Combining Organocatalysis with Central-to-Axial Chirality Conversion: Atroposelective Hantzsch-Type Synthesis of 4-Arylpyridines

Enantioselective Construction of Quinoxaline-Based Heterobiaryls and P,N-Ligands via Chirality Transfer Strategy

Central-to-axial chirality transfer via C-N single bond oxidation was first achieved as a versatile and conceptually distinct strategy to prepare a new family of axially chiral heteroaromatic biaryl backbones and P,N-ligands (named as Quinoxalinaps) in gram scale. Two atropisomers of Quinoxalinaps (ee >99%) were readily accessed from the same precursor enantiomer by a simple dehydrogenative oxidation with MnO₂ and t-BuOOH under mild conditions. Phosphine could be introduced into the ligands before or after the chirality control process. Moreover, these Quinoxalinap P,N-ligands performed well for both asymmetric reactions of the CuBr-catalyzed alkyne conjugate addition with up to -94% ee and AgOAc-catalyzed glycinate imine [3 + 2] annulation with 90% ee, respectively.

Recent Progress towards Organocatalyzed Asymmetric (Hetero)Arene Formation

Owing to the importance of arene moieties in organic chemistry, methods for arene construction attract great attention. Besides the traditional substitution strategy from pre-existing arenes, the straightforward formation of arene cores can also provide significant shortcuts towards a wide array of target molecules with different substitution patterns. Among direct arene formation reactions, applying environmentally benign organocatalysis to access arene moieties continues to attract increasing attention. This short review provides a brief summary of recent progress on organocatalyzed de novo (hetero)arene formation and applications in enantioselective synthesis.

1 Introduction

2 Arene Formation with Non-Covalent Organocatalysts

2.1 Arene Formation with Acid Organocatalysts

2.2 Arene Formation with Base Organocatalysts

2.3 Arene Formation with Hydrogen-Bonding Organocatalysts

3 Arene Formation with Covalent Organocatalysts

3.1 Arene Formation with Lewis Base Organocatalysts

3.1.1 Arene Formation with Secondary Amine Organocatalysts

3.1.2 Arene Formation with Tertiary Amine, Tertiary Phosphine and Sulfide Organocatalysts

3.1.3 Arene Formation with N-Heterocyclic Carbene Organocatalysts

3.2 Arene Formation with Lewis Acid Organocatalysts

4 Conclusion

A one-pot stepwise approach to axially chiral quinoline-3-carbaldehydes enabled by iminium–allenamine cascade catalysis

An organocatalytic atroposelective annulation between 2-(tosylamino)aryl ketones and 2-alkynals for the construction of enantioenriched axially chiral 4-arylquinoline-3-carbaldehydes is achieved.

Catalytic asymmetric C-Si bond activation via torsional strain-promoted Rh-catalyzed aryl-Narasaka acylation

Atropisomers are important organic frameworks in bioactive natural products, drugs as well as chiral catalysts. Meanwhile, silanols display unique properties compared to their alcohol analogs, however, the catalytic synthesis of atropisomers bearing silanol groups is challenging. Here, we show a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers. The reaction features a dynamic kinetic resolution of C(Ar)-Si bond cleavage, whose stereochemistry was controlled by a phosphoramidite ligand derived from (S)-3-methyl-1-((2,4,6-triisopropylphenyl)sulfonyl)piperazine. This work is a demonstration of an aryl-Narasaka acylation, where the C(Ar)-Si bond cleavage is promoted by the torsional strain of α, α’-disubstituted silafluorene.

Methods to access atropisomers bearing silanol groups are of value in organic synthesis. Here, the authors report a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers.

Construction of Axially Chiral Compounds via Central-to-Axial Chirality Conversion

Central-to-axial chirality conversion represents a fascinating class of chemical processes consisting of the destruction of stereogenic centers and the simultaneous installation of axial chiral elements, which provides efficient methods for the preparation of axially chiral compounds. Using the strategy, a wide range of axially chiral compounds, including biaryls, heterobiaryls, aromatic amides, allenes and vinyl arenes, have been synthesized with high efficiency and excellent enantioselectivity. In addition, central-to-axial chirality conversion strategy has been applied to the synthesis of natural products. The strategy has undoubtedly become and will continue to be a hot research topic in the field of asymmetric catalysis and synthesis. In this minireview, we selected some examples to introduce the developments and trends in the central-to-axial chirality conversion strategy up to April 2020.

