Iminium catalysis

Design and Evolution of an Enzyme for the Asymmetric Michael Addition of Cyclic Ketones to Nitroolefins by Enamine Catalysis

Consistent introduction of novel enzymes is required for developing efficient biocatalysts for challenging biotransformations. Absorbing catalytic modes from organocatalysis may be fruitful for designing new-to-nature enzymes with novel functions. Herein we report a newly designed artificial enzyme harboring a catalytic pyrrolidine residue that catalyzes the asymmetric Michael addition of cyclic ketones to nitroolefins through enamine activation with high efficiency. Diverse chiral γ-nitro cyclic ketones with two stereocenters were efficiently prepared with excellent stereoselectivity (up to 97 % e.e., >20 : 1 d.r.) and good yield (up to 86 %). This work provides an efficient biocatalytic strategy for cyclic ketone functionalization, and highlights the usefulness of artificial enzymes for extending biocatalysis to further non-natural reactions.

Organocatalytic enantioselective construction of Si-stereocenters: recent advances and perspectives

Silicon-stereogenic chiral organosilanes have found increasing applications in synthetic chemistry, medicinal chemistry, and materials science. In this context, various asymmetric catalytic methods have been established for the diverse synthesis of silicon-stereogenic silanes. In particular, asymmetric organocatalysis is emerging as an important and complementary synthetic tool for the enantioselective construction of silicon-stereocenters, along with the rapid development of chiral-metal catalyzed protocols. Its advent provides a powerful platform to achieve functionalized silicon-stereogenic organosilanes with structural diversity, and should lead to great development in chiral organosilicon chemistry. In this Tutorial Review, we highlight these latest achievements from two aspects: desymmetrizations of prochiral tetraorganosilanes and dynamic kinetic asymmetric transformations of racemic organosilanes by employing five organocatalytic activation modes. The advantages, limitations and synthetic value of each protocol, as well as the synthetic opportunities still open for further exploration, are also discussed.

Synthesis and application of chiral cis-2,5-disubstituted pyrrolidine organocatalysts

The first synthesis of chiral cis-2,5-disubstituted pyrrolidine organocatalysts is described. Their application for the enantioselective Michael addition of nitromethane to α,β-unsaturated aldehydes with excellent yield (up to 91%) and enantioselectivity (up to >99% ee) is demonstrated. Another catalytic application for the reaction of 2,4-hexadienal with nitrostyrene is also showcased.

One-pot chemo- and photo-enzymatic linear cascade processes

The combination of chemo- and photocatalyses with biocatalysis, which couples the flexible reactivity of the photo- and chemocatalysts with the highly selective and environmentally friendly nature of enzymes in one-pot linear cascades, represents a powerful tool in organic synthesis. However, the combination of photo-, chemo- and biocatalysts in one-pot is challenging because the optimal operating conditions of the involved catalyst types may be rather different, and the different stabilities of catalysts and their mutual deactivation are additional problems often encountered in one-pot cascade processes. This review explores a large number of transformations and approaches adopted for combining enzymes and chemo- and photocatalytic processes in a successful way to achieve valuable chemicals and valorisation of biomass. Moreover, the strategies for solving incompatibility issues in chemo-enzymatic reactions are analysed, introducing recent examples of the application of non-conventional solvents, enzyme-metal hybrid catalysts, and spatial compartmentalization strategies to implement chemo-enzymatic cascade processes.

Asymmetric Organocatalytic 1,3-Dipolar Cycloaddition of Azomethine Ylides with β-Substituted Cyclic Enones

The enantioselective and diastereoselective control of 1,3-dipolar cycloaddition reactions to β-substituted cyclic enones has been developed. The 1,3-dipolar cycloaddition of phthalazinium dicyanomethanides with cyclic dienones affords chiral tetrahydropyrrolo[2,1-a]phthalazine derivatives 3 through vinylogous iminium ion activation by combining a cinchona-based primary amine C3 and a chiral camphorsulfonic acid additive. Conversely, with a weaker 3,5-bis(trifluoromethyl)benzoic acid additive, the 1,3-dipolar cycloaddition of phthalazinium dicyanomethanides with β-substituted cyclic enones leads to chiral hexahydroisoindolo[1,2-a]phthalazin-10(8H)-one derivatives 4 with excellent stereocontrol via endo-dienamine activation.

