Controlling Stereoselectivity with Noncovalent Interactions in Chiral Phosphoric Acid Organocatalysis

Chiral phosphoric acids (CPAs) have emerged as highly effective Brønsted acid catalysts in an expanding range of asymmetric transformations, often through novel multifunctional substrate activation modes. Versatile and broadly appealing, these catalysts benefit from modular and tunable structures, and compatibility with additives. Given the unique types of noncovalent interactions (NCIs) that can be established between CPAs and various reactants─such as hydrogen bonding, aromatic interactions, and van der Waals forces─it is unsurprising that these catalyst systems have become a promising approach for accessing diverse chiral product outcomes. This review aims to provide an in-depth exploration of the mechanisms by which CPAs impart stereoselectivity, positioning NCIs as the central feature that connects a broad spectrum of catalytic reactions. Spanning literature from 2004 to 2024, it covers nucleophilic additions, radical transformations, and atroposelective bond formations, highlighting the applicability of CPA organocatalysis. Special emphasis is placed on the structural and mechanistic features that govern CPA-substrate interactions, as well as the tools and techniques developed to enhance our understanding of their catalytic behavior. In addition to emphasizing mechanistic details and stereocontrolling elements in individual reactions, we have carefully structured this review to provide a natural progression from these specifics to a broader, class-level perspective. Overall, these findings underscore the critical role of NCIs in CPA catalysis and their significant contributions to advancing asymmetric synthesis.

Recent advances in organocatalytic atroposelective reactions

Axial chirality is present in a variety of naturally occurring compounds, and is becoming increasingly relevant also in medicine. Many axially chiral compounds are important as catalysts in asymmetric catalysis or have chiroptical properties. This review overviews recent progress in the synthesis of axially chiral compounds via asymmetric organocatalysis. Atroposelective organocatalytic reactions are discussed according to the dominant catalyst activation mode. For covalent organocatalysis, the typical enamine and iminium modes are presented, followed by N-heterocyclic carbene-catalyzed reactions. The bulk of the review is devoted to non-covalent activation, where chiral Brønsted acids feature as the most prolific catalytic structure. The last part of the article discusses hydrogen-bond-donating catalysts and other catalyst motifs such as phase-transfer catalysts.

Asymmetric Synthesis of Axially Chiral N, N-1,2-Pentatomic Heterobiaryl Diamines by an Organocatalytic Arylation Reaction

The utilization of axially chiral biaryl diamines has been widely acknowledged as highly advantageous structures for the advancement of chiral catalysts and ligands. This highlights their extensive range of applications in asymmetric catalysis and synthesis. Herein, we devised a direct arylation reactions of 5-aminopyrazoles with azonaphthalenes, utilizing chiral phosphoric acid as the catalyst. This method delivers structurally novel atroposelective N, N-1,2-azole heteroaryl diamines with high yields (up to >98 %) and good to excellent enantiomeric ratios while exhibiting a wide range of substrate compatibility.

Asymmetric synthesis of atropisomeric arylpyrazoles via direct arylation of 5-aminopyrazoles with naphthoquinones

Construction of axially chiral arylpyrazoles represents an attractive challenge due to the relatively low rotational barrier of biaryl structures containing five-membered heterocycles. This work describes the catalytic asymmetric construction of axially chiral arylpyrazoles using 5-aminopyrazoles and naphthoquinone derivatives. The chiral axis could be formed through a central-to-axial chirality relay step of the chiral phosphoric acid-catalyzed arylation reaction, which features excellent yields and enantioselectivities with a broad substrate scope under mild reaction conditions.

Catalytic Enantioselective Synthesis of Axially Chiral Imidazoles by Cation-Directed Desymmetrization

Axially chiral five-membered heterobiaryls synthesized by enantioselective catalysis typically feature large ortho-substituents or a heteroatom in the chiral axis to maintain a stable configuration. Herein we report a cation-directed catalytic enantioselective desymmetrization method that enables rapid access to axially chiral imidazoles with the basic nitrogen at the ortho position and efficiently integrates π-stacking moieties to ensure a stable axial configuration for further applications. The process is operationally simple, is highly enantioselective, and can be performed on the gram scale. The majority of the products are obtained in >90% ee, but interestingly even those with only moderate ee can readily be enriched to near optical purity by selective racemate crystallization. Together with a mild phosphine oxide reduction method, axially chiral imidazoles such as StackPhos and its derivatives are readily prepared in high yield and excellent enantioselectivity on the gram scale. The method also enables the preparation of new chiral non-phosphine-bearing imidazoles.

