Efficient Asymmetric α-Oxyamination of Aldehydes by Resin-Supported Peptide Catalyst in Aqueous Media

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1-Oxyl to Promote Desulfonylative α-Oxyamination Reactions of α-Sulfonylketones

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI⁺-ArO^-), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI⁺-ArO^- and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations.

Synergistic Peptide and Gold Catalysis: Enantioselective Addition of Branched Aldehydes to Allenamides

The combination of a peptide catalyst and a gold catalyst is presented for enantioselective addition reactions between branched aldehydes and allenamides. The two catalysts act in concert to provide γ,δ-enamide aldehydes bearing a fully substituted, benzylic stereogenic center - a structural motif common in many natural products and therapeutically active compounds - with good yields and enantioselectivities. The reaction tolerates a variety of alkyl and alkoxy substituted aldehydes and the products can be elaborated into several chiral building blocks bearing either 1,4- or 1,5- functional group relationships. Mechanistic studies showed that the conformational features of the peptide are important for both the catalytic efficiency and stereochemistry, while a balance of acid/base additives is key for ensuring formation of the desired product over undesired side reactions.

Synthesis of aliphatic α-ketoamides from α-substituted methyl ketones via a Cu-catalyzed aerobic oxidative amidation

α-Ketoamides are an important key functional group and have been used as versatile and valuable intermediates and synthons in a variety of functional group transformations. Synthetic methods for making aryl α-ketoamides as drug candidates have been greatly improved through metal-catalyzed aerobic oxidative amidations. However, the preparation of alkyl α-ketoamides through metal-catalyzed aerobic oxidative amidations has not been reported because generating α-ketoamides from aliphatic ketones with two α-carbons theoretically provides two distinct α-ketoamides. Our strategy is to activate the α-carbon by introducing an N-substituent at one of the two α-positions. The key to this strategy is how heterocyclic compounds such as triazoles and imidazoles affect the selectivity of the synthesis of the alkyl α-ketoamides. From this basic concept, and by optimizing the reaction and elucidating the mechanism of the synthesis of aryl α-ketoamides via a copper-catalyzed aerobic oxidative amidation, we prepared fourteen aliphatic α-ketoamides in high yields (48-84%).

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

N-Primary-amine tetrapeptide-catalyzed highly asymmetric Michael addition of aliphatic aldehydes to maleimides

The highly asymmetric Michael addition reaction between maleimides and aliphatic aldehydes catalyzed by low-loading β-turn tetrapeptides with excellent yields and enantioselectivities at room temperature was reported. α-Branched and α-unbranched aldehydes both are suitable nucleophiles. N-Aryl, alkyl and hydrogen maleimides all are well tolerated and led to high yields and enantioselectivities. The transformation can be enlarged to the gram scale without decrease in the yield and enantioselectivity. Furthermore, the succinimides were converted into γ-lactams and γ-lactones, showing good practicality of this work. Some reaction intermediates in the proposed reaction mechanism can be captured with the HR-MS method.

Peptide-Catalyzed Highly Asymmetric Cross-Aldol Reaction of Aldehydes to Biomimetically Synthesize 1,4-Dicarbonyls

β-Turn tetrapeptides were demonstrated to catalyze asymmetric aldol reaction of α-branched aldehydes and α-carbonyl aldehydes, i.e. glyoxylates and α-ketoaldehydes, to biomimetically synthesize acyclic all-carbon quaternary center-bearing 1,4-dicarbonyls in high yield and excellent enantioselectivity under mild conditions. The spatially restricted environment of the tetrapeptide warrants high enantioselectivity and yield with broad substrates. Using this protocol, (R)-pantolactone, the key intermediate of vitamin B5, was readily accessed in a practical, efficient, and environmentally benign process from inexpensive starting materials.

Organocatalysis and Beyond: Activating Reactions with Two Catalytic Species

Since the beginning of the millennium, organocatalysis has been gaining a predominant role in asymmetric synthesis and it is, nowadays, a foundation of catalysis. Synergistic catalysis, combining two or more different catalytic cycles acting in concert, exploits the vast knowledge acquired in organocatalysis and other fields to perform reactions that would be otherwise impossible. Merging organocatalysis with photo-, metallo- and organocatalysis itself, researchers have ingeniously devised a range of activations. This feature review, focusing on selected synergistic catalytic approaches, aims to provide a flavor of the creativity and innovation in the area, showing ground-breaking examples of organocatalysts, such as proline derivatives, hydrogen bond-mediated, Cinchona alkaloids or phosphoric acids catalysts, which work cooperatively with different catalytic partners.