Nonsilyl Bicyclic Secondary Amine Catalysts for the Asymmetric Transfer Hydrogenation of α,β-Unsaturated Aldehydes

The first chiral synthesis of nonsilyl bicyclic secondary amine organocatalysts and their application to the asymmetric transfer hydrogenation of α,β-unsaturated aldehydes are disclosed. A lower catalytic loading (5 mol %) is demonstrated for the reduction of a wide range of α,β-unsaturated aldehydes (up to 97% yield and up to 99% ee). The application of this scalable methodology is showcased for the asymmetric synthesis of bioactive molecules such as phenoxanol, citronellol, ramelteon, and terikalant.

Formation of iminium ions during the processing of metal halide perovskites

Ammonium ions are a key component of many organic-inorganic metal halide materials. We show that the hexagonal perovskite (Me2NH2)PbI3 is rapidly transformed to iminium-based perovskites (Me2CNMeR)PbI3 (R = Me, Et), simply by stirring the material in the respective ketone at room temperature, triggering clear changes in the materials' photophysical properties.

An Artificial Enzyme for Asymmetric Nitrocyclopropanation of α,β-Unsaturated Aldehydes-Design and Evolution

The introduction of an abiological catalytic group into the binding pocket of a protein host allows for the expansion of enzyme chemistries. Here, we report the generation of an artificial enzyme by genetic encoding of a non-canonical amino acid that contains a secondary amine side chain. The non-canonical amino acid and the binding pocket function synergistically to catalyze the asymmetric nitrocyclopropanation of α,β-unsaturated aldehydes by the iminium activation mechanism. The designer enzyme was evolved to an optimal variant that catalyzes the reaction at high conversions with high diastereo- and enantioselectivity. This work demonstrates the application of genetic code expansion in enzyme design and expands the scope of enzyme-catalyzed abiological reactions.

Secondary Amine Catalysis in Enzyme Design: Broadening Protein Template Diversity through Genetic Code Expansion

Secondary amines, due to their reactivity, can transform protein templates into catalytically active entities, accelerating the development of artificial enzymes. However, existing methods, predominantly reliant on modified ligands or N-terminal prolines, impose significant limitations on template selection. In this study, genetic code expansion was used to break this boundary, enabling secondary amines to be incorporated into alternative proteins and positions of choice. Pyrrolysine analogues carrying different secondary amines could be incorporated into superfolder green fluorescent protein (sfGFP), multidrug-binding LmrR and nucleotide-binding dihydrofolate reductase (DHFR). Notably, the analogue containing a D-proline moiety demonstrated both proteolytic stability and catalytic activity, conferring LmrR and DHFR with the desired transfer hydrogenation activity. While the LmrR variants were confined to the biomimetic 1-benzyl-1,4-dihydronicotinamide (BNAH) as the hydride source, the optimal DHFR variant favorably used the pro-R hydride from NADPH for stereoselective reactions (e.r. up to 92 : 8), highlighting that a switch of protein template could broaden the nucleophile option for catalysis. Owing to the cofactor compatibility, the DHFR-based secondary amine catalysis could be integrated into an enzymatic recycling scheme. This established method shows substantial potential in enzyme design, applicable from studies on enzyme evolution to the development of new biocatalysts.