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

Organocatalytic Enantioselective Synthesis of Atropisomeric Aryl-p-Quinones: Platform Molecules for Diversity-Oriented Synthesis of Biaryldiols

Presented here is a class of novel axially chiral aryl-p-quinones as platform molecules for the preparation of non-C₂ symmetric biaryldiols. Two sets of aryl-p-quinone frameworks were synthesized with remarkable enantiocontrol by means of chiral phosphoric acid catalyzed enantioselective arylation of p-quinones by central-to-axial chirality conversion. These aryl-p-quinones were then used to access a wide spectrum of highly functionalized non-C₂ symmetric biaryldiols with excellent retention of the enantiopurity.

Michael Reaction Inspired Atroposelective Construction of Axially Chiral Biaryls

The first copper-catalyzed atroposelective Michael-type addition between azonaphthalenes and arylboronic acids for the construction of biaryl atropisomers was established using a novel BINOL-derived phosphoramidite as a chiral ligand. A broad range of atropisomeric biaryls were obtained with good efficiency, and the practicality of this approach was verified by versatile transformations toward axially chiral ligands, catalysts, and other functional atropisomers. This set of catalytic systems successfully inhibited the routine 1,2-addition and promoted the formation of an aryl-aryl chiral axis. Meanwhile, this strategy bypassed the use of an oxidant as well as the harsh conditions normally necessary for transition-metal-mediated arene C-H coupling with arylboronic acids as an arylation counterpart, offering a straightforward alternative to access optically active biaryls.

Chiral phosphoric acid catalyzed atroposelective and diastereoselective synthesis of 9-aryltetrahydroacridines

A chiral phosphoric acid catalyzed atroposelective annulation between 2-aminoaryl ketones and alicyclic ketones has been developed to access a new library of enantioenriched axially chiral 9-aryltetrahydroacridines.

Bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C-C stereogenic axes

We report the bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C–C stereogenic axes. These are controlled by a two-fold central-to-axial chirality conversion upon oxidative aromatization.

We report the bidirectional enantioselective synthesis of bis-benzofuran atropisomeric oligoarenes featuring two distal C–C stereogenic axes obtained by a two-fold central-to-axial chirality conversion upon oxidative aromatization. The key enantioenriched centrally chiral bis-dihydrobenzofuran precursors were synthesized via a bidirectional diastereo- and enantio-selective organocatalyzed domino reaction between simple achiral and easily accessible dihydroxylated aromatics and chloronitroalkenes. Moreover, the stereodivergent nature of the methodology was established by synthesizing both diastereomers of a non-symmetrically functionalized bis-axially chiral oligoarene.

Chiral Phosphoric Acid-Catalyzed Enantioselective Direct Arylation of Iminoquinones: A Case Study of the Model Selectivity

Chiral phosphoric acid (CPA)-catalyzed enantioselective arylation reactions have attracted immense attention recently. However, the preferential activation model in the stereodetermining step is controversial, and hence, the origin of enantioselectivity is still far from being understood. Two stereochemical models are provided on the basis of the asymmetric arylations of iminoquinones with naphthylamines (reaction 1) or naphthols (reaction 2) catalyzed by (R/S)-TRIP to explain the high enantioselectivity and the effect of CPAs scaffolds. Unexpectedly, our calculations reveal that substrate naphthylamines or naphthols prefer enantioselective aminal formation model II or 1,4-addition model I, respectively, which is the reverse of Tan's and Xu's model. The different noncovalent and steric interactions between catalysts and substrates are responsible for the observed model preference. Moreover, the enantioselectivity arises from distortion (reaction 1) and noncovalent interactions (reaction 2) that discriminate between the diastereomeric transition states. We further investigated the effect of SPINOL-based CPAs on the enantioselectivity and found that the more rigid skeleton and a smaller binding pocket lead to lower enantioselectivity as compared with that of BINOL-based CPA. The new insights into the reaction activation model rationalize the stereoselectivity outcome of direct asymmetric arylation reactions, and our general model can be extended to related transformations.

