Recent Development and Application of Chiral Phase-Transfer Catalysts

N-Heterocyclic Carbene-Catalyzed Asymmetric SN2 Alkylation via Noncovalent Activation

The field of asymmetric catalysis has been developed by exploring noncovalent interactions, particularly within N-heterocyclic carbene-mediated processes. Despite challenges due to the limited number of compatible electrophiles (predominantly π-acceptors), this study introduces the first asymmetric α-alkylation of 3-aryl oxindoles using Csp3 electrophiles. The innovative protocol integrates diverse oxindoles and alkyl, allyl, and propargyl electrophiles, achieving high yields and enantioselectivities. Preliminary mechanistic explorations support a noncovalent catalytic mechanism, enhancing the tool kit for constructing complex chiral molecules with potential applications.

Structural Chemistry of C-N Axially Chiral Compounds

In the last several years, atropisomers owing to the rotational restriction around a C-N single bond (C-N axially chiral compounds) have attracted significant attention in the field of synthetic organic chemistry. In particular, the highly enantioselective synthesis of various C-N axially chiral compounds and their application to asymmetric reactions have been reported by many groups. On the other hand, studies on the structural chemistry of C-N axially chiral compounds have attracted scant attention in comparison with synthetic studies. For over 25 years, our group has explored asymmetric synthesis of C-N axially chiral compounds and their synthetic application. In the course of these synthetic studies, we found several notable structural properties in relation to the C-N bond rotation and an association of enantiomers (the relationship between the rotational stability and the structure or electronic effect, the chirality-dependent halogen bond, and the self-disproportionation of enantiomers). Furthermore, on the basis of these structural properties, the development of acid-mediated molecular rotors and the synthesis of isotopic atropisomers possessing high stereochemical purity and rotational stability were achieved. Through this Perspective, I wish to make the chemistry community aware that C-N axially chiral compounds are attractive molecules from the viewpoints of both synthetic organic chemistry and structural chemistry.

A Type of Chiral C2-Symmetric Arylthiol Catalyst for Highly Enantioselective Anti-Markovnikov Hydroamination

The development of chiral hydrogen donor catalysts is fundamental in the expansion and innovation of asymmetric organocatalyzed reactions via an enantioselective hydrogen atom transfer (HAT) process. Herein, an unprecedented type of chiral C2-symmetric arylthiol catalysts derived from readily available enantiomeric lactate ester was developed. With these catalysts, an asymmetric anti-Markovnikov alkene hydroamination-cyclization reaction was established, affording a variety of pharmaceutically interesting 3-substituted piperidines with moderate to high enantioselectivity. Results of the designed control experiments and theoretical computation rationalized the origin of stereocontrol and disclosed the spatial effect of the moiety of chiral thiols on the enantioselectivity. We believed the facile synthesis, flexible tunability, and effective enantioselectivity-controlling capability of these catalysts would shed light on the development of versatile chiral HAT catalysts and related asymmetric reactions.

Enantioselective Total Synthesis of the Neurotoxin Caramboxin

Herein, we report the first and efficient asymmetric total synthesis of the neurotoxin (-)-caramboxin. The key to success is the creation of a stereogenic center by using enantioselective catalytic phase-transfer α-alkylation of glycine imines, affording this unusual α-amino acid in good yields and up to 99% ee. This work validates the S configuration of the natural product.

Stable 2-Azaallyl Salts as a Bridge between 2H-Azirines and Densely Functionalized 2H-Pyrroles

An efficient protocol for the synthesis of stable 2-azaallyl anion salts by the reaction of alkyl 2-bromo-2H-azirine-2-carboxylates with trimethylsilyl cyanide/Bu4NF has been developed. The domino reaction proceeds in four steps via the cleavage of the azirine C-C bond to provide the tetrabutylammonium salts of stereochemically pure 2-azaallyl anions having U-configuration relative to the cyano groups. The anions with an ortho-substituted aryl group or styryl group exist as a mixture of two geometrical isomers across the N2-C3 bond. 2-Azaallyl anion salts have been shown to be convenient substrates for the one-pot synthesis of densely functionalized 2H-pyrroles by the alkylation-cyclization sequence.

Water-Tolerant Superbase Polyoxometalate [H2(Nb6O19)]6- for Homogeneous Catalysis

Here, doubly protonated Lindqvist-type niobium oxide cluster [H2(Nb6O19)]6-, fabricated by microwave-assisted hydrothermal synthesis, exhibited superbase catalysis for Knoevenagel and crossed aldol condensation reactions accompanied by activating C-H bond with pKa >26 and proton abstraction from a base indicator with pKa=26.5. Surprisingly, [H2(Nb6O19)]6- exhibited water-tolerant superbase properties for Knoevenagel and crossed aldol condensation reactions in the presence of water, although it is well known that the strong basicity of metal oxides and organic superbase is typically lost by the adsorption of water. Density functional theory calculation revealed that the basic surface oxygens that share the corner of NbO6 units in [H2(Nb6O19)]8- maintained the negative charges even after proton adsorption. This proton capacity and the presence of un-protonated basic sites led to the water tolerance of the superbase catalysis.