1,5-Hydrogen atom transfer of α-iminyl radical cations: a new platform for relay annulation for pyridine derivatives and axially chiral heterobiaryls

The exploitation of new reactive species and novel transformation modes for their synthetic applications have significantly promoted the development of synthetic organic methodology, drug discovery, and advanced functional materials. α-Iminyl radical cations, a class of distonic ions, exhibit great synthetic potential for the synthesis of valuable molecules. For their generation, radical conjugate addition to α,β-unsaturated iminium ions represents a concise yet highly challenging route, because the in situ generated species are short-lived and highly reactive and they have a high tendency to cause radical elimination (β-scission) to regenerate the more stable iminium ions. Herein, we report a new transformation mode of the α-iminyl radical cation, that is to say, 1,5-hydrogen atom transfer (1,5-HAT). Such a strategy can generate a species bearing multiple reactive sites, which serves as a platform to realize (asymmetric) relay annulations. The present iron/secondary amine synergistic catalysis causes a modular assembly of a broad spectrum of new structurally fused pyridines including axially chiral heterobiaryls, and exhibits good functional group tolerance. A series of mechanistic experiments support the α-iminyl radical cation-induced 1,5-HAT, and the formation of several radical species in the relay annulations. Various synthetic transformations of the reaction products demonstrate the usefulness of this relay annulation protocol for the synthesis of significant molecules.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Metal-free β,γ-C(sp3)-H difunctionalization of propanols: DMP-initiated asymmetric spirocyclopropanation

A DMP-initiated metal-free effective β,γ-asymmetric spirocyclopropanation of propanols strategy using oxidative iminium activation is described. This process has been realized by a synergistic amine-catalyzed one-pot cascade oxidation-Michael addition cyclopropanation for "one-pot" access to various spirocyclopropyl propionaldehydes/propanols from diverse 3-arylpropanols and α-brominated active methylene compounds under mild conditions and with high enantioselectivity (ee up to >99%).

Catalytic Atroposelective Synthesis of Axially Chiral Heterobiaryl Oxime Ethers via the One-Step Dynamic Kinetic Condensation Reaction

The catalytic atroposelective synthesis of axially chiral heterobiaryls was first developed through the direct one-step dynamic kinetic condensation reaction with the simple transformation of the C═O bond to the C═N bond, delivering a series of novel axially chiral heterobiaryl oxime ethers.

Lewis Acid-Mediated Friedel-Crafts-Type Formylation of Alkenes with Dichloromethyl Methyl Ether in the Presence of Pyridines

Various alkenes are formylated with dichloromethyl methyl ether (MOMCl2) by the combined use of SnCl4/2,6-dibromopyridine (B1) or AgOTf/pyridine (B4) via Friedel-Crafts-type reaction. The former reagent combination is mainly applied to α,α-diarylalkenes, while the latter one is applied not only to arylalkenes but also to some alkylalkenes. Vinyl aldehydes are exclusively obtained from alkenes that can possibly afford both allyl and vinyl aldehydes.

Click Organocatalysis: Acceleration of Azide-Alkyne Cycloadditions with Mutually Orthogonal Click Reactions

"Click organocatalysis" uses mutually orthogonal click reactions to organocatalyze a click reaction. We report the development of an isobenzofuran organocatalyst that increases the rate and regioselectivity of an azide-alkyne cycloaddition. The organocatalytic cycle consists of (1) a Diels-Alder reaction of an alkyne with a diarylisobenzofuran to form a benzooxanorbornadiene, (2) a 1,3-dipolar cycloaddition with an azide to form a 4,5-dihydro-1,2,3-triazole, and (3) a retro-Diels-Alder reaction that releases the triazole product and regenerates the diarylisobenzofuran organocatalyst. The diarylisobenzofuran organocatalyst was computationally designed to catalyze the reaction of perfluorophenyl azide and methyl propiolate to selectively form a 1,4-triazole product. Experimental validation of the designed organocatalyst was obtained with methyl 4-azido-2,3,5,6-tetrafluorobenzoate and methyl propiolate.

Proline-catalyzed synthesis of α-substituted (E)-α,β-unsaturated aldehydes from epoxides

A novel, simple and metal-free tandem approach for synthesizing α-substituted (E)-α,β-unsaturated aldehyde derivatives through acid-catalyzed epoxide rearrangement and organocatalyzed aldol condensation processes has been described. This transformation has a broad substrate scope under mild conditions, including epoxides and aldehydes containing diverse functional groups, resulting in moderate to high yields of the desired products. Eventually, large-scale reactions and the synthesis of some bioactive molecules are used to demonstrate the potential applicability of the developed method.

