Non-bonding 1,5-S···O interactions govern chemo- and enantioselectivity in isothiourea-catalyzed annulations of benzazoles

Organocatalytic Enantioselective [1,2]-Stevens Rearrangement of Azetidinium Salts

The first organocatalyzed enantioselective [1,2]-Stevens rearrangement is reported. 4-Alkylideneproline derivatives are produced in up to 86% yield and in up to 90:10 er, with recrystallization enhancing er up to >99.5:0.5. Product configuration was opposite that predicted by existing stereochemical models for this organocatalyst class, and DFT calculations revealed a novel mode of asymmetric induction. The adaptability of this catalytic strategy for asymmetric [1,2]-Stevens rearrangements of other heterocyclic amines was demonstrated.

De-epimerizing DyKAT of β-lactones generated by isothiourea-catalysed enantioselective [2 + 2] cycloaddition

An enantioselective isothiourea-catalysed [2 + 2] cycloaddition of C(1)-ammonium enolates with pyrazol-4,5-diones is used to construct spirocyclic β-lactones in good yields, excellent enantioselectivity (99 : 1 er) but with modest diastereocontrol (typically 70 : 30 dr). Upon ring-opening with morpholine or alternative nucleophilic amines and alcohols β-hydroxyamide and β-hydroxyester products are generated with enhanced diastereocontrol (up to >95 : 5 dr). Control experiments show that stereoconvergence is observed in the ring-opening of diastereoisomeric β-lactones, leading to a single product (>95 : 5 dr, >99 : 1 er). Mechanistic studies and DFT analysis indicate a substrate controlled Dynamic Kinetic Asymmetric Transformation (DyKAT) involving epimerisation at C(3) of the β-lactone under the reaction conditions, coupled with a hydrogen bond-assisted nucleophilic addition to the Si-face of the β-lactone and stereodetermining ring-opening. The scope and limitations of a one-pot protocol consisting of isothiourea-catalysed enantio-determining [2 + 2] cycloaddition followed by diastereo-determining ring-opening are subsequently developed. Variation within the anhydride ammonium enolate precursor, as well as N(1) and C(3) within the pyrazol-4,5-dione scaffold is demonstrated, giving a range of functionalised β-hydroxyamides with high diastereo- and enantiocontrol (>20 examples, up to >95 : 5 dr and >99 : 1 er) via this DyKAT.

Enantioselective Synthesis in Continuous Flow: Polymer-Supported Isothiourea-Catalyzed Enantioselective Michael Addition-Cyclization with α-Azol-2-ylacetophenones

A packed reactor bed incorporating a polymer-supported isothiourea HyperBTM catalyst derivative has been used to promote the enantioselective synthesis of a range of heterocyclic products derived from α-azol-2-ylacetophenones and -acetamides combined with alkyl, aryl, and heterocyclic α,β-unsaturated homoanhydrides in continuous flow via an α,β-unsaturated acyl-ammonium intermediate. The products are generated in good to excellent yields and generally in excellent enantiopurity (up to 97:3 er). Scale-up is demonstrated on a 15 mmol scale, giving the heterocyclic product in 68% overall yield with 98:2 er after recrystallization.

Weak Interaction Activates Esters: Reconciling Catalytic Activity and Turnover Contradiction by Tailored Chalcogen Bonding

The activation of esters by strong Lewis acids via the formation of covalent adducts is a classic strategy to give reactivity; however, this approach frequently incurs limited turnover due to the low efficiency in the dissociation of catalyst from a stable catalyst-product complex. While the use of some weak interaction catalysts that can easily dissociate from any bonding complexes in the reaction system would solve this catalyst turnover problem, the poor catalytic activity in the ester activation that can be provided by these noncovalent forces in turn sets up a formidable challenge. Herein, we describe the activation and catalytic transformation of esters by weak interactions, which provides a promising platform to reconcile the catalytic activity and turnover problems. Several tailored chalcogen-bonding catalysts were developed for the activation of esters, enabling achieving several inherently low reactive Diels-Alder reactions as well as the ring-opening polymerization of lactones through weak chalcogen bonding interactions. This supramolecular catalysis approach is particularly highlighted by its capability to promote some uncommon Diels-Alder reactions involving using dienes bearing electron-withdrawing groups coupled by α,β-unsaturated ester as dienophiles and substrate incorporating competitive Lewis basic sites, in which typical strong Lewis acids showed low catalytic efficiency, while representative hydrogen and halogen bonding catalysts were inactive.