Application of sSPhos as a Chiral Ligand for Palladium-Catalyzed Asymmetric Allylic Alkylation

Palladium-catalyzed asymmetric allylic alkylation is a versatile method for C-C bond formation. Many established classes of chiral ligands can perform allylic alkylation reactions enantioselectively, but identification of new ligand classes remains important for future development of the field. We demonstrate that enantiopure sSPhos, a bifunctional chiral monophosphine ligand, when used as its tetrabutyl ammonium salt, is a highly effective ligand for a benchmark Pd-catalyzed allylic alkylation reaction. We explore the scope and limitations and perform experiments to probe the origin of selectivity. In contrast with reactions previously explored using enantiopure sSPhos, it appears that steric bulk around the sulfonate group is responsible for the high enantioselectivity in this case, rather than attractive noncovalent interactions.

Tertiary Amides as Directing Groups for Enantioselective C-H Amination using Ion-Paired Rhodium Complexes

Enantioselective C-H amination at a benzylic methylene is a vital disconnection towards chiral benzylamines. Here we disclose that butyric and valeric acid-derived tertiary amides can undergo highly enantioselective benzylic amination using an achiral anionic Rh complex that is ion-paired with a Cinchona alkaloid-derived chiral cation. A broad scope of compounds can be aminated encompassing numerous arene substitutions, amides, and two different chain lengths. Excellent tolerance of ortho substituents was observed, which has not been achieved before in asymmetric intermolecular C-H amination with Rh. We speculate that the tertiary amide group of the substrate engages in hydrogen bonding interactions directly with the chiral cation, enabling a high level of organisation at the transition state for C-H amination. This is in contrast with our previous work where a substrate bearing a hydrogen bond donor was required. Control experiments led to the discovery that methyl ethers also function as proficient directing groups under the optimised conditions, potentially also acting as hydrogen bond acceptors. This finding has the promise to dramatically expand the applicability of our ion-paired chiral catalysts.

Chiral Aldehyde Catalysis-Enabled Asymmetric α-Functionalization of Activated Primary Amines

ConspectusThe development of catalytic activation modes provides a reliable and effective platform for designing new enantioselective reactions and preparing chiral molecules with diverse structures. Chiral aldehyde catalysis is an attractive concept in asymmetric catalysis, which utilizes a chiral aldehyde catalyst to promote the asymmetric hydroamination of allylic amines, the asymmetric α-functionalization of primary amines, or the asymmetric transamination of α-keto esters. Typically, the chiral aldehyde-catalyzed asymmetric α-functionalization of primary amines provides an efficient and straightforward method for the synthesis of α-functionalized chiral amines, which does not require any additional protection or deprotection manipulations of the amine group. However, achieving catalytic stereoselective transformations with high efficiency and enantioselectivity by this strategy has remained an intractable challenge.This Account summarizes our endeavors in the development and application of chiral aldehyde catalysis. Using a chiral aldehyde as a catalyst, we reported the catalytic asymmetric α-C alkylation of 2-aminomalonate with 3-indolylmethanol in 2014, which represents the first chiral aldehyde-catalyzed asymmetric α-functionalization of an activated primary amine. Subsequently, several axially chiral aldehyde catalysts were continuously prepared by using chiral BINOL as the starting material, and their applications in asymmetric synthesis were explored. On the one hand, they were used as organocatalysts to realize the various transformations of α-amino acid esters, such as asymmetric 1,4-addition toward conjugated enones/α,β-unsaturated diesters and cyclic 1-azadienes as well as asymmetric α-arylation/allylation and benzylation with corresponding halohydrocarbons. Notably, taking advantage of the difference in the distribution of catalytic sites between two chiral aldehyde catalysts, we disclosed chiral aldehyde-catalyzed diastereodivergent 1,6-conjugated addition and Mannich reactions. On the other hand, the potential for the cooperative catalysis of a chiral aldehyde with a transition metal has also been demonstrated. Enabled by the combination of a chiral aldehyde, a palladium complex, and a Lewis acid, the enantioselective α-allylation of amino acid esters with allyl alcohol esters was established. Moreover, the ternary catalytic system has been successfully used for the α-functionalization of amino acid esters with 1,3-dienes, allenes, allenylic alcohol esters, 1,3-disubstituted allyl alcohol esters, and arylmethanol esters as well as the asymmetric cascade Heck-alkylation reaction. The combination of a chiral aldehyde and nickel complex allows for the asymmetric α-propargylation of amino acid esters with propargylic alcohol esters and provides excellent enantioselectivities. These transformations provide a large library of optically active amines and amino acids. With those chiral amino acid esters as key building blocks, the synthesis or formal synthesis of multiple natural products and biologically significant unnatural molecules was accomplished. This includes the stereodivergent synthesis of natural pyrrolizidine alkaloid NP25302 and the formal synthesis of natural product (S)-hypoestestatin 1 and manzacidin C, clinical candidate compound (+)-AG-041R, and somatostatin mimetics. It is fully anticipated that chiral aldehyde catalysis will soon witness rapid expansion both in the development of novel asymmetric transformations and in innovative applications for constructing optically active nitrogen-containing molecules with significant values.