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

Novel asymmetric aminocatalytic cycloadditions are described between formyl cycloheptatrienes and 6,6-dimethylfulvene that lead to [4 + 2], [6 + 2], and [4 + 6] cycloadducts. The unprecedented reaction course is dependent on the position of the formyl functionality in the cycloheptatriene core, and each formyl cycloheptatriene isomer displays a distinct reactivity pattern. The formyl cycloheptatriene isomers are activated by a chiral primary diamine catalyst, and the activation mode is dependent on the position of the formyl functionality relative to the cycloheptatriene core. The [4 + 2] and [6 + 2] cycloadducts are formed via rare iminocatalytic inverse electron-demand cycloadditions, while the [4 + 6] cycloadduct is formed by a normal electron-demand cycloaddition. The reactivity displayed by the different formyl cycloheptatrienes was investigated by DFT calculations. These computational studies account for the different reaction paths for the three isomeric formyl cycloheptatrienes. The aminocatalytic [4 + 2], [6 + 2], and [4 + 6] cycloadditions proceed by stepwise processes, and the interplay between conjugation, substrate distortion, and dispersive interactions between the fulvene and aminocatalyst mainly defines the outcome of each cycloaddition.

Enamine/Iminium-based Dual Organocatalytic Systems for Asymmetric Catalysis and Synthesis

The rational combination of two catalysts to expedite the construction of chiral complex biologically and pharmacologically relevant chiral compounds has widely gained momentum over the past decade. In particular, enamine or iminium catalysis ensuing from the activation of aldehyde or ketone by chiral amine catalysts in conjugation with other organocatalytic cycles has facilitated several asymmetric transformations to yield the enantioenriched products. Regardless of the considerable discussion on the various dual catalytic approaches, literature lacks a comprehensive review focusing on the enamine and iminium-based dual organocatalytic systems. Thus, this review article has discussed the noteworthy achievements in the field of asymmetric catalysis and synthesis catalyzed by the enamine and iminium-based dual organocatalytic systems.

Organocatalytic Enantioselective Intramolecular Michael Addition by In Situ Generated Aminoisobenzofulvenes: Construction of Spiro Quaternary Carbon Stereocenters

An unprecedented enantioselective organocatalytic spirocyclization strategy is presented by in situ generation of aminoisobezofulvenes. The reaction sequence involves a reductive Michael/aldol-condensation/Michael addition cascade by iminium-enamine catalysis. The key success of this spirocyclization was the formation of intermediatory nucleophilic aminoisobenzofuvenes accountable for intramolecular Michael addition. Benzospirononanes featuring an all carbon qauternary spirocenter were obtained using proline-derived amino-organocatalyst in moderate to good yields and excellent diastereo- and enantioselectivities (up to >20 : 1 dr, and 99 % ee). Post-methodological manipulation of benzospirononanes was also demonstrated.

Diiminium Nucleophile Adducts Are Stable and Convenient Strong Lewis Acids

Strong Lewis acids are essential tools for manifold chemical procedures, but their scalable deployment is limited by their costs and safety concerns. We report a scalable, convenient, and inexpensive synthesis of stable diiminium-based reagents with a Lewis acidic carbon centre. Coordination with pyridine donors stabilises these centres; the 2,2'-bipyridine adduct shows a chelation effect at carbon. Due to high fluoride, hydride, and oxide affinities, the diiminium pyridine adducts are promising soft and hard Lewis acids. They effectively produce acylpyridinium salts from carboxylates that can acylate amines to give amides and imides even from electronically intractable coupling partners.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Mechanism to model: a physical organic chemistry approach to reaction prediction