Bifunctional Squaramide-Catalyzed Asymmetric Cascade Reaction of Benzothiazoles with 2-Nitroallylic Acetates or Nitroenynes

We describe here an organocatalytic asymmetric cascade formal [3 + 3] cycloaddition of benzothiazoles with 2-nitroallylic acetates and nitroenynes. This dearomative methodology provided a facile and efficient strategy for the construction of a broad range of valuable benzothiazolopyridines bearing two adjacent stereogenic centers in moderate to good yields with good to excellent stereocontrol.

Understanding divergent substrate stereoselectivity in the isothiourea-catalysed conjugate addition of cyclic α-substituted β-ketoesters to α,β-unsaturated aryl esters

The development of enantioselective synthetic methods capable of generating vicinal stereogenic centres, where one is tetrasubstituted (such as either an all-carbon quaternary centre or where one or more substituents are heteroatoms), is a recognised synthetic challenge. Herein, the enantioselective conjugate addition of a range of carbo- and heterocyclic α-substituted β-ketoesters to α,β-unsaturated aryl esters using the isothiourea HyperBTM as a Lewis base catalyst is demonstrated. Notably, divergent diastereoselectivity is observed through the use of either cyclopentanone-derived or indanone-derived substituted β-ketoesters with both generating the desired stereodefined products with high selectivity (>95 : 5 dr, up to 99 : 1 er). The scope and limitations of these processes are demonstrated, alongside application on gram scale. The origin of the divergent substrate selectivity has been probed through the use of DFT-analysis, with preferential orientation driven by dual stabilising CH⋯O interactions. The importance of solvation with strongly polar transition-states is highlighted and the SMD solvation model is demonstrated to capture solvation effects reliably.

Dual Chalcogen-Bonding Interactions for the Conformational Control of Urea

Dual chalcogen-bonding interactions is proposed as a novel means for the conformational control of urea derivatives. The formation of a chalcogen-bonding interaction at both sides of the urea carbonyl group was unambiguously confirmed by X-ray diffraction as well as computational studies including non-covalent interaction (NCI) plot index analysis, quantum theory of atoms in molecules (QTAIM) analysis, and natural bond orbital (NBO) analysis via DFT calculations. By virtue of this dual interaction, urea derivatives that bear chalcogen atoms (X=S and Se) adopt a planar structure via the carbonyl oxygen (O) with an X⋅⋅⋅O⋅⋅⋅X arrangement on the same side of the molecule. The rigidity of the conformational lock was evaluated using the molecular arrangement in the crystal and the rotational barrier of benzochalcogenophene ring, which indicated a stronger conformational lock in benzoselenophene than in benzothiophene urea derivatives. Furthermore, the acidity of the urea derivatives increases according to the Lewis-acidic properties of the chalcogen-bonding interactions, whereby benzoselenophene urea is more acidic than benzothiophene urea. Tweezer-shaped urea derivatives were prepared, and their stereostructure proved the viability of the conformational control for defining the location of the substituents on the urea framework.

Supramolecular catalysis with ethers enabled by dual chalcogen bonding activation

The activation of ethers by weak interactions is a long-standing objective in supramolecular catalysis, but yet it remains an underdeveloped topic. The obstacles towards solving this problem are prominent since it is difficult for a weak interaction to cleave a relatively strong C-O σ-bond and moreover, the ionic intermediate composing of an alkoxide ion and an electrophilic carbocation would deactivate weak interaction donors. Herein, we describe a distinctive activation mode, dual Se···π and Se···O bonding, that could activate benzylic as well as allylic ether C-O σ-bonds to achieve cyclization, coupling and elimination reactions. This dual Se···π and Se···O bonding catalysis approach could tolerate various alkoxide leaving groups, while the other representative weak interaction donors showed no catalytic activity.