Asymmetric Synthesis of Axially Chiral Arylpyrazole via an Organocatalytic Arylation Reaction

Herein, a highly enantioselective arylation reaction of 3-aryl-5-aminopyrazoles and quinone derivatives was realized using a chiral phosphoric acid catalyst under mild conditions. The reaction has a broad scope with respect to both arylation reaction partners and hence offers rapid access to an array of axially chiral arylpyrazoles with pretty outcomes (up to 95% yield and 99% ee). Notably, the reaction is very efficient, as the catalyst loadings for the model reaction can be reduced to 1 mol % and the enantioselectivity is still maintained. Besides, the synthetic utility of the protocol was proven by a gram-scale reaction and the transformation of the product.

Towards Axially Chiral Pyrazole-Based Phosphorus Scaffolds by Dipeptide-Phosphonium Salt Catalysis

Given the comparatively lower rotational barriers, the catalytic asymmetric construction of axially chiral biaryl structures, especially those containing a five-membered heterocycle, still remains a challenge. Herein, we described a general and modular protocol to access atropisomeric arylpyrazole scaffolds containing a phosphorus unit by a dipeptide phosphonium salt catalyzed reaction involving an oxidative central-to-axial chirality conversion. This reaction features excellent yields and enantioselectivities, broad substrate scope, and a low catalyst loading, delivering axially chiral phosphine compounds.

Enantioselective construction of triaryl-substituted all-carbon quaternary stereocenters via organocatalytic arylation of oxindoles with azonaphthalenes

Azonaphthalenes have been verified as a class of effective arylation reagents in a variety of asymmetric transformations. Here a highly efficient approach to construct triaryl-substituted all-carbon quaternary stereocenters through chiral phosphoric acid-catalyzed enantioselective arylation of 3-aryl-2-oxindoles with azonaphthalenes is disclosed. This chemistry is scalable and displays excellent functional group tolerance, furnishing a series of 3,3-disubstituted 2-oxindole derivatives in good yields with excellent enantiocontrol. Preliminary mechanistic data suggest that the initially formed direct addition intermediate undergoes intramolecular annulation under acidic reaction conditions.

Diversity synthesis of indole derivatives via catalyst control cyclization reaction of 2-indolylmethanols and azonaphthalene

A series of indole-fused scaffolds and derivatives was synthesized via the cyclization reaction of 2-indolylmethanols with azonaphthalene. These reactions were realized under mild reaction conditions through catalyst control, providing structurally diverse indole derivatives with moderate to excellent yields. This protocol also shows good substrate adaptability, especially in six-membered ring products.

Catalytic asymmetric amination of azlactones with azobenzenes

We reported an efficient asymmetric amination of azlactones with N-aryl-N-aroyldiazenes through a chiral N,N'-dioxide-based Lewis acid catalyst. The multicoordination ability of Nd(III) enabled it to simultaneously activate and to locate the two reactants for N-selective addition. Hydrazine-bearing azlactone derivatives were obtained in moderate to good yields with high enantioselectivity.

Copper-Catalyzed Cascade Annulation of o-Hydroxyphenyl Propargylamines with Pyrazolin-5-ones to Access Pyrano[2,3-c]pyrazoles

An efficient copper-catalyzed cascade annulation of o-hydroxyphenyl propargylamines and pyrazolin-5-ones is described. This methodology leads to the rapid assembly of a series of valuable pyrano[2,3-c]pyrazoles with good yields across a wide range of substrates in a simple fashion. This novel reaction involves the formation of alkynyl ortho-quinone methides, a 1,4-conjugate addition, and a subsequent 6-endo cyclization process. The mechanistic elucidation is well supported by control experiment and literature precedents.