α-Methylbenzylamine Functionalized Crown-Ether-Appended Calix[4]arene Phase Transfer Catalyst for Enantioselective Henry Reaction

In this letter, we designed a highly selective α-methylbenzylamine functionalized crown-ether-appended calix[4]arene derived phase transfer catalyst for asymmetric nitroaldol reaction to provide the desired nitroaldol adducts in high yields (up to 99 % yield) with good to excellent enantioselectivities (up to 99.8 % ee).

The Development of Aldehyde Catalytic System

Aldehyde catalysts have proven to be highly effective in facilitating and accelerating a wide range of challenging transformations in organic chemistry. This article is structured into three main sections, focusing on the utilization of aldehydes as organocatalysts, the aldehydes/transition metals catalytic systems, and photochemical initiators. Finally, we provide a concise summary of the advancements in this fascinating research field, offering our perspectives and insights.

Interrogating the Crucial Interactions at Play in the Chiral Cation-Directed Enantioselective Borylation of Arenes

Rendering a common ligand scaffold anionic and then pairing it with a chiral cation represents an alternative strategy for developing enantioselective versions of challenging transformations, as has been recently demonstrated in the enantioselective borylation of arenes using a quinine-derived chiral cation. A significant barrier to the further generalization of this approach is the lack of understanding of the specific interactions involved between the cation, ligand, and substrate, given the complexity of the system. We have embarked on a detailed computational study probing the mechanism, the key noncovalent interactions involved, and potential origin of selectivity for the desymmetrizing borylation of two distinct classes of substrate. We describe a deconstructive, stepwise approach to tackling this complex challenge, which involves building up a detailed understanding of the pairwise components of the nominally three component system before combining together into the full 263-atom reactive complex. This approach has revealed substantial differences in the noncovalent interactions occurring at the stereodetermining transition state for C-H oxidative addition to iridium for the two substrate classes. Each substrate engages in a unique mixture of diverse interactions, a testament to the rich and privileged structure of the cinchona alkaloid-derived chiral cations. Throughout the study, experimental support is provided, and this culminates in the discovery that prochiral phosphine oxide substrates, lacking hydrogen bond donor functionality, can also give very encouraging levels of enantioselectivity, potentially through direct interactions with the chiral cation. We envisage that the findings in this study will spur further developments in using chiral cations as controllers in asymmetric transition-metal catalysis.

Impact of Catalyst Deuteration on the Reactivity of Chiral Phase-Transfer Organocatalysts

Site-specifically deuterated organocatalysts were prepared and found to show improved reactivity over the non-deuterated analogs. Two privileged C2 -symmetric chiral binaphthyl-modified tetraalkylammonium salts were selected for this study. The stability of these phase-transfer catalysts was generally improved by site-specific deuteration, though the degree of improvement was structure dependent. In particular, a large secondary kinetic isotope effect was observed for the tetradeuterated phase-transfer catalyst. The performance of these deuterated catalysts in the asymmetric catalytic alkylation of amino acid derivatives was better than that of non-deuterated analogs at low catalyst loadings. The results suggest that catalyst deuteration is a promising strategy for enhancing the stability and performance of organocatalysts.

Desymmetrization of Cyclohexadienones through Phase-Transfer-Catalyzed Stereoselective Intramolecular Aza-Michael Addition with Chiral Sulfinamide Nucleophiles

This paper reports the desymmetrization of cyclohexadienones through stereoselective intramolecular aza-Michael addition with a tethered chiral sulfinamide nucleophile. The reaction was facilitated by phase-transfer catalysis and produced various nitrogen-containing bicyclic compounds with a yield of up to 93% and a diastereomeric ratio of up to >20:1.

Enantioselective Transformations in the Synthesis of Therapeutic Agents

The proportion of approved chiral drugs and drug candidates under medical studies has surged dramatically over the past two decades. As a consequence, the efficient synthesis of enantiopure pharmaceuticals or their synthetic intermediates poses a profound challenge to medicinal and process chemists. The significant advancement in asymmetric catalysis has provided an effective and reliable solution to this challenge. The successful application of transition metal catalysis, organocatalysis, and biocatalysis to the medicinal and pharmaceutical industries has promoted drug discovery by efficient and precise preparation of enantio-enriched therapeutic agents, and facilitated the industrial production of active pharmaceutical ingredient in an economic and environmentally friendly fashion. The present review summarizes the most recent applications (2008-2022) of asymmetric catalysis in the pharmaceutical industry ranging from process scales to pilot and industrial levels. It also showcases the latest achievements and trends in the asymmetric synthesis of therapeutic agents with state of the art technologies of asymmetric catalysis.