Enantioselective isothiourea-catalysed reversible Michael addition of aryl esters to 2-benzylidene malononitriles

Catalytic enantioselective transformations usually rely upon optimal enantioselectivity being observed in kinetically controlled reaction processes, with energy differences between diastereoisomeric transition state energies translating to stereoisomeric product ratios. Herein, stereoselectivity resulting from an unusual reversible Michael addition of an aryl ester to 2-benzylidene malononitrile electrophiles using an isothiourea as a Lewis base catalyst is demonstrated. Notably, the basicity of the aryloxide component and reactivity of the isothiourea Lewis base both affect the observed product selectivity, with control studies and crossover experiments indicating the feasibility of a constructive reversible Michael addition from the desired product. When this reversible addition is coupled with a crystallisation-induced diastereomer transformation (CIDT) it allows isolation of products in high yield and stereocontrol (14 examples, up to 95 : 5 dr and 99 : 1 er). Application of this process to gram scale, plus derivatisations to provide further useful products, is demonstrated.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

Probing Regio- and Enantioselectivity in the Formal [2 + 2] Cycloaddition of C(1)-Alkyl Ammonium Enolates with β- and α,β-Substituted Trifluoromethylenones

The isothiourea-catalyzed regio- and enantioselective formal [2 + 2] cycloaddition of C(1)-alkyl and C(1)-unsubstituted ammonium enolates with β- and α,β-substituted trifluoromethylenones has been developed. In all cases, preferential [2 + 2]-cycloaddition over the alternative [4 + 2]-cycloaddition is observed, giving β-lactones with excellent diastereo- and enantioselectivity (34 examples, up to >95:5 dr, >99:1 er). The regioselectivity of the process was dictated by the nature of the substituents on both reaction components. Solely [2 + 2] cycloaddition products are observed when using α,β-substituted trifluoromethylenones or α-trialkylsilyl acetic acid derivatives; both [2 + 2] and [4 + 2] cycloaddition products are observed when using β-substituted trifluoromethylenones and α-alkyl-α-trialkylsilyl acetic acids as reactants, with the [2 + 2] cycloaddition as the major reaction product. The beneficial role of the α-silyl substituent within the acid component in this protocol has been demonstrated by control experiments.

Strategies for the Design of PEDOT Analogues Unraveled: the Use of Chalcogen Bonds and σ-Holes

In this theoretical study, we set out to demonstrate the substitution effect of PEDOT analogues on planarity as an intrinsic indicator for electronic performance. We perform a quantum mechanical (DFT) study of PEDOT and analogous model systems and demonstrate the usefulness of the ωB97X-V functional to simulate chalcogen bonds and other noncovalent interactions. We confirm that the chalcogen bond stabilizes the planar conformation and further visualize its presence via the electrostatic potential surface. In comparison to the prevalent B3LYP, we gain 4-fold savings in computational time and simulate model systems of up to a dodecamer. Implications for design of conductive polymers can be drawn from the results, and an example for self-doped polymers is presented where modulation of the strength of the chalcogen bond plays a significant role.

Chalcogen Bonding Catalysis with Phosphonium Chalcogenide (PCH)

