingly correlate FTIR surface-chemical adsorption data of the active catalyst state and FTIR gas phase data with complementary reactivity data obtained via mass spectrometry in situ. The full set of catalytic operation modes (recirculating static and flow reactor conditions) is accessible and can be complemented with a variety of temperature-programmed reaction modes or thermal desorption. Due to the unique transfer process involving a home-built portable glovebox to avoid air exposure, a variety of complementary quasi in situ characterization methods for the pre- and post-reaction catalyst states become accessible. We exemplify the capabilities for additional x-ray photoelectron spectroscopy characterization of surface-chemical states, highlighting the unique strength of combining adsorption, electronic structure, and reactivity data to gain detailed insight into the reactive state of a Cu/ZrO2 heterogeneous catalyst during methanol steam reforming operation.

Visible-light-mediated aerobic oxidation of toluene via V2O5@CN boosting benzylic C(sp3)[sbnd]H bond activation

Incorporating photocatalysis into benzylic C(sp3)H oxidation represents a recent advance of organic synthesis in a sustainable manner. Although photoinduced charge carriers enable the C(sp3)H bond to be activated under mild conditions, the reaction scale and activity remain hindered by the photoactivation efficiency. Herein, we present that the 1.0% V2O5@CN photocatalyst, a visible-light-active C3N4-based material, is striking active for the aerobic oxidation of benzylic C(sp3)H bonds, affording a conversion rate 11.6 times higher than that of independent CN. The enhanced activity and C(sp3)H bond photoactivation, as well as the induction period shortening and active period acceleration, are rationalized by the low-EVB hole injection from V2O5 into CN and the boosted charge separation. The durability and utility are affirmed in multiple catalytic cycles, scalable long-term reactions, and available substrate scopes. Reactive intermediate studies by EPR demonstrate the involvement of CH2Ph, O2−, and OH radicals in photoredox catalysis. The mechanism that causes the change of main product species is attributed to the formation of OH in the later reaction stage. This work indicates a unique photoactive V2O5@CN catalysis mode, opening up the efficient use of CN in scalable photocatalytic toluene oxidation.