Asymmetric Azidation under Hydrogen Bonding Phase-Transfer Catalysis: A Combined Experimental and Computational Study

Asymmetric phase-transfer catalysis

Over the past three decades, chiral phase-transfer catalysts (PTCs) have emerged as highly successful organocatalysts in a diverse range of asymmetric reactions. A substantial number of chiral PTCs have now already been discovered and utilized in dependable routes to enantioenriched products. These extend beyond the classical cationic PTCs with the emergence of anionic phase-transfer catalysis and hydrogen-bonding phase-transfer catalysis providing new asymmetric synthetic approaches. Nevertheless, the application level of chiral PTCs in both academic and industrial processes is below our expectation. This Review highlights the notable advances in chiral PTCs, including challenges, limitations and efforts to overcome them. Following this, the potential for sustainable chiral PTCs is described with a focus on using photocatalysed, flow and electrochemical synthesis.

Origins of High-Activity Cage-Catalyzed Michael Addition

Cage catalysis continues to create significant interest, yet catalyst function remains poorly understood. Herein, we report mechanistic insights into coordination-cage-catalyzed Michael addition using kinetic and computational methods. The study has been enabled by the detection of identifiable catalyst intermediates, which allow the evolution of different cage species to be monitored and modeled alongside reactants and products. The investigations show that the overall acceleration results from two distinct effects. First, the cage reaction shows a thousand-fold increase in the rate constant for the turnover-limiting C-C bond-forming step compared to a reference state. Computational modeling and experimental analysis of activation parameters indicate that this stems from a significant reduction in entropy, suggesting substrate coencapsulation. Second, the cage markedly acidifies the bound pronucleophile, shifting this equilibrium by up to 6 orders of magnitude. The combination of these two factors results in accelerations up to 109 relative to bulk-phase reference reactions. We also show that the catalyst can fundamentally alter the reaction mechanism, leading to intermediates and products that are not observable outside of the cage. Collectively, the results show that cage catalysis can proceed with very high activity and unique selectivity by harnessing a series of individually weak noncovalent interactions.

Enantioselective copper-catalyzed azidation/click cascade reaction for access to chiral 1,2,3-triazoles

Chiral 1,2,3-triazoles are highly attractive motifs in various fields. However, achieving catalytic asymmetric click reactions of azides and alkynes for chiral triazole synthesis remains a significant challenge, mainly due to the limited catalytic systems and substrate scope. Herein, we report an enantioselective azidation/click cascade reaction of N-propargyl-β-ketoamides with a readily available and potent azido transfer reagent via copper catalysis, which affords a variety of chiral 1,2,3-triazoles with up to 99% yield and 95% ee under mild conditions. Notably, chiral 1,5-disubstituted triazoles that have not been accessed by previous asymmetric click reactions are also prepared with good functional group tolerance.

Synthesis and Cycloaddition Reactions of 1-Azido-1,1,2,2-tetrafluoroethane

A new fluorinated azidoethane─1-azido-1,1,2,2-tetrafluoroethane─was prepared in quantitative yield by the addition of an azide anion to tetrafluoroethylene in a protic medium. The title azide was shown to be thermally stable and insensitive to impact. Copper(I)-catalyzed [3 + 2] cycloaddition with alkynes afforded 4-substituted N-tetrafluoroethyl-1,2,3-triazoles which underwent rhodium(II)-catalyzed transannulation with nitriles to novel N-tetrafluoroethylimidazoles or the reaction with triflic acid to enamido triflates. [3 + 2] Cycloaddition of the title azide with primary amines afforded novel 5-difluoromethyl tetrazoles.

Continuous addition kinetic elucidation: catalyst and reactant order, rate constant, and poisoning from a single experiment

Kinetic analysis of catalytic reactions is a powerful tool for mechanistic elucidation but is often challenging to perform, limiting understanding and therefore development of these reactions. Establishing order in a catalyst is usually achieved by running several reactions at different loadings, which is both time-consuming and complicated by the challenge of maintaining consistent run-to-run experimental conditions. Continuous addition kinetic elucidation (CAKE) was developed to circumvent these issues by continuously injecting a catalyst into a reaction, while monitoring reaction progress over time. For reactions that are mth order in a single yield-limiting reactant and nth order in catalyst, a plot of reactant concentration against time has a shape dependent only on the orders m and n. Therefore, fitting experimental CAKE data (using open access code or a convenient web tool) allows the reactant and catalyst orders, rate constant, and the amount of complete catalyst inhibition to be determined from a single experiment. Kinetic information obtained from CAKE experiments showed good agreement with the literature.

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.