Formal [3 + 2] Cycloaddition of α-Imino Esters with Azo Compounds: Facile Construction of Pentasubstituted 1,2,4-Triazoline Skeletons

1,2,4-Triazole and 1,2,4-triazoline are important components of bioactive molecules and catalysts employed in organic synthesis. Therefore, the efficient synthesis of these components has received significant research attention. However, studies on their structural diversity remain lacking. Previously, we developed chiral phase-transfer-catalyzed asymmetric reactions of α-imino carbonyl compounds with α,β-unsaturated carbonyl compounds and haloalkanes. In this study, we demonstrate the formal [3 + 2] cycloaddition reaction of α-imino esters with azo compounds under Brønsted base catalysis, resulting in the corresponding 1,2,4-triazolines in high yields. The results revealed that a wide range of substrates and reactants can be applied, irrespective of their steric and electronic characteristics. The present reaction made the general preparation of 3-aryl pentasubstituted 1,2,4-triazolines possible for the first time. Furthermore, a mechanistic study suggested that the reaction proceeds without isomerization into the aldimine form.

Copper(I)-catalyzed asymmetric alkylation of α-imino-esters

Asymmetric alkylation of enolates is one of the most direct and important reactions to prepare α-chiral carbonyl compounds. Except for the classical methods that rely on the use of chiral auxiliaries, asymmetric catalysis emerged as a powerful tool, especially asymmetric phase-transfer catalysis. However, in the field of transition metal catalysis, only limited success with asymmetric alkylation of enolates was achieved. Hereby, we disclose a copper(I)-catalyzed asymmetric alkylation of α-imino-esters with various alkyl halides, including allyl bromides, propargyl bromide, benzyl bromides, α-bromo carbonyl compounds, and alkyl iodides. Both linear and cyclic α-imino-esters serve as competent pronucleophiles in the alkylation, which affords α-amino acid derivatives bearing either a trisubstituted or a tetrasubstituted stereogenic carbon center in high to excellent enantioselectivity. Control experiments indicate that the α-imino-ester is activated by a chiral copper(I)-phosphine complex through coordination, thus enabling facile deprotonation to provide a stabilized copper(I)-enolate in the presence of a mild base. Finally, the mildly basic nature allows the asymmetric alkylation of chiral dipeptides with excellent both chemo- and enantioselectivities.

Stereoselective Assembly of Hydrogen-Bonded Anionic Cages Dictated by Organophosphate-Based Chiral Nodes

Inspired by the signal transduction function of organophosphates in biological systems, bioactive organophosphates were utilized for the first time as chiral nodes to dictate the stereoselective assembly of hydrogen-bonded anionic cages. Phosphonomycin (antibiotics), tenofovir (antivirals), adenosine monophosphate (natural product, AMP) and clindamycin phosphate (antibiotics) were assembled with an achiral bis-monourea ligand, thereby leading to the stereoselective formation of quadruple or triple helicates. The extent of the stereoselectivity could be enhanced by either lowering the temperature or adding stronger-binding cations as templates. With the chiral anionic cages as the host, some enantioselectivity was achieved when binding chiral quaternary ammonium cations.

Regio- and stereoselective syntheses of chiral α-quaternary (Z)-trisubstituted allylic amino acids via synergistic Pd/Cu catalysis

Synergistic palladium/copper catalysis for asymmetric allylic alkylation of vinylethylene carbonates with aldimine esters has been developed for the synthesis of α-quaternary (Z)-trisubstituted allylic amino acids under mild conditions. This methodology features broad substrate compatibilities in yields of up to 87% and up to 94% ee. A facile scale-up and straightforward conversion to 1,2,3,5-tetrasubstituted pyrrole and 1,2,5,6-tetrahydropyridine bearing chiral quaternary carbon centers verifies the synthetic utility of this method.

Recent Asymmetric Phase-Transfer Catalysis with Chiral Binaphthyl-Modified and Related Phase-Transfer Catalysts over the Last 10 Years

In this personal account, we describe our recent advances in the three types of phase-transfer catalysis for various transformations including asymmetric induction: Firstly, asymmetric phase-transfer catalysis with Maruoka-type C₂ -symmetric chiral biaryl-modified tetraalkylammonium salts and phosphonium salts; Secondly, asymmetric phase-transfer catalysis under base-free and neutral conditions; Thirdly, hydrogen-bonding catalysis using tetraalkylammonium and trialkylsulfonium salts. These three different strategies are illustrated by using various phase-transfer catalyzed transformations.