Differentiation between enamines and tautomerizable imines in the oxidation reaction with TEMPO

Enamine and imine represent two of the most common reaction intermediates in syntheses, and the imine intermediates containing α-hydrogen often exhibit the similar reactivity to enamines due to their rapid tautomerization to enamine tautomers. Herein, we report that the minor structural difference between the enamine and the enamine tautomer derived from imine tautomerization results in the different chemo- and regioselectivity in the reaction of cyclohexanones, amines and TEMPO: the reaction of primary amines furnishes the formal oxygen 1,2-migration product, α-amino-enones, while the reaction of secondary amines under similar conditions generates exclusively arylamines via consecutive dehydrogenation on the cyclohexyl rings. The ¹⁸O-labeling experiment for α-amino-enone formation revealed that TEMPO served as oxygen transfer reagent. Experimental and computational studies of reaction mechanisms revealed that the difference in chemo- and regioselectivity could be ascribed to the flexible imine-enamine tautomerization of the imine intermediate containing an α-hydrogen.

Tautomerization of imines into enamines is the basis of their similar reactivity; however, minor structural changes may lead to different outcomes. Here, the authors show that the reaction of cyclohexanone and amines in presence of TEMPO affords either α-amino-enones or arylamines depending on the intermediate imine structure.

Asymmetric one-pot reactions using heterogeneous chemical catalysis: recent steps towards sustainable processes

Asymmetric one-pot reactions applying heterogeneous chemical catalysts and unifying the benefits of these catalytic materials with the advantages of one-pot methods, are surveyed.

Development of Selective Peptide Catalysts with Secondary Structural Frameworks

Enzymes are biogenic catalysts that enable the vital activity of organisms. Enzymes promote reactions in a selective manner with a high level of substrate recognition ability. The development of such a sophisticated catalyst has been one of the goals for chemists. A synthetic peptide is the prime candidate to realize an enzyme-like catalyst. Considering that the catalytic function of enzymes derives from their molecular structures, the key for the creation of a peptide catalyst might be the introduction of a specific three-dimensional structure. Our motivation was to find a peptide catalyst with a versatile secondary structural framework and apply the peptide to a variety of selective reactions. Although helical-peptide-catalyzed asymmetric epoxidation of enones is popular, no other highly enantioselective reaction with a helical peptide has been reported. It was found that resin-supported α-helical polyleucine promoted asymmetric conjugate addition of a carbon nucleophile to enones via the formation of an iminium intermediate at the N-terminal amino group. By changing the helical chain to a repetitive Leu-Leu-Aib (Aib = α-aminoisobutyric acid) sequence and introducing a few amino acids to the N-terminus, a highly enantioselective peptide catalyst was obtained. The helical peptide catalyst was applicable for a tandem enamine/iminium-mediated reaction and asymmetric epoxidation of enones. Although the extension of the helical peptide to conjugate addition of a nucleophile to an enal was not successful simply by attaching proline to the N-terminus of the helix, the incorporation of a β-turn motif was effective to improve the catalytic performance. In the sequence of such a turn-helix-type peptide, the helical part was seemingly distant from the N-terminal amino group; however, the hydrophobicity, structure, and chirality of the helix largely affected the reaction. The turn-helix-type peptide promoted a wide range of asymmetric reactions: conjugated additions of hydride and carbon nucleophiles to enals via the iminium activation and α-oxyamination of aldehydes via the enamine activation. The peptides with turn-helix and helix frameworks were also employed for several reactions that were difficult to achieve with low-molecular-weight catalysts: enzyme-cocatalyzed asymmetric oxidation in water, diastereo- and enantioselective cyclopropanation, regioselective reduction of dienals, kinetic resolution of planar-chiral compounds, and desymmetrization to induce planar chirality. To explore other types of peptide catalysts, a combinatorial library screening was performed. On the way, it was revealed that a histidyl residue assisted to accelerate a reaction via reversible addition to an iminium intermediate. Through the screening of random peptide libraries, novel peptide sequences for efficient and enantioselective conjugate addition were discovered. Although we have no information about the molecular structure of the newly found peptides, they can be an entry point for establishing a versatile framework of peptide catalysts.