The application of mechanistic generalizations is at the core of chemical reaction development and application. These strategies are rooted in physical organic chemistry where mechanistic understandings can be derived from one reaction and applied to explain another. Over time these techniques have evolved from rationalizing observed outcomes to leading experimental design through reaction prediction. In parallel, significant progression in asymmetric organocatalysis has expanded the reach of chiral transfer to new reactions with increased efficiency. However, the complex and diverse catalyst structures applied in this arena have rendered the generalization of asymmetric catalytic processes to be exceptionally challenging. Recognizing this, a portion of our research has been focused on understanding the transferability of chemical observations between similar reactions and exploiting this phenomenon as a platform for prediction. Through these experiences, we have relied on a working knowledge of reaction mechanism to guide the development and application of our models which have been advanced from simple qualitative rules to large statistical models for quantitative predictions. In this feature article, we describe the models acquired to generalize organocatalytic reaction mechanisms and demonstrate their use as a powerful approach for accelerating enantioselective synthesis.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

1,3-Diamine-Derived Catalysts: Design, Synthesis, and the Use in Enantioselective Mannich Reactions of Ketones

1,3-Diamine-derived catalysts were designed, synthesized, and used in asymmetric Mannich reactions of ketones. The reactions catalyzed by one of the 1,3-diamine derivatives in the presence of acids afforded the Mannich products with high enantioselectivities under mild conditions. In most cases, bond formation occurred at the less-substituted α-position of the ketone carbonyl group. Our results indicate that the primary and the tertiary amines of the 1,3-diamine derivative cooperatively act for the catalysis.

Cyclic Allene Approach to the Manzamine Alkaloid Keramaphidin B

We report an approach to the core of the manzamine alkaloid keramaphidin B that relies on the strain-promoted cycloaddition of an azacyclic allene with a pyrone trapping partner. The cycloaddition is tolerant of nitrile and primary amide functional groups and can be complemented with a subsequent retro-Diels-Alder step. These efforts demonstrate that strained cyclic allenes can be used to build significant structural complexity and should encourage further studies of these fleeting intermediates.

Aromatic C-H bond functionalization through organocatalyzed asymmetric intermolecular aza-Friedel-Crafts reaction: a recent update

The aza-Friedel-Crafts reaction allows an efficient coupling of electron-rich aromatic systems with imines for the facile incorporation of aminoalkyl groups into the aromatic ring. This reaction has a great scope of forming aza-stereocenters which can be tuned by different asymmetric catalysts. This review assembles recent advances in asymmetric aza-Friedel-Crafts reactions mediated by organocatalysts. The mechanistic interpretation with the origin of stereoselectivity is also explained.

Going Full Circle with Organocatalysis and Biocatalysis: The Latent Potential of Cofactor Mimics in Asymmetric Synthesis

Many enzymes work in tandem with small molecule cofactors, which have inspired organocatalyst designs. Chemical modification of cofactor scaffolds has increased organocatalytic reactivity and reaction scope. This synopsis presents a selection of recent advances in the use of cofactors (native and mimics) in organocatalysis and biocatalysis. We aim to highlight the benefits of combining fundamental knowledge gained in both bio- and organo-catalysis for asymmetric biocatalysis.

Synthesis of pyrrolidin-2-ylidenes and pyrrol-2-ylidenes via 1,3-dipolar cycloaddition of H-bond-assisted azomethine ylides to nitrostyrenes

Hydrogen-bond-assisted azomethine ylides, generated from 2-(benzylamino)-2-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)acetonitriles, undergo a formal Huisgen 1,3-dipolar cycloaddition with β-bromo-β-nitrostyrenes to afford a diastereoselective synthesis of highly substituted pyrrolidin-2-ylidene derivatives. When β-nitrostyrenes were used as the alkene component, 2-(4,5-diaryl-1,5-dihydro-2H-pyrrol-2-ylidene)-1H-indene-1,3(2H)-diones were obtained. Efficient conversion of pyrrolidene-2-ylidenes to the corresponding pyrrol-2-ylidenes takes place in refluxing 1-propanol in the presence of excess Et₃N. Also, the structure of the pyrrolidene-2-ylidene derivative was determined by X-ray crystallography.