Ionic Liquids: Advances and Applications in Phase Transfer Catalysis

Ionic liquids are a family of liquids that are composed entirely of ions and usually have melting points lower than 100 °C. Extensive research, along with the ever-growing interest of the scientific community, allowed for the development of a multitude of ionic liquids with low melting points. Such compounds are considered neoteric materials as well as ideal, custom-made solvents for a variety of different chemical transformations. In this regard, the importance of phase transfer catalysis is evident in a diversity of substrates and reactions. The use of phase transfer catalysts allows the reaction to proceed, facilitating the transfer of otherwise insoluble reactants to the desired phase. Recent scientific advances led to the emergence of ionic liquids, which are excellent candidates as phase transfer catalysts. The inherent fine-tuning capability of these molecules, along with the potential of phase transfer catalytic reactions, epitomize the sustainable aspect of this field of research. Herein, a cohesive report of such applications will be presented, including the period from the last decade of the 20th century up to date.

Ion Pair Catalyst - Pentanidinium

In this account, we further describe our already developed N-sp² hybrid guanidinium as an efficient phase-transfer catalyst and ion pair catalysis based on N-sp² hybrid pentanidinium and its application in some new reactions. The sp³ hybrid quaternary ammonium salt has a tetrahedral structure, which means that three sides of it can be effectively steric, allowing the remaining side to be close to the substrate. However, the sp² hybrid ammonium salt allows the substrate to form ion pairs from both directions respectively, so it is a greater challenge to control the stereoselectivity of the reaction. Van der Waals forces, such as hydrogen bonds and π - π ${\pi -\pi }$ interactions, have been used to make electrophiles approach from a certain direction, leading to a higher enantioselectivity. Based on the above idea, we designed an N-sp² hybrid phase-transfer catalyst, pentanidinium. Pentanidinium has five conjugated nitrogen atoms, one of which has a formal positive charge, which is necessary for it to become an ion pair catalyst. We have confirmed that pentanidinium can catalyze α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins, desymmetrization of pro-chiral sulfinate to afford enantioenriched sulfinate esters. Pentanidinium with side chain structure changes can also be catalyzed efficiently with enantioconvergent halogenophilic nucleophilic substitution, including azidation and thioesterification. In the reaction catalyzed by pentanidinium, it always attracts us with the advantages of low catalytic load and good enantioselectivity.

Molecular Understanding and Practical In Silico Catalyst Design in Computational Organocatalysis and Phase Transfer Catalysis-Challenges and Opportunities

Through the lens of organocatalysis and phase transfer catalysis, we will examine the key components to calculate or predict catalysis-performance metrics, such as turnover frequency and measurement of stereoselectivity, via computational chemistry. The state-of-the-art tools available to calculate potential energy and, consequently, free energy, together with their caveats, will be discussed via examples from the literature. Through various examples from organocatalysis and phase transfer catalysis, we will highlight the challenges related to the mechanism, transition state theory, and solvation involved in translating calculated barriers to the turnover frequency or a metric of stereoselectivity. Examples in the literature that validated their theoretical models will be showcased. Lastly, the relevance and opportunity afforded by machine learning will be discussed.

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.

Stereodivergent Construction of Csp3 -Csp3 Bonds Bearing Vicinal Stereocenters by Synergistic Palladium and Phase-Transfer Catalysis

Synergistic catalysis has emerged as one of the most powerful tools for stereodivergent formation of Csp³ -Csp³ bonds bearing vicinal stereocenters. Despite the many successes that have been achieved in this field, stereodivergent Csp³ -Csp³ coupling reactions involving stabilized nucleophiles remain challenging because of the competing single-catalysis pathway. Herein, we report a synergistic palladium/phase-transfer catalyst system that enables diastereodivergent Csp³ -Csp³ coupling reactions of 1,3-dienes with stabilized nucleophile oxindoles. Both the syn and anti coupling products, bearing quaternary and tertiary vicinal stereocenters, could be selectively produced in good yields with high enantio- and diastereoselectivities. Non-covalent activation of the stabilized nucleophile via chiral ion pair in a biphasic system is a crucial success factor in achieving diastereodivergence.

Fundamentals of chirality, resolution, and enantiopure molecule synthesis methods

The chirality of molecules is a concept that explains the interactions in nature. We may observe the same formula but different organizations revolving around the chiral center. Since Pasteur's meticulous observation of sodium ammonium tartrate crystals' structure, scientists have discovered many features of chiral molecules. The number of newly approved single enantiomeric drugs increases every year and takes place in the market. Thus, separation or resolution methods of racemic mixtures are of continued importance in the efficacy of drugs, installation of affordable production processes, and convenient synthetic chemistry practice. This article presents the asymmetric synthesis approaches and the classification of direct resolution methods of chiral molecules.