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.

The dicarbonylation of indoles via Friedel–Crafts reaction with dicarbonyl nitrile generated in situ and retro-cyanohydrination

The reaction of indole and β-carbonyl nitrile to generate dicarbonyl indoles has been developed. This process involves α-oxonation of the β-carbonyl nitrile, Friedel–Crafts reaction with indoles and retro-cyanohydrination form dicarbonyl indoles.

Friedel–Crafts reaction of indoles with vicinal tricarbonyl compounds generated in situ from 1,3-dicarbonyl compounds and TEMPO: highly selective synthesis of tertiary alcohols

A novel Friedel–Crafts reaction of indoles with vicinal tricarbonyl compounds generated in situ has been developed, forming indolyl tertiary alcohols. The process involves disproportionation of TEMPO, α-oxyamination, N–O bond cleavage and, finally, addition of the indole.

Cerium(IV) ammonium nitrate mediated three-component α-allylation of imine surrogates

A general and practical CAN-mediated oxidative radical α-coupling reaction of various imine surrogates with allylsilanes has been described. This multicomponent process affords β-allylated α-carbamido ethers as stable imine precursors in respectable yields under mild conditions.

Polymer-immobilized chiral catalysts

This review illustrates the current strategies and potential of polymer-immobilized chiral catalysts for highly enantioselective asymmetric synthesis.

Asymmetric α-oxyamination of aldehydes by synergistic catalysis of imidazolethiones and metal salts

The novel and efficient imidazolethione catalysts combined with metal salts were successfully introduced to the asymmetric α-oxyamination of aldehydes.

Asymmetric one-pot sequential Friedel-Crafts-type alkylation and α-oxyamination catalyzed by a peptide and an enzyme

In the presence of a peptide catalyst and the oxidative enzyme laccase, a one-pot sequential reaction including a Friedel–Crafts-type alkylation of α,β-unsaturated aldehydes followed by an α-oxyamination was realized. The reaction in aqueous solvent to promote the enzymatic oxidation, and the use of a peptide catalyst compatible with such conditions, were essential. The present sequential reaction afforded oxygen-functionalized indole or pyrrole derivatives in a highly enantioselective manner.

α-Aminoxylation of ketones and β-chloro-α-aminoxylation of enones with TEMPO and chlorocatecholborane

Oxidation of various cyclic and acyclic ketones under mild conditions with chlorocatecholborane, a bulky pyridine base, and TEMPO to the corresponding α-aminoxylated products in good to excellent yields (52-99%) is described. For enones as substrates, products of a β-chloro-α-aminoxylation are obtained (70-89%).

N719 dye-sensitized organophotocatalysis: enantioselective tandem Michael addition/oxyamination of aldehydes

A remarkably efficient photosensitizer, N719 dye, was used in asymmetric tandem Michael addition/oxyamination of aldehydes, rendering α,β-substituted aldehydes in good yields with excellent levels of enantioselectivity and diastereoselectivity. This is the first report of a multiorganocatalytic reaction involving iminium catalysis and photoinduced singly occupied molecular orbital (SOMO) catalysis. This reaction is expected to expand the scope of tandem organocatalytic reactions.

Asymmetric catalysis with peptides

Over the past decade several peptides have been developed as effective asymmetric catalysts for a range of synthetically useful reactions. Many have properties that are difficult to achieve with other catalysts. The article highlights features that render peptidic organocatalysts unique and attractive for future applications. Challenges such as the design of peptidic catalysts are discussed.

Nitroxides: applications in synthesis and in polymer chemistry

This Review describes the application of nitroxides to synthesis and polymer chemistry. The synthesis and physical properties of nitroxides are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis. The oxidation of alcohols and carbanions, as well as oxidative C-C bond-forming reactions are presented along with other typical oxidative transformations. A section is also dedicated to the extensive use of nitroxides as trapping reagents for C-centered radicals in radical chemistry. Alkoxyamines derived from nitroxides are shown to be highly useful precursors of C-centered radicals in synthesis and also in polymer chemistry. The last section discusses the basics of nitroxide-mediated radical polymerization (NMP) and also highlights new developments in the synthesis of complex polymer architectures.