Synthesis of Tetracyclic C2-Symmetric Ketiminium Salts and Evaluation as Catalysts for Epoxidation

Short synthetic routes to the nonplanar tetracyclic ketiminium salt A and two close analogs are described. These are helical nonplanar structures as shown by X-ray crystallography.

Roughness-Dependent Electro-Reductive Coupling of Nitrobenzenes and Aldehydes on Copper Electrodes

The electro-reductive coupling of nitro and carbonyl compounds enables a facile, environmentally friendly and energy benign transformation toward value-added nitrones or imines, but the selectivity is still challenging. Here, the surface roughness of Cu electrodes is introduced for the first time as the determinant to switch products from nitrones to imines owing to the controllable reduction of nitroarenes to hydroxylamines or amines on tailored Cu^I /Cu⁰ interfaces. The roughness-dependent selectivity, that is the decrease of nitrones and the increase of imines with enhanced roughness, is visible in the electro-reductive coupling of nitrobenzene and furfural. Thus, the high selectivity of nitrone (98 %) and imine (80 %) can be achieved on a surface smooth Cu foil and the one electrochemically roughened in the presence of I^- , respectively. Such roughness-dependence of nitrone/imine selectivity on Cu electrodes is further verified in a wide substrate scope, highlighting the promise of surface/interfacial engineering for electrochemical synthesis.

Harnessing Protonated 2,2'-Bipyridinium Salts as Powerful Brønsted Acid Catalysts in Organic Reactions

It is the first time that the readily available protonated 2,2'-bipyridinium salts are used as Brønsted acid catalysts to accelerate a series of organic transformations that included the hydration of aromatic alkynes, etherification of alcohols, cyclotrimerization of aliphatic aldehydes, Ritter reaction, Mannich reaction, Biginelli reaction, preparation of substituted alkenes from alcohols, synthesis of spirooxindole, bisindolylmethane, and noncyclized tetraketone with good to excellent yields. These results strongly suggest that there exists enormous potentiality in the development of the protonated 2,2'-bipyridinium catalytic system.

Efficient Synthesis of Imine-Carboxylic Acid Functionalized Compounds: Single Crystal, Hirshfeld Surface and Quantum Chemical Exploration

Two aminobenzoic acid based crystalline imines (HMBA and DHBA) were synthesized through a condensation reaction of 4-aminobenzoic acid and substituted benzaldehydes. Single-crystal X-ray diffraction was employed for the determination of structures of prepared Schiff bases. The stability of super molecular structures of both molecules was achieved by intramolecular H-bonding accompanied by strong, as well as comparatively weak, intermolecular attractive forces. The comparative analysis of the non-covalent forces in HMBA and DHBA was performed by Hirshfeld surface analysis and an interaction energy study between the molecular pairs. Along with the synthesis, quantum chemical calculations were also accomplished at M06/6-311G (d, p) functional of density functional theory (DFT). The frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), natural bond orbitals (NBOs), global reactivity parameters (GRPs) and natural population (NPA) analyses were also carried out. The findings of FMOs found that Egap for HMBA was examined to be smaller (3.477 eV) than that of DHBA (3.7933 eV), which indicated a greater charge transference rate in HMBA. Further, the NBO analysis showed the efficient intramolecular charge transfer (ICT), as studied by Hirshfeld surface analysis.

Skeletal Transformation of Oxindoles into Quinolinones Enabled by Synergistic Copper/Iminium Catalysis

We describe a synergistic Cu/secondary amine catalysis for skeletal transformation of an oxindole core into a quinolinone skeleton, which generates several structurally new pyridine-fused quinolinones. The synergistic reactions allow expansion of a five-membered lactam ring by radical cation-triggered C-C bond cleavage and enable a further intramolecular cyclization with the aim to construct totally distinct core skeletons.