Substrate-Dependent Regioselectivity: Pd/PTC Cooperatively Catalyzed Domino Heck/Allylation of Allenamides with α-Carbon of Carbonyl Compounds

A Pd/phase-transfer catalyst cooperatively catalyzed domino Heck/allylation reaction is first reported, which represents interesting substrate-dependent regioselectivity. Under the same conditions, Ts-protected N-(2-iodophenyl)allenamides produced only linear allylation products, while Cbz, Ac, or Boc-protected N-(2-iodophenyl)allenamides and N-(2-iodobenzoyl)allenamides with various compounds generated branch allylation products with an exocylic C═C bond and two vicinal stereocenters. Up-scale syntheses and diverse fused cyclization transformations of products were then carried out. The enantioselective version for the domino process was studied.

Cyclopropenium Ions in Catalysis

Cyclopropenium ions are the smallest class of aromatic compounds, satisfying Hückel's rules of aromaticity with two π electrons within a three-membered ring. First prepared by Breslow in 1957, cyclopropenium ions have been found to possess extraordinary stability despite being both cationic and highly strained. In the 65 years since their first preparation, cyclopropenium ions have been the subject of innumerable studies concerning their synthesis, physical properties, and reactivity. However, prior to our work, the reactivity of these unique carbocations had not been exploited for reaction promotion or catalysis.Over the past 13 years, we have been exploring aromatic ions as unique and versatile building blocks for the development of catalysts for organic chemistry. A major portion of this work has been focused on leveraging the remarkable properties of the smallest of the aromatic ions─cyclopropeniums─as a design element in the invention of highly reactive catalysts. Indeed, because of its unique profile of hydrolytic stability, compact geometry, and relatively easy oxidizability, the cyclopropenium ring has proven to be a highly advantageous construction module for catalyst invention.In this Account, we describe some of our work using cyclopropenium ions as a key element in the design of novel catalysts. First, we discuss our early work aimed at promoting dehydrative reactions, starting with Appel-type chlorodehydrations of alcohols and carboxylic acids, cyclic ether formations, and Beckmann rearrangements and culminating in the realization of catalytic chlorodehydrations of alcohols and a catalytic Mitsunobu-type reaction. Next, we describe the development of cyclopropenimines as strong, neutral organic Brønsted bases and, in particular, the use of chiral cyclopropenimines for enantioselective Brønsted catalysis. We also describe the development of higher-order cyclopropenimine superbases. The use of tris(amino)cyclopropenium (TAC) ions as a novel class of phase-transfer catalysts is discussed for the reaction of epoxides with carbon dioxide. Next, we describe the formation of a cyclopropenone radical cation that has a portion of its spin density on the oxygen atom, leading to some peculiar metal ligand behavior. Finally, we discuss recent work that employs TAC electrophotocatalysts for oxidation reactions. The key intermediate for this chemistry is a TAC radical dication, which as an open-shell photocatalyst has remarkably strong excited-state oxidizing power. We describe the application of this strategy to transformations ranging from the oxidative functionalization of unactivated arenes to the regioselective derivatization of ethers, C-H aminations, vicinal C-H diaminations, and finally aryl olefin dioxygenations. Collectively, these catalytic platforms demonstrate the utility of charged aromatic rings, and cyclopropenium ions in particular, to enable unique advances in catalysis.

Unravelling the Development of Non-Covalent Organocatalysis in India

This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field.

1 Introduction

2 Non-Covalent Catalysis and Its Various Activation Modes

3 Hydrogen-Bonding Catalysis

3.1 Urea- and Thiourea-Derived Organocatalysts

3.1.1 Thiourea-Derived Organocatalysts

3.1.2 Urea-Derived Organocatalysts

3.2 Squaramide-Derived Organocatalysts

3.2.1 Michael Reactions

3.2.2 C-Alkylation Reactions

3.2.3 Mannich Reactions

3.2.4 [3+2] Cycloaddition Reactions

3.3 Cinchona-Alkaloid-Derived Organocatalysts

3.3.1 Michael Reactions

3.3.2 Aldol Reactions

3.3.3 Friedel–Crafts Reactions

3.3.4 Vinylogous Alkylation of 4-Methylcoumarins

3.3.5 C-Sulfenylation Reactions

3.3.6 Peroxyhemiacetalisation of Isochromans

3.3.7 Diels–Alder Reactions

3.3.8 Cycloaddition Reactions

3.3.9 Morita–Baylis–Hilman Reactions

4 Brønsted Acid Derived Organocatalysts

4.1 Chiral Phosphoric Acid Catalysis

4.1.1 Diels–Alder Reactions

4.1.2 Addition of Ketimines

4.1.3 Annulation of Acyclic Enecarbamates

5 Conclusion

Construction of Non-Biaryl Atropisomeric Amide Scaffolds Bearing a C-N Axis via Enantioselective Catalysis