Synthesis of Polycyclic Spirocarbocycles via Acid-Promoted Ring-Contraction/Dearomative Ring-Closure Cascade of Oxapropellanes

We report herein the development of an acid-promoted rearrangement of oxa[4.3.2]propellanes to afford polyaromatic-fused spiro[4.5]carbocycles. DFT calculations suggest that the reaction pathway involves generation of a cyclobutyl cation, ring contraction to the cyclopropylcarbinyl cation, and dearomative ring closure by an internal 2-naphthol moiety. The resulting spirocarbocycles are synthetically valuable, as they could be transformed into two different polycyclic aromatic hydrocarbons via skeletal rearrangement. Syntheses of optically pure spirocarbocycles via a central-to-axial-to-central chirality transfer are also described.

Organocatalytic atroposelective construction of axially chiral arylquinones

Atropisomeric biaryl motifs are ubiquitous in chiral catalysts and ligands. Numerous efficient strategies have been developed for the synthesis of axially chiral biaryls. In contrast, the asymmetric construction of o-quinone-aryl atropisomers has yet to be realized. Inspired by the rapid progress of the chemistry of biaryls, here we present our initial investigations about the atroposelective construction of axially chiral arylquinones by a bifunctional chiral phosphoric acid-catalyzed asymmetric conjugate addition and central-to-axial chirality conversion. With o-naphthoquinone as both the electrophile and the oxidant, three types of arylation counterparts, namely 2-naphthylamines, 2-naphthols and indoles, are utilized to assemble a series of atropisomeric scaffolds in good yields and excellent enantioselectivities. This approach not only expands the axially chiral library but also offers a route to a class of potential, chiral biomimetic catalysts.

Axially chiral compounds have widespread use in asymmetric catalysis. Here, the authors disclose a highly enantioselective synthesis of axially chiral o-naphthoquinones by chiral phosphoric acid catalysis and central-to-axial chirality conversion.

Conversion of two stereocenters to one or two chiral axes: atroposelective synthesis of 2,3-diarylbenzoindoles

Central-to-axial chirality conversion provides efficient access to axially chiral compounds, and several examples regarding the conversion of one, two or four stereocenters to one axis have been reported. Herein, we report the conversion of two stereocenters to one or two chiral axes for the first time. In this study, a new class of enantiomerically enriched 2,3-diarylbenzoindoles was efficiently synthesized using a chiral phosphoric acid-catalyzed [3 + 2] formal cycloaddition and a mild DDQ oxidation strategy. Moreover, a speculative model of the central-to-axial chirality conversion outcome was proposed based on preliminary mechanistic studies and DFT calculations. Potentially, using this strategy, useful chiral phosphine ligand can be synthesized smoothly (99% ee).

Catalytic Kinetic Resolution by Enantioselective Aromatization: Conversion of Racemic Intermediates of the Barton-Zard Reaction into Enantioenriched 3-Arylpyrroles

Racemic 3,4-dihydro-2H-pyrroles, hypothetical intermediates of the Barton-Zard reaction, were synthesized in a highly diastereoselective manner and fully characterized for the first time. Kinetic resolution of the dihydropyrroles with a quinine-derived thiourea afforded the (+)-3-arylpyrrole products and recovered (+)-3,4-dihydro-2H-pyrroles with high efficiency (s-factor up to 153). The resolved (+)-3,4-dihydro-2H-pyrroles underwent subsequent aromatization with a quinidine-derived thiourea catalyst to afford (-)-3-arylpyrroles with excellent central-to-axial chirality transfer. In contrast to the well-accepted Barton-Zard mechanism, the aromatization of the 3,4-dihydro-2H-pyrroles in the presence of a bifunctional catalyst is believed to proceed by an unprecedented sequence involving syn elimination of HNO₂ and aromatization.