ConspectusThe exploration of new catalysis concepts and strategies to drive chemical reactions is of vital importance for the sustainable development of organic synthesis. Recently, chalcogen bonding catalysis has emerged as a new concept for organic synthesis and has been demonstrated to be an important synthetic tool capable of addressing elusive reactivity and selectivity issues. This Account describes our progress in the research field of chalcogen bonding catalysis, including (1) the discovery of phosphonium chalcogenide (PCH) as highly efficient chalcogen bonding catalyst; (2) the development of "chalcogen-chalcogen bonding catalysis" and "chalcogen···π bonding catalysis" modes; (3) the demonstration that chalcogen bonding catalysis with PCH can activate hydrocarbons to achieve cyclization and coupling reactions of alkenes; (4) the discovery of unusual results that chalcogen bonding catalysis with PCH can solve elusive reactivity and selectivity issues that are inaccessible by classic catalysis approaches; and (5) the elucidation of chalcogen bonding mechanisms.With PCH catalysts, we systematically studied their chalcogen bonding properties, the relationship between structure and catalysis, and their application in facilitating a diverse array of reactions. Enabled by chalcogen-chalcogen bonding catalysis, an efficient assembly reaction of three molecules of β-ketoaldehyde and one indole derivative in a single operation was realized, delivering heterocycles with a newly constructed seven-membered ring. In addition, a Se···O bonding catalysis approach achieved an efficient synthesis of calix[4]pyrroles. We developed a "dual chalcogen bonding catalysis" strategy to solve reactivity and selectivity issues in the Rauhut-Currier-type reactions and related cascade cyclizations, thus shifting conventionally covalent Lewis base catalysis to a cooperative Se···O bonding catalysis approach. This strategy enables the cyanosilylation of ketones to take place in the presence of a ppm-level amount of PCH catalyst loading. Furthermore, we established chalcogen···π bonding catalysis for catalytic transformation of alkenes. In the research field of supramolecular catalysis, the activation of hydrocarbons such as alkenes by weak interactions is a highly interesting unresolved topic. We showed that the Se···π bonding catalysis approach could efficiently activate alkenes to achieve both coupling and cyclization reactions. Chalcogen···π bonding catalysis with PCH catalysts is particularly highlighted by the capability of facilitating strong Lewis-acid inaccessible transformations, such as the controlled cross coupling of triple alkenes. Overall, this Account presents a panoramic view of our research on chalcogen bonding catalysis with PCH catalysts. The works described in this Account provide a significant platform to solve synthetic problems.

Catalytic Enantioselective Steglich-Type Rearrangement of Enol Lactones: Asymmetric Synthesis of Spirocyclic 1,3-Diketones

An example of asymmetric Steglich-type rearrangement of enol lactones is reported. This highly enantioselective acyl transfer reaction is catalyzed by chiral isothiourea at ambient temperature and provides a useful synthetic approach to access enantioenriched spirotricyclic β,β'-diketones from a broad range of indanone or tetralone-derived lactones. Preliminary mechanistic studies suggest the initial formation of an N-acylated iminium cation intermediate that induces a following facial selective condensation.

Probing the Directionality of S···O/N Chalcogen Bond and Its Interplay with Weak C-H···O/N/S Hydrogen Bond Using Molecular Electrostatic Potential

The directionality of the chalcogen bond (Ch-bond) formed by S and its interplay with other weak interactions have important chemical and biological implications. Here, dimers made of CH₃-S-X and O/N containing nucleophiles are studied and found to be stabilized by coexisting S···O/N and C-H···O/N interactions. Based on experimentally accessible electron density and molecular electrostatic potentials (MESPs), we showed that reciprocity between S···O/N and C-H···O/N interactions in the stability of cumulative molecular interaction (ΔE) was dependent on the strength of the σ-hole on S (V_s,max). Direct correlation between ΔE of dimers with V_s,max of S supports the electrostatic nature of the Ch-bond. Such interplay of the Ch-bond is necessary for its directionality in complex nucleophiles (carbonyl groups) with multiple electron-rich centers, which is explained using MESP. A correlation between the MESP minima in the π-region and the strength of the S-π interaction explains the directional selectivity of the Ch-bond.

A Desilylative Approach to Alkyl Substituted C(1)‐Ammonium Enolates: Application in Enantioselective [2+2] Cycloadditions

The catalytic generation of C(1)‐ammonium enolates from the corresponding α‐silyl‐α‐alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α‐unsubstituted and α‐alkyl substituted C(1)‐ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]‐cycloaddition processes with perfluoroalkylketones (31 examples, up to 96 % yield and >99 : 1 er), as well as selective [2+2]‐cycloaddition with trifluoromethyl enones (4 examples, up to 75 % yield and >99 : 1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)‐α‐silyl‐α‐alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero‐order kinetics.