The significant scaffold offered by atropisomeric amides with a C–N chiral axis has been extensively utilized for pharmaceuticals, agricultural science, and organic syntheses. As a result, the field of atropisomer synthesis has attracted considerable interest within chemistry communities. To date, a range of catalytic atroposelective approaches has been reported for the efficient construction of these challenging scaffolds. However, greatly concise and highly useful methodologies for the synthesis of these atropisomeric compounds, focusing on transition-metal, chiral amine, and phosphoric acid catalysis reactions, etc., are still desirable. Hence, it is indispensable to succinctly and systematically present all such reports by means of disclosing the mechanistic analysis and application, as well as the challenges and issues associated with the establishment of these atropisomers. In this review, we summarize the development of catalytic asymmetric synthetic strategies to access non-biaryl atropisomers rotating around a C–N chiral axis, including the reaction methods, mechanism, late-stage transformations, and applications.

Highly Efficient Darzens Reactions Mediated by Phosphazene Bases under Mild Conditions

The highly basic and poorly nucleophilic phosphazene base P1 -t-Bu promotes the Darzens condensation of α-halo esters with aromatic aldehydes affording α,β-epoxy esters in nearly quantitative yields under mild conditions and in short reaction times. The more basic P4 -t-Bu phosphazene was found useful with low reactivity aldehydes. These reactions can be performed in aprotic organic solvents of low polarity, thus minimizing the hydrolysis of α,β-epoxy esters which often accompanies the base-promoted Darzens condensations.

(Thio)urea containing chiral ammonium salt catalysts

(Thio)-urea-containing bifunctional quaternary ammonium salts emerged as powerful non-covalently interacting organocatalysts over the course of the last decade. The most commonly employed catalysts in this field are either based on Cinchona alkaloids, α-amino acids, or trans-cyclohexane-1,2-diamine. Our group has been heavily engaged in the design and use of such catalysts, i. e. trans-cyclohexane-1,2-diamine-based ones for around 10 years now, and it is therefore the intention of this short personal account to provide an overview of the, at least in our opinion, most significant and pioneering achievements in this field by looking on catalyst design and asymmetric method development, with a special focus on our own contributions.

Synthesis and Stereochemical Characterization of a Novel Chiral α-Tetrazole Binaphthylazepine Organocatalyst

A novel α-tetrazole-substituted 1,1′-binaphthylazepine chiral catalyst has been synthesized and its absolute configuration has been determined by DFT computational analysis of the vibrational circular dichroism (VCD) spectrum of its precursor. The VCD analysis, carried out through the model averaging method, allowed to assign the absolute configuration of a benzylic stereocenter in the presence of a chiral binaphthyl moiety. The 1,1′-binaphthylazepine tetrazole and the nitrile its immediate synthetic precursor, have been preliminarily tested as chiral organocatalysts in the asymmetric intramolecular oxa-Michael cyclization of 2-hydroxy chalcones for the synthesis of chiral flavanones obtaining low enantioselectivity.

Highly diastereo- and enantioselective synthesis of multisubstituted allylic amino acid derivatives by allylic alkylation of a chiral glycine-based nickel complex and vinylethylene carbonates

The asymmetric synthesis of multisubstituted allylic amino acid derivatives was accomplished by the allylic alkylation of a chiral glycine-based nickel complex with vinylethylene carbonates. High enantioselectivities and diastereoselectivities were obtained under mild reaction conditions. The gram-scale synthesis was carried out with a good yield and high enantioselectivity, indicating that the method is a highly efficient route to chiral multisubstituted allylic amino acid derivatives.

Understanding and Manipulating Helical Nanofilaments in Binary Systems with Achiral Dopants

Here, we report the relationship between helical pitch of the helical nanofilament (HNF) phase formed by bent-core molecule NOBOW and the concentration of achiral dopants 5CB and octane, using linearly polarized resonant soft X-ray scattering (RSoXS). Utilizing theory-based simulation, which fits well with the experiments, the molecular helices in the filament were probed and the superstructure of helical 5CB directed by groove of HNFs was observed. Quantitative pitch determination with RSoXS reveals that helical pitch variation is related to 5CB concentration with no temperature dependence. Doping rodlike immiscible 5CB led to a pitch shortening of up to 30%, which was attributed to a change in interfacial tension. By shedding light not only on phase behavior of binary systems but also enabling control over pitch length, our work may benefit various applications of HNF-containing binary systems, including optical rotation devices, circularly polarized light emitters, and chirality transfer agents.

Photochemical Deracemization of a Medicinally-Relevant Benzopyran using an Oscillatory Flow Reactor

Dynamic deracemization processes, such as crystallization-induced diastereomer transformations (CIDTs), offer the opportunity to combine racemization and resolution processes, to provide high yields of enantiomerically pure compounds. To date, few of these processes have incorporated photochemical racemization. By combining batch crystallization with a flow photoreactor for efficient irradiation, it is possible to perform such deracemization in an effective, scalable and high yielding manner. After applying design of experiment (DoE) principles and mathematical modelling, the most efficient parameter set could be identified, leading to excellent results in just 4 h reaction time: isolated yield of 82 % and assay ee of 96 %. Such photochemical racemization methods can serve to open new avenues for preparation of enantiomerically pure functional molecules on both small and industrially-relevant scales.