Organocatalytic atroposelective construction of axially chiral N, N- and N, S-1,2-azoles through novel ring formation approach

1,2-Azoles are privileged structures in ligand/catalyst design and widely exist in many important natural products and drugs. In this report, two types of axially chiral 1,2-azoles (naphthyl-isothiazole S-oxides with a stereogenic sulfur center and atropoisomeric naphthyl pyrazoles) are synthesized via modified vinylidene ortho-quinone methide intermediates. Diverse products are acquired in satisfying yields and good to excellent enantioselectivities. The vinylidene ortho-quinone methide intermediates bearing two hetero atoms at 5-position have been demonstrated as a platform molecule for the atroposelective synthesis of axially chiral 1,2-azoles. This finding not only enrich our knowledge of vinylidene ortho-quinone methide chemistry but also provide the easy preparation method for diverse atropisomeric heterobiaryls that were inaccessible by existing methodologies. The obtained chiral naphthyl-isothiazole S-oxides and naphthyl-pyrazoles have demonstrated their potential application in further synthetic transformations and therapeutic agents.

Recent advances in the applications of pyrazolone derivatives in enantioselective synthesis

Pyrazolones and pyrazoles, featuring nitrogen-nitrogen bonds, are two of the most important classes of heterocycles, owing to their widespread occurrence in medicinal chemistry and functional materials. The last decade has witnessed a rapid increase in the construction of chiral pyrazolone and pyrazole derivatives, with the application of pyrazolone derivatives as powerful synthons. Since our last review in 2018, a large number of new achievements has emerged in this area, requiring a timely update. Thus, this review summarizes these elegant achievements based on the multiple reactive sites of different pyrazolone synthons. In addition, important mechanisms and interesting biological investigations relating to the corresponding products are also discussed.

Facile synthesis of N-aryl phenothiazines and phenoxazines via Brønsted acid catalyzed C-H amination of arenes

N-Aryl phenothiazines and phenoxazines are of significant importance in various disciplines throughout academia and industry. The conventional synthetic strategy for the construction of these structures centers on the transition-metal-catalyzed cross-coupling of aryl halides with phenothiazines or phenoxazines. Here we present an organocatalytic approach to access N-naphthyl phenothiazine and phenoxazine scaffolds through a straightforward C-H amination of arenes as enabled by an azo group. This reaction features operational simplicity, adequate substrate generality and excellent functional group compatibility. Notably, the efficiency of the catalyst could be perfectly preserved after 5 catalytic cycles.

Synthesis of axially chiral N-aryl benzimidazoles via chiral phosphoric acid catalyzed enantioselective oxidative aromatization

Hantzsch ester and benzothiazoline have been extensively used as H2 donors in enantioselective transfer hydrogenation of C=N bond during the past several decades. However, the corresponding Hantzsch pyridine and benzothiazoles...

Nitrosobenzene-Enabled Chiral Phosphoric Acid Catalyzed Enantioselective Construction of Atropisomeric N-Arylbenzimidazoles

Described herein is an imidazole ring formation strategy for the synthesis of axially chiral N-arylbenzimidazoles by means of chiral phosphoric acid catalysis. Two sets of conditions were developed to transform two classes of 2-naphthylamine derivatives into structurally diverse N-arylbenzimidazole atropisomers with excellent chemo- and regioselectivity as well as high levels of enantiocontrol. It is worth reflecting on the unique roles played by the nitroso group in this domino reaction. It functions as a linchpin by first offering an electrophilic site (N) for the initial C-N bond formation while the resulting amine performs the nucleophilic addition to form the second C-N bond. Additionally, it could facilitate the final oxidative aromatization as an oxidant. The atropisomeric products could be conveniently elaborated to a series of axially chiral derivatives, enabling the exploitation of N-arylbenzimidazoles for their potential utilities in asymmetric catalysis.

Recent advances in the asymmetric phosphoric acid-catalyzed synthesis of axially chiral compounds

In recent years, the synthesis of axially chiral compounds has received considerable attention due to their extensive application as biologically active compounds in medicinal chemistry and as chiral ligands in asymmetric catalysis. Chiral phosphoric acids are recognized as efficient organocatalysts for a variety of enantioselective transformations. In this review, we summarize the recent development of chiral phosphoric acid-catalyzed synthesis of a wide range of axially chiral biaryls, heterobiaryls, vinylarenes, N-arylamines, spiranes, and allenes with high efficiency and excellent stereoselectivity.