Isothiourea-Catalyzed [2 + 2] Cycloaddition of C(1)-Ammonium Enolates and N-Alkyl Isatins

Enantioselective [2 + 2] cycloaddition of C(1)-ammonium enolates generated catalytically using the isothiourea HyperBTM with N-alkyl isatins gives spirocyclic β-lactones. In situ ring opening with an amine nucleophile generates isolable highly enantioenriched products in up to 92:8 dr and in >99:1 er.

Cooperative chalcogen bonding interactions in confined sites activate aziridines

The activation of aziridines typically involves the use of strong Lewis acids or transition metals, and methods relying on weak interactions are rare. Herein, we report that cooperative chalcogen bonding interactions in confined sites can activate sulfonyl-protected aziridines. Among the several possible distinct bonding modes, our experiments and computational studies suggest that an activation mode involving the cooperative Se···O and Se···N interactions is in operation. The catalytic reactions between weakly bonded supramolecular species and nonactivated alkenes are considered as unfavorable approaches. However, here we show that the activation of aziridines by cooperative Se···O and Se···N interactions enables the cycloaddition of weakly bonded aziridine-selenide complex with nonactivated alkenes in a catalytic manner. Thus, weak interactions can indeed enable these transformations and are an alternative to methods relying on strong Lewis acids.

The activation of aziridines is typically achieved via reaction with strong Lewis acids or transition metals. Here, the authors report that cooperative Se ∙ ∙∙O and Se ∙ ∙∙N noncovalent interactions can activate sulfonyl-protected aziridines, which enables their use in cycloaddition reactions with nonactivated alkenes.

Cooperative Palladium/Isothiourea Catalyzed Enantioselective Formal (3+2) Cycloaddition of Vinylcyclopropanes and α,β-Unsaturated Esters

A protocol for the enantioselective synthesis of substituted vinylcyclopentanes has been realised using cooperative palladium and isothiourea catalysis. Treatment of vinylcyclopropanes with Pd(PPh₃ )₄ generates a zwitterionic π-allyl palladium intermediate that intercepts a catalytically generated α,β-unsaturated acyl ammonium species prepared from the corresponding α,β-unsaturated para-nitrophenyl ester and the isothiourea (R)-BTM. Intermolecular formal (3+2) cycloaddition between these reactive intermediates generates functionalised cyclopentanes in generally good yields and excellent diastereo- and enantiocontrol (up to >95 : 5 dr, 97 : 3 er), with the use of LiCl as an additive proving essential for optimal stereocontrol. To the best of our knowledge a dual transition metal/organocatalytic process involving α,β-unsaturated acyl ammonium intermediates has not been demonstrated previously.

Isothiourea-catalyzed formal enantioselective conjugate addition of benzophenone imines to β-fluorinated α,β-unsaturated esters

The isothiourea-catalyzed formal enantioselective conjugate addition of 2-hydroxybenzophenone imine derivatives to α,β-unsaturated para-nitrophenyl esters has been developed. Investigations of the scope and limitations of this procedure showed that β-electron withdrawing substituents within the α,β-unsaturated ester component are required for good product yield, giving rise to a range of β-imino ester and amide derivatives in moderate to good isolated yields with excellent enantioselectivity (20 examples, up to 81% yield and 97 : 3 er).

Isothiourea-Catalyzed Enantioselective Michael Addition of Malonates to α,β-Unsaturated Aryl Esters

An enantioselective Michael addition of malonates to α,β-unsaturated para-nitrophenyl esters was achieved using the Lewis basic isothiourea HyperBTM, giving excellent levels of product enantioselectivity (up to >99:1 enantiomeric ratio) in good yields and with complete regioselectivity (>20:1 regioselectivity ratio) in the presence of alternative (phenyl ketone and ethyl ester) Michael acceptors. Density functional theory calculations indicate that N-acylation is rate-limiting. This constitutes a rare example of a highly enantioselective addition of simple, readily available malonates to α,β-unsaturated esters.