Optical resolution of 1,16-dihydroxytetraphenylene by chiral gold(iii) complexation and its applications as chiral ligands in asymmetric catalysis

We report herein a novel approach involving optical resolution of (±)-1,16-dihydroxytetraphenylene (DHTP) by chiral gold(iii) complexation. This method features several key advantages, i.e., recyclability of chiral resolution reagents, feasibility of scaling up to gram quantities, and operational simplicity. On the basis of this method, which led to optically pure DHTP, a library of 2,15-diaryl (S)-DHTPs and several (S)-DHTP-derived phosphoramidite ligands were synthesized. Finally, the superior performance of a (S)-DHTP phosphoramidite ligand was demonstrated by efficient iridium-catalyzed asymmetric allylic alkynylation reactions.

Phase-transfer catalyzed Michael/ammonolysis cascade reactions of enaminones and olefinic azlactones: a new approach to structurally diverse quinoline-2,5-diones

Michael/ammonolysis cascade reactions between cyclohexane-1,3-dione-derived enaminones and olefinic azlactones via phase-transfer catalysis have been developed. This method provides rapid access to a suite of architecturally complex and diverse quinoline-2,5-diones bearing a secondary amide group at the C-3 position in moderate to excellent yields (53-94%) and with excellent diastereoselectivities (>99 : 1 dr in most cases). The achievement of a preparative-scale reaction and the diverse product derivatization that can be obtained highlight the application potential of this protocol both in academic and industrial settings. An investigation of the reaction mechanism implies that tetrabutylammonium hydroxide may be the actual catalyst during this cascade reaction.

Stereoselective 1,2-cis Furanosylations Catalyzed by Phenanthroline

Stereoselective formation of the 1,2-cis furanosidic linkage, a motif of many biologically relevant oligosaccharides and polysaccharides, remains an important synthetic challenge. We herein report a new stereoselective 1,2-cis furanosylation method promoted by phenanthroline catalysts under mild and operationally simple conditions. NMR experiments and density functional theory calculations support an associative mechanism in which the rate-determining step occurs from an inverted displacement of the faster-reacting phenanthrolinium ion intermediate with an alcohol nucleophile. The phenanthroline catalysis system is applicable to a number of furanosyl bromide donors to provide the challenging 1,2-cis substitution products in good yield with high anomeric selectivities. While arabinofuranosyl bromide provides β-1,2-cis products, xylo- and ribofuranosyl bromides favor α-1,2-cis products.

Hydrogen Bonding Phase-Transfer Catalysis with Alkali Metal Fluorides and Beyond

Phase-transfer catalysis (PTC) is one of the most powerful catalytic manifolds for asymmetric synthesis. Chiral cationic or anionic PTC strategies have enabled a variety of transformations, yet studies on the use of insoluble inorganic salts as nucleophiles for the synthesis of enantioenriched molecules have remained elusive. A long-standing challenge is the development of methods for asymmetric carbon-fluorine bond formation from readily available and cost-effective alkali metal fluorides. In this Perspective, we describe how H-bond donors can provide a solution through fluoride binding. We use examples, primarily from our own research, to discuss how hydrogen bonding interactions impact fluoride reactivity and the role of H-bond donors as phase-transfer catalysts to bring solid-phase alkali metal fluorides in solution. These studies led to hydrogen bonding phase-transfer catalysis (HB-PTC), a new concept in PTC, originally crafted for alkali metal fluorides but offering opportunities beyond enantioselective fluorination. Looking ahead, the unlimited options that one can consider to diversify the H-bond donor, the inorganic salt, and the electrophile, herald a new era in phase-transfer catalysis. Whether abundant inorganic salts of lattice energy significantly higher than those studied to date could be considered as nucleophiles, e.g., CaF₂, remains an open question, with solutions that may be found through synergistic PTC catalysis or beyond PTC.

Asymmetric Phase-Transfer Catalytic aza-Michael Addition to Cyclic Enone: Highly Enantioselective and Diastereoselective Synthesis of Cyclic 1,3-Aminoalcohols

The highly enantioselective aza-Michael reaction of tert-butyl β-naphthylmethoxycarbamate to cyclic enones has been accomplished by using a new cinchona alkaloid derived C(9)-urea ammonium catalyst under phase-transfer catalysis conditions with up to 98% ee at 0 °C. The resulting aza-Michael adducts can be converted to versatile intermediates by selective deprotection and the cyclic 1,3-aminoalcohols by diastereoselective reduction with up to 32:1, which have been widely used as important pharmacophores in pharmaceutical development.