Asymmetrische Organokatalyse: α,β‐ungesättigte Acylammoniumsalze erweisen sich als immer nützlicher

Design, Preparation, and Implementation of Axially Chiral Benzotetramisoles as Lewis Base Catalysts for Asymmetric Cycloadditions

Developing novel catalysts with potent activity is of great importance in organocatalysis. In this study, we designed and prepared a new class of benzotetramisole Lewis base catalysts (AxBTM) that have both central and axial chirality. This unique feature of these catalysts results in a three-dimensional microenvironment with multi-layers of chirality. The performance of the developed catalysts was tested in a series of cycloaddition reactions. These included the AxBTM-catalyzed (2+2) cycloaddition between α-fluoro-α-aryl anhydride with imines or oxindoles, and the sequential gold/AxBTM-catalyzed (4+2) cycloaddition of enynamides with pentafluorophenyl esters. The interplay between axial and central chirality had a collaborative effect in regulating the stereochemistry in these cycloadditions, leading to high levels of stereoselectivity that would otherwise be challenging to achieve using conventional BTM catalysts. However, the (2+2) and (4+2) cycloadditions have different predilections for axial and central chirality combinations.

Lewis Base Catalyzed Synthesis of Sulfur Heterocycles via the C1‐Pyridinium Enolate

While the addition of C1‐Lewis base enolates to carbonyls and related structures are well established, the related addition to thiocarbonyls compounds are unknown. Herein, we report a reaction cascade in which a C1‐pyridinium enolate undergos addition to dithioesters, trithiocarbonates and xanthates. The reaction provides access to a range of dihydrothiophenes and dihydrothiopyrans (28‐examples). Mechanistic investigations, including isolation of intermediates, electronic correlation, and kinetic isotope effect studies support the viability of an activated acid intermediate giving rise to the C1‐pyridinium enolate which undergoes turnover limiting cyclization. Subsequent formation of a β‐thiolactone regenerates the catalyst with loss of carbon oxysulfide providing the observed products.

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.

Revealing the Similarities of α,β-Unsaturated Iminiums and Acylazoliums in Organocatalysis

The secondary amine-catalyzed reactions proceeding via α,β-unsaturated iminiums and the N-heterocyclic carbene (NHC)-catalyzed transformations taking place via α,β-unsaturated acylazoliums are the two widely used electrophilic intermediates in organocatalysis. Over the last two decades, these two intermediates are extensively utilized for the enantioselective construction of valuable molecules. Both intermediates are generated by the covalent binding of catalysts to the substrates leading to LUMO activation of α,β-unsaturated carbonyls. A variety of soft nucleophiles are known to add to the α,β-unsaturated iminiums and acylazoliums in a conjugate fashion, and in many cases, striking similarity in reactivity has been observed. Having said this, there are few cases where these intermediates exhibit difference in reactivity. This Minireview is aimed at highlighting the resemblances in reactivity between α,β-unsaturated iminiums and acylazoliums thereby shedding light on the unnoticed parallels of the two intermediates in organocatalysis.

N-Heterocyclic Carbene (NHC)-Organocatalyzed Kinetic Resolutions, Dynamic Kinetic Resolutions, and Desymmetrizations

The last couple of decades have witnessed tremendous development within N-heterocyclic carbene (NHC) organocatalysis. NHCs have been used as powerful organic catalysts in asymmetric synthesis. Although great achievements have been made in asymmetric NHC catalysis, their applications in kinetic resolution (KR), dynamic kinetic resolution (DKR), and desymmetrization processes have been relatively less developed. Moreover, limited activation modes have been involved in these processes. This review provides an overview of the NHC-organocatalyzed KR, DKR, and desymmetrization reactions in the preparation of chiral functional molecules. The aim is to highlight both the importance and elegance of these methods in the construction of challenging chiral compounds and to provide our own perspective on future development in this direction.

Enantioselective Synthesis of Medium-Sized Lactams via Chiral α,β-Unsaturated Acylammonium Salts

Medium-sized lactams are important structural motifs found in a variety of bioactive compounds and natural products but are challenging to prepare, especially in optically active form. A Michael addition/proton transfer/lactamization organocascade process is described that delivers medium-sized lactams, including azepanones, benzazepinones, azocanones, and benzazocinones, in high enantiopurity through the intermediacy of chiral α,β-unsaturated acylammonium salts. An unexpected indoline synthesis was also uncovered, and the benzazocinone skeleton was transformed into other complex heterocyclic derivatives, including spiroglutarimides, isoquinolinones, and δ-lactones.

Aryloxide-Facilitated Catalyst Turnover in Enantioselective α,β-Unsaturated Acyl Ammonium Catalysis

A new general concept for α,β-unsaturated acyl ammonium catalysis is reported that uses p-nitrophenoxide release from an α,β-unsaturated p-nitrophenyl ester substrate to facilitate catalyst turnover. This method was used for the enantioselective isothiourea-catalyzed Michael addition of nitroalkanes to α,β-unsaturated p-nitrophenyl esters in generally good yield and with excellent enantioselectivity (27 examples, up to 79 % yield, 99:1 er). Mechanistic studies identified rapid and reversible catalyst acylation by the α,β-unsaturated p-nitrophenyl ester, and a recently reported variable-time normalization kinetic analysis method was used to delineate the complex reaction kinetics.

Construction of Fused Pyrrolidines and β-Lactones by Carbene-Catalyzed C-N, C-C, and C-O Bond Formations

A carbene-catalyzed intermolecular C-N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon-heteroatom bond-forming reactions.