Recent progress in asymmetric synergistic catalysis - the judicious combination of selected chiral aminocatalysts with achiral metal catalysts

In this review we survey recent synergistic applications of a chiral organocatalyst with an achiral metal to perform stereoselective transformations of synthetic utility (since 2016). The transformations are classified by the modes of reactivity deployed, focussing on organocatalytic activation of carbonyl substrates as chiral nucleophiles via the α-position (e.g., as enamines) and as chiral electrophiles via the β-position (e.g., as iminium ions) combined with complementary activation of their reaction partners by an achiral metal co-catalyst (e.g., Pd or Cu-based). Corresponding radical reactions are also presented in which photocatalysis mediated by achiral metal complexes replaces the metal co-catalyst. Certain privileged structures are revealed and opportunities to develop this exciting field are highlighted.

A critical survey of recent synergistic applications of a chiral organocatalyst with an achiral metal to perform stereoselective transformations of synthetic utility.

A Se···O bonding catalysis approach to the synthesis of calix[4]pyrroles

Described herein is a chalcogen bonding catalysis approach to the synthesis of calix[4]pyrrole derivatives. The Se···O bonding interactions between selenide catalysts and ketones gave rise to the catalytic activity in the condensation reactions between pyrrole and ketones, leading to the generation of calix[4]pyrrole derivatives in moderate to high yields. This chalcogen bonding catalysis approach was efficient since only 5 mol % catalyst loading was used to promote the consecutive condensation processes while the reactions could be carried out at room temperature, thus highlighting the potential of this type of nonclassical interactions in catalyzing relative complex transformations.

Isothiourea-Catalysed Enantioselective Radical Conjugate Addition under Batch and Flow Conditions

The photocatalytic generation of α-amino radicals is combined with chiral isothiourea derived α,β-unsaturated acyl ammonium intermediates. The reaction proceeds via a [3+2] radical-polar crossover mechanism to generate γ-lactams in good...

Impact of the Difluoromethylene Group in the Organocatalyzed Acylative Kinetic Resolution of α,α-Difluorohydrins

Due to the omnipresence of chiral organofluorine compounds in pharmaceutical, agrochemical, and material chemistry, the development of enantioselective methods for their preparation is highly desirable. In the present study, the enantioselective organocatalyzed acylation of α,α-difluorohydrins using a commercially available chiral isothiourea is reported through a kinetic resolution (KR) process. It reveals that the difluoromethylene moiety (C(sp³ )F₂ ) can serve as a directing group through electrostatic fluorine-cation interactions, greatly improving the enantioselectivity of the KR. In this context, a broad range of fluorinated alcohols such as valuable 4,4-difluoro-1,3-diols could be synthesized with exquisite enantiocontrol (typically >99:1 er). Turning to 2,2-difluoro-1,3-diols, we also demonstrated that aromatic and fluorinated groups were mutually compatible to provide the expected enantioenriched adducts with >99:1 er.

Isothiourea-Catalyzed Enantioselective α-Alkylation of Esters via 1,6-Conjugate Addition to para-Quinone Methides

The isothiourea-catalyzed enantioselective 1,6-conjugate addition of para-nitrophenyl esters to 2,6-disubstituted para-quinone methides is reported. para-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the para-nitrophenyl ester, is proposed to facilitate catalyst turnover in this transformation. A range of para-nitrophenyl ester products can be isolated, or derivatized in situ by addition of benzylamine to give amides at up to 99% yield. Although low diastereocontrol is observed, the diastereoisomeric ester products are separable and formed with high enantiocontrol (up to 94:6 er).