Diarylureas: New Promising Small Molecules against Streptococcus mutans for the Treatment of Dental Caries

Dental caries is a biofilm-mediated disease that represents a worldwide oral health issue. Streptococcus mutans has been ascertained as the main cariogenic pathogen responsible for human dental caries, with a high ability to form biofilms, regulated by the quorum sensing. Diarylureas represent a class of organic compounds that show numerous biological activities, including the antimicrobial one. Two small molecules belonging to this class, specifically to diphenylureas, BPU (1,3-bis[3,5-bis(trifluoromethyl)phenyl]urea) and DMTU (1,3-di-m-tolyl-urea), showed interesting results in studies regarding the antimicrobial activity against the cariogenic bacterium S. mutans. Since there are not many antimicrobials used for the prevention and treatment of caries, further studies on these two interesting compounds and other diarylureas against S. mutans may be useful to design new effective agents for the treatment of caries with generally low cytotoxicity.

Elucidating the Infrared Spectral Properties of Succinic Molecular Acid Crystals: Illustration of the Structure and the Hydrogen Bond Energies of the Crystal and Its Deuterated Analogs

Herein, the infrared spectroscopic properties of molecular succinic acid crystals (SA) and their four isotopic analogs [C₂H₄(COOH)₂, h₆-SA; C₂H₄(COOD)₂, d₂-SA; C₂D₄(COOH)₂, d₄-SA; C₂D₄(COOD)₂, d₆-SA] are reported. The correlation between the structure of succinic acid molecules and their corresponding hydrogen bond energies is elucidated. The effects related to the isotopic dilution as well as the changes in the spectrum recording temperature on the fine structures of the v_O-H and v_O-D bands are interpreted. The infrared spectral anomalies detected in the spectra of isotopically neat succinic nanocrystal acids are confirmed by theoretical calculations using density functional theory (DFT). According to previous spectroscopic studies of succinic acid and those carried out for α,ω-dicarboxylic acids, a decent agreement between the experimental results and the theoretical DFT simulations is obtained. Moreover, the spectra of single crystals of the h₆ and d₄ succinic acid variants prove that the vibrational coupling mechanism between the (COOH)₂ cycles is rigorously convergent to that detected in the spectra of aromatic carboxylic acids, suggesting thereby that the promotion of symmetry-forbidden high stretching IR transitions plays a crucial role. Furthermore, the obtained experimental results reveal that the succinic acid shows a spectral behavior significantly different from that characteristic of hydrogen associations of other acids of homologous series, such as the glutaric, adipic, malonic, and pimelic acid crystals. The results obtained herein shed light on the way to explore the revealed structure of isotopic derivatives of succinic acid crystals and may prove to be useful results for understanding the nature of unconventional interactions as well as the macroscopic energy effects directing the development of hydrogen associations.

Activation Modes in Asymmetric Anion-Binding Catalysis

Over the past two decades, enantioselective anion-binding catalysis has emerged as a powerful strategy for the induction of chirality in organic transformations. The stereoselectivity is achieved in a range of different reactions by using non-covalent interactions between a chiral catalyst and an ionic substrate or intermediate, and subsequent formation of a chiral contact ion-pair upon anion-binding. This strategy offers vast possibilities in catalysis and the constant development of new reactions has led to various substrate activation approaches. This review provides an overview on the different activation modes in asymmetric anion-binding catalysis by looking at representative examples and recent advances made in this field.

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.

Organocatalytic asymmetric azidation of sulfoxonium ylides: mild synthesis of enantioenriched α-azido ketones bearing a labile tertiary stereocenter

Disclosed here is a catalytic asymmetric azidation reaction for the efficient synthesis of α-azido ketones bearing a labile tertiary stereocenter. With a superb chiral squaramide catalyst, a mild asymmetric formal H–N₃ insertion of α-carbonyl sulfoxonium ylides proceeded with excellent efficiency and enantioselectivity. This organocatalytic process not only complements the previous α-azidation approaches for the formation of quaternary stereocenters and mostly for 1,3-dicarbonyl compounds, but also has advantages over the well-known metal-catalyzed asymmetric carbene insertion chemistry using α-diazocarbonyl compounds. Detailed mechanistic studies via control reactions and NMR studies provided important insights into the reaction pathway, which features reversible protonation and dynamic kinetic resolution. The curiosity in mechanism also led to the development of a simplified alternative protocol with a cheaper HN₃ source.

An organocatalytic asymmetric H–N₃ insertion of α-carbonyl sulfoxonium ylides has been developed, providing efficient access to α-azido ketones bearing labile tertiary stereocenters and complementing the metal carbene insertion chemistry.