On-Demand Release of Secondary Amine Bases for the Activation of Catalysts and Crosslinkers

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

2-[2,6-Diisopropylphenyl]-4-phenyl-5H-5,9b[1′,2′]-benzonaphtho[1,2-b]pyrrol-2-ium Tetrafluoroborate

A novel α,β-unsaturated iminium salt (3) incorporated into a rigid dibenzobarrelene backbone was synthesized by heating N-(anthracen-9-ylmethyl)-2,6-diisopropylaniline (2) and 3-phenyl-2-propynal in THF in the presence of excess amounts of magnesium sulfate and 0.5 equivalents of an HBF4-Et2O complex. The molecular structure of 3 was characterized unambiguously by NMR spectroscopy and single-crystal X-ray diffraction (SCXRD) analyses. Compound 3 exhibits yellow luminescence in CH2Cl2 (λem = 516 nm) and in the solid state (λem = 517 nm) with relatively high to moderate quantum yields (ΦF(CH2Cl2) = 0.63; ΦF(solid) = 0.34).

Iridium-Catalyzed Enantioconvergent Borrowing Hydrogen Annulation of Racemic 1,4-Diols with Amines

We present an enantioconvergent access to chiral N-heterocycles directly from simple racemic diols and primary amines, through a highly economical borrowing hydrogen annulation. The identification of a chiral amine-derived iridacycle catalyst was the key for achieving high efficiency and enantioselectivity in the one-step construction of two C-N bonds. This catalytic method enabled a rapid access to a wide range of diversely substituted enantioenriched pyrrolidines including key precursors to valuable drugs such as aticaprant and MSC 2530818.

A recyclable stereoauxiliary aminocatalyzed strategy for one-pot synthesis of indolizine-2-carbaldehydes

Indolizine-carbaldehydes with the easily modifiable carbaldehyde group are important synthetic targets as versatile precursors for distinct indolizines. However, the efficient one-pot construction of trisubstituted indolizine-2-carbaldehydes represents a long-standing challenge. Herein, we report an unprecedented recyclable stereoauxiliary aminocatalytic approach via aminosugars derived from biomass, which enable the efficient one-pot synthesis of desired trisubstituted indolizine-2-carbaldehydes via [3+2] annulations of acyl pyridines and α,β-unsaturated aldehyde. Compared to the steric shielding effect from α-anomer, a stereoauxiliary effect favored by β-anomer of D-glucosamine is supported by control experiments. Furthermore, polymeric chitosan containing predominantly β-D-anhydroglucosamine units also shows excellent catalytic performance in aqueous solutions for the conversion of various substrates, large-scale synthesis and catalytic cycling experiments. Thus, our approach advances the existing methodologies by providing a rich library of indolizine-2-aldehydes. In addition, it delivers an efficient protocol for a set of late-stage diversification and targeted modifications of bioactive molecules or drugs, as showcased with 1,2,3-trisubstituted indolizine-2-carbaldehydes.

Design and fabrication of superhydrophobic cellulose nanocrystal films by combination of self-assembly and organocatalysis

Cellulose nanocrystals, which have unique properties of high aspect ratio, high surface area, high mechanical strength, and a liquid crystalline nature, constitute a renewable nanomaterial with great potential for several uses (e.g., composites, films and barriers). However, their intrinsic hydrophilicity results in materials that are moisture sensitive and exhibit poor water stability. This limits their use and competitiveness as a sustainable alternative against fossil-based materials/plastics in packaging, food storage, construction and materials application, which cause contamination in our oceans and environment. To make cellulose nanocrystal films superhydrophobic, toxic chemicals such as fluorocarbons are typically attached to their surfaces. Hence, there is a pressing need for environmentally friendly alternatives for their modification and acquiring this important surface property. Herein, we describe the novel creation of superhydrophobic, fluorocarbon-free and transparent cellulose nanocrystal films with functional groups by a bioinspired combination of self-assembly and organocatalytic surface modification at the nanoscale using food approved organic acid catalysts. The resulting film-surface is superhydrophobic (water contact angle > 150°) and has self-cleaning properties (the lotus effect). In addition, the superhydrophobic cellulose nanocrystal films have excellent water stability and significantly decreased oxygen permeability at high relative humidity with oxygen transmission rates better than those of commonly used plastics.