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Enantioselective Synthesis of α-Aryl-β2 -Amino-Esters by Cooperative Isothiourea and Brønsted Acid Catalysis

The synthesis of α-aryl-β2 -amino esters through enantioselective aminomethylation of an arylacetic acid ester in high yields and enantioselectivity via cooperative isothiourea and Brønsted acid catalysis is demonstrated. The scope and limitations of this process are explored (25 examples, up to 94 % yield and 96:4 er), with applications to the synthesis of (S)-Venlafaxine⋅HCl and (S)-Nakinadine B. Mechanistic studies are consistent with a C(1)-ammonium enolate pathway being followed rather than an alternative dynamic kinetic resolution process. Control studies indicate that (i) a linear effect between catalyst and product er is observed; (ii) an acyl ammonium ion can be used as a precatalyst; (iii) reversible isothiourea addition to an in situ generated iminium ion leads to an off-cycle intermediate that can be used as a productive precatalyst.

Chalcogen⋅⋅⋅π Bonding Catalysis

While the presence of sulfur⋅⋅⋅π bonding interaction is a general phenomenon in the biological systems, the exploitation of this noncovalent force in a chemical process yet remains elusive. Herein, we describe the concept of chalcogen⋅⋅⋅π bonding catalysis that activates molecules of π systems through the interaction between chalcogen and π-electron cloud. The proof-of-concept studies using a vinylindole-based Diels-Alder benchmark reaction demonstrate that S⋅⋅⋅π and Se⋅⋅⋅π bonding interaction can drive the cycloaddition reaction efficiently. Experimental results suggest that a simultaneously double Se⋅⋅⋅π bonding interaction directs the stereoselectivity in this cycloaddition process.

α,β-Unsaturated acyl ammonium species as reactive intermediates in organocatalysis: an update

α,β-Unsaturated acyl ammonium species are versatile intermediates that have been applied in a variety of transformations including Michael additions, domino reactions and cycloadditions. Many of these transformations are promoted by chiral Lewis base catalysts, enabling the rapid generation of molecular complexity with high stereochemical control. This review highlights recent developments in the generation and application of α,β-unsaturated acyl ammonium intermediates reported since a previous review of this area in 2016. Particular emphasis will be placed on reports providing mechanistic insight into catalytic transformations and observed selectivities. A perspective on current challenges and potential future developments in the field of α,β-unsaturated acyl ammonium catalysis is also provided.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Isothiourea-Catalyzed Acylative Kinetic Resolution of Tertiary α-Hydroxy Esters

A highly enantioselective isothiourea-catalyzed acylative kinetic resolution (KR) of acyclic tertiary alcohols has been developed. Selectivity factors of up to 200 were achieved for the KR of tertiary alcohols bearing an adjacent ester substituent, with both reaction conversion and enantioselectivity found to be sensitive to the steric and electronic environment at the stereogenic tertiary carbinol centre. For more sterically congested alcohols, the use of a recently-developed isoselenourea catalyst was optimal, with equivalent enantioselectivity but higher conversion achieved in comparison to the isothiourea HyperBTM. Diastereomeric acylation transition state models are proposed to rationalize the origins of enantiodiscrimination in this process. This KR procedure was also translated to a continuous-flow process using a polymer-supported variant of the catalyst.

Exploring the Scope of Tandem Palladium and Isothiourea Relay Catalysis for the Synthesis of α-Amino Acid Derivatives

The scope and limitations of a tandem N-allylation/[2,3]-rearrangement protocol are investigated through the incorporation of a variety of functional groups within an allylic phosphate precursor. This method uses readily accessible N,N-dimethylglycine aryl esters and functionalized allylic phosphates, forming quaternary ammonium salts in situ in the presence of a palladium catalyst. Subsequent enantioselective [2,3]-sigmatropic rearrangement, promoted by the chiral isothiourea tetramisole, generates α-amino acid derivatives with two contiguous stereocenters. The incorporation of electron-withdrawing ester and amide groups gave the best results, furnishing the desired products in moderate to good yields (29–70%), with low diastereocontrol (typically 60:40 dr) but high enantioselectivity (up to 90:10 er). These results indicate that substrate–catalyst interactions in the proposed transition state are sensitive to the substitution pattern of the substrates.