Access to Axially and Centrally Chiral Sulfinamides via Asymmetric Allylic Alkylation

Herein, access to axially and centrally chiral sulfinamides via asymmetric allylic alkylation was reported. A series of sulfinamides were obtained with good outcomes (up to 99% yield, >19:1 dr, and 98:2 er). The synthetic utility of the reaction was demonstrated by scaled-up synthesis, product transformation, and application as a catalyst in asymmetric catalysis.

Enantioselective Aromatic Amination?

The atroposelective formation of C-N bonds has recently emerged within the field of amination reactions. On first sight, it may seem quite surprising that such an ancient class of organic coupling reactions (Gabriel, Ullmann, Goldberg, Buchwald, Hartwig and many others) has so few enantioselective solutions, and this in spite of asymmetric synthesis being now a mature concept and field. Why should enantioselective C-N bond formation be so difficult? This question and some of the first examples that promise an imminent change of paradigm are herein discussed.

Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Atropisomerism is a stereochemical behavior portrayed by three-dimensional molecules that bear rotationally restricted σ bond. Akin to the well-represented point-chiral molecules, atropisomerically chiral compounds are finding increasing utilities in many disciplines where molecular asymmetry is influential. This provides steady demand on atroposelective synthesis, where numerous synthetic pursuits have been rewarded with conceptually novel and streamlined methods while expanding the structural diversity of atropisomers. This review summarizes key achievements in stereoselective preparation of biaryl, heterobiaryl, and nonbiaryl atropisomers documented between 2015 and 2020. Emphasis is placed on the synthetic strategies for each structural class, while examples are cited to illustrate the potential applications of the accessed atropochiral targets.

Organocatalytic Regioselective [3+2] Annulation of Morita-Baylis-Hillman Carbonates with Azonaphthalenes: An Efficient Access to 3-Spiropyrazole- 2-oxindoles

Background:

A cheap and commercially available organocatalyst, 4-dimethylaminopyridine was successfully employed in the regioselective [3+2] cycloaddition of isatin-derived Morita-Baylis- Hillman carbonates with azonaphthalenes for the construction of 3-spiropyrazole-2-oxindoles in excellent yields under mild conditions.

Methods:

In the presence of 4-dimethylaminopyridine with a loading of 10 mol%, a series of isatinderived Morita-Baylis-Hillman carbonates reacted smoothly with azonaphthalenes in dichloromethane at room temperature to furnish 3-spiropyrazole-2-oxindoles in 72-98% yield.

Result and Conclusion:

In summary, we have developed reasonably cheap and commercially available 4-dimethylaminopyridine-mediated regioselective [3+2] annulations between isatin-derived Morita- Baylis-Hillman carbonates and azonaphthalenes for the construction of 3-spiropyrazole-2-oxindoles under mild conditions.

Organocatalytic Asymmetric Methodologies towards the Synthesis of Atropisomeric N-Heterocycles

A perspective on the literature dealing with the organocatalytic asymmetric preparation of axially chiral N-heterocycles is provided. A particular focus is devoted to rationalize the synthetic strategies employed in each case. Moreover, specific classes of organocatalysts are shown to stand out as privileged motives for the stereoselective preparation of such synthetically challenging molecular architectures. Finally, an overview of the main trends in the field is given.

1 Introduction

2 Five-Membered Rings

2.1 Arylation

2.2 Dynamic Kinetic Resolution

2.3 Ring Construction

2.4 Central-to-Axial Chirality Conversion and Chirality Transfer

2.5 Desymmetrization

3 Six-Membered Rings

3.1 Desymmetrization

3.2 (Dynamic) Kinetic Resolution

3.3 Ring Construction

3.4 Central-to-Axial Chirality Conversion

4 Conclusion

Enantioselective dearomative [3 + 2] annulation of 3-hydroxy chromanones with azonaphthalenes

Enantioselective dearomative [3 + 2] annulation of 3-hydroxy chromanones with azonaphthalenes provided a large variety of chromanone fused indolines in moderate to good yields with generally good enantioselectivities.

A Rh(ii)/phosphoric acid co-catalyzed three-component reaction of diazo-ketones with alcohols and azonaphthalenes: access to indole derivatives via a formal [3 + 2]-cycloaddition

Rh(ii)/Phosphoric Acid co-catalyzed muti-component annulation was well established as a new efficient route to construct 1-amino-indole derivatives via dearomatization/rearomatization/cyclization process with in situ formed oxonium ylide.