Highly Enantioselective Construction of Multifunctional Silicon-Stereogenic Silacycles by Asymmetric Enamine Catalysis

We report the first highly enantioselective construction of silicon-stereocenters by asymmetric enamine catalysis. An unprecedented desymmetric intramolecular aldolization of prochiral siladials was thus developed for the facile access of multifunctional silicon-stereogenic silacycles in high to excellent enantioselectivity. With an enal moiety, these adducts could be readily elaborated for the diverse synthesis of silicon-stereogenic compounds, and for late-stage modification.

Mechanism of a Dually Catalyzed Enantioselective Oxa-Pictet-Spengler Reaction and the Development of a Stereodivergent Variant

Enantioselective oxa-Pictet-Spengler reactions of tryptophol with aldehydes proceed under weakly acidic conditions utilizing a combination of two catalysts, an indoline HCl salt and a bisthiourea compound. Mechanistic investigations revealed the roles of both catalysts and confirmed the involvement of oxocarbenium ion intermediates, ruling out alternative scenarios. A stereochemical model was derived from density functional theory calculations, which provided the basis for the development of a highly enantioselective stereodivergent variant with racemic tryptophol derivatives.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

Asymmetric Organocatalysis Under Mechanochemical Conditions

Asymmetric organocatalysis is a robust methodology providing access to numerous valuable compounds while having green chemistry principles in mind. The realization of organocatalytic transformation under solvent-free mechanochemical conditions brings additional benefits in terms of yields, selectivities, and, last but not least overall improved sustainability. This overview describes developments in the use of mechanochemistry as a vehicle for asymmetric organocatalytic transformations. The material is organized according to main catalytic activation modes, starting with covalent activation and proceeding to non-covalent activation modes. The advantages of mechanochemical organocatalytic reactions are particularly highlighted, but in some cases also, limitations are mentioned. Possibilities for target compound synthesis are also discussed.

Copper-catalyzed multicomponent assembly of γ-butenolides via the interception of carbonyl ylides with iminium ions

A Cu(I)-catalyzed three-component reaction of cyclopropenes, enamines and aldehydes has been realized. This reaction proceeds via the interception of carbonyl oxonium ylide intermediates with α, β-unsaturated iminium ions that are in situ generated from enamines and aldehydes under the catalysis of Cu(MeCN)₄PF₆, leading to the desired γ-butenolide derivatives in good yields and with moderate diastereoselectivities. Access to these derivatives with tethered ketone and alkynal groups will expand the structural diversity of multi-substituted butenolides.

Construction of Covalent Organic Frameworks via Multicomponent Reactions

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Streptocyanine as an activation mode of amine catalysis for the conversion of pyridine rings to benzene rings

Amine catalysts have emerged as an invaluable tool in organic synthesis. Iminium, enamine, and enamine radical cation species are representative activation modes of amine catalysis. However, the development of new amine catalysis activation modes that enable novel synthetic strategies remains highly desirable. Herein, we report streptocyanine as a new amine catalysis activation mode, which enables the skeletal editing of pyridine rings to benzene rings. N-Arylation of pyridines bearing an alkenyl substituent at the 3-position generates the corresponding N-arylpyridiniums. The resulting pyridinum reacts with a catalytic amount of piperidine to afford a streptocyanine intermediate. Catalytically generated streptocyanine forms a benzene ring via a ring-closing reaction, thereby releasing the amine catalyst. Consequently, the alkene moiety in the starting pyridines is incorporated into the benzene ring of the products. Pyridiniums bearing various alkene moieties were efficiently converted to formyl-substituted benzene derivatives. Mechanistic studies support the postulation that the present catalytic process was intermediated by streptocyanine. In this reaction system, streptocyanine could be regarded as a new activation mode of amine catalysis.