Enantio- and Diastereoselective Carbonyl-Ene Cyclization–Acetalization Tandem Reaction Catalyzed by Tris(pentafluorophenyl)borane-Assisted Chiral Phosphoric Acids

B(C6F5)3/Chiral Phosphoric Acid Promoted Asymmetric C-3 gem-Difluoroalkylation of Quinoxalin-2-ones with Difluoroenoxysilanes

The asymmetric Mannich-type reaction of quinoxalin-2-ones with difluoroenoxysilanes has been developed for the synthesis of chiral gem-difluoroalkylated quinoxalin-2-ones. The reaction worked in the presence of chiral phosphoric acid CPA 1 and B(C6F5)3 in THF at room temperature. The reaction exhibited a good substrate scope furnishing the products in good yields (up to 97%) with up to 96% ee.

B(C6F5)3/CPA-Catalyzed Aza-Diels-Alder Reaction of 3,3-Difluoro-2-Aryl-3H-indoles and Unactivated Dienes

Here we report B(C6F5)3/CPA-catalyzed enantioselective aza-Diels-Alder reaction of 3,3-difluoro-2-Aryl-3H-indoles with unactivated dienes to access chiral 10,10-difluoro-tetrahydropyrido[1,2-a]indoles. This protocol allows the formation of pyrazole-based C2-quaternary indolin-3-ones with high enantioselectivities and regioselectivities. Moreover, gram-scale synthesis of the 10,10-difluoro-tetrahydropyrido[1,2-a]indole skeleton was successfully achieved without any reduction in both yield and enantioselectivity.

Valence-isomer selective cycloaddition reaction of cycloheptatrienes-norcaradienes

The rapid and precise creation of complex molecules while controlling multiple selectivities is the principal objective in synthetic chemistry. Combining data science and organic synthesis to achieve this goal is an emerging trend, but few examples of successful reaction designs are reported. We develop an artificial neural network regression model using bond orbital data to predict chemical reactivities. Actual experimental verification confirms cycloheptatriene-selective [6 + 2]-cycloaddition utilizing nitroso compounds and norcaradiene-selective [4 + 2]-cycloaddition reactions employing benzynes. Additionally, a one-pot asymmetric synthesis is achieved by telescoping the enantioselective dearomatization of non-activated benzenes and cycloadditions. Computational studies provide a rational explanation for the seemingly anomalous occurrence of thermally prohibited suprafacial [6 + 2]-cycloaddition without photoirradiation.

Concerted Hydrosilylation Catalysis by Silica-Immobilized Cyclic Carbonates and Surface Silanols

Developing a method for creating a novel catalysis of organic molecules is essential because of the growing interest in organocatalysis. In this study, we found that cyclic carbonates immobilized on a nonporous or mesoporous silica support showed catalytic activity for hydrosilylation, which was not observed for the free cyclic carbonates, silica supports, or their physical mixture. Analysis of the effects of linker lengths and pore sizes on the catalytic activity and carbonate C=O stretching frequency revealed that the proximity of carbonates and surface silanols was crucial for synergistic hydrosilylation catalysis. A carbonate and silanol concertedly activated the silane and aldehyde for efficient hydride transfer. Density functional theory calculations on a model reaction system demonstrated that both the carbonate and silanol contributed to the stabilization of the transition state of hydride transfer, which resulted in a reasonable barrier height of 16.8 kcal mol-1. Furthermore, SiO2/carbonate(C4) enabled the hydrosilylation of an aldehyde with an amino group without catalyst poisoning, owing to the weak acidity of surface silanols, in sharp contrast to previously developed acid catalysts. This study demonstrates that immobilization on a solid support can convert inactive organic molecules into active and heterogeneous organocatalysts.

Synergistic effect of hydrogen bonds and π-π interactions of B(C6F5)3·H2O/amides complex: Application in photoredox catalysis

B(C₆F₅)₃·H₂O has been long recognized as a common Brønsted acid. The lack of X-ray crystal structure of B(C₆F₅)₃·H₂O with other substrates has greatly limited the development of a new catalytic mode. In this work, a complex of B(C₆F₅)₃·H₂O and amide 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one with hydrogen bonds and π-π interactions is characterized by X-ray diffraction. Such noncovalent interactions in solution also exist, as verified by NMR, UV-Vis absorption, and fluorescence emission measurements. Moreover, the mixture of amide 2-phenyl-3,4-dihydroisoquinolin-1(2H)-one and B(C₆F₅)₃·H₂O, instead of other tested Brønsted acids, shows a tailing absorption band in the visible light region (400-450 nm). Based on the photoactive properties of the complex, a photoredox catalysis is developed to construct α-aminoamides under mild conditions.

Hydrogen Bond Assisted Three-Component Tandem Reactions to Access N-Alkyl-4-Quinolones

Hydrogen-bonding catalytic reactions have gained great interest. Herein, a hydrogen-bond-assisted three-component tandem reaction for the efficient synthesis of N-alkyl-4-quinolones is described. This novel strategy features the first proof of polyphosphate ester (PPE) as a dual hydrogen-bonding catalyst and the use of readily available starting materials for the preparation of N-alkyl-4-quinolones. The method provides a diversity of N-alkyl-4-quinolones in moderate to good yields. The compound 4h demonstrated good neuroprotective activity against N-methyl-ᴅ-aspartate (NMDA)-induced excitotoxicity in PC12 cells.

Catalytic asymmetric synthesis of cannabinoids and menthol from neral

The selective conversion of natural or synthetic neral to (1R,6S)-trans-isopiperitenol would enable and expedite sustainable routes to menthol1,2 and cannabinoids3-5. However, this reaction has been considered impossible because its product is more reactive to the required acid catalysts than its starting material, resulting in several side products6-9. We now show that an unsymmetric, strong and confined chiral acid, a highly fluorinated imino-imidodiphosphate, catalyses this process with excellent efficiency and selectivity. Expanding the method to other α,β-unsaturated aldehydes could enable access to new cannabinoids and menthol derivatives not readily accessible previously. Mechanistic studies suggest that the confined catalyst accomplishes this reaction by binding the product in an unreactive conformation, thereby preventing its decomposition. We also show how (1R,6S)-trans-isopiperitenol can be readily converted to pharmaceutically useful cannabinoids and menthol, each in the shortest and most atom-economic routes so far.

Asymmetric Hydrophosphinylation of Alkynes: Facile Access to Axially Chiral Styrene-Phosphines

A Cu/CPA co-catalytic system has been developed for achieving the direct hydrophosphinylation of alkynes with phosphine oxides in delivering novel axially chiral phosphorus-containing alkenes in high yields and excellent enantioselectivities (up to 99 % yield and 99 % ee). DFT calculations were performed to elucidate the reaction pathway and the origin of enantiocontrol. This streamlined and modular methodology establishes a new platform for the design and application of new axially chiral styrene-phosphine ligands.

Recent Advances in Asymmetric Catalysis Associated with B(C6F5)3

The prevalence and significance of asymmetric catalysis in the modern medicinal industry has been witnessed in recent years, which have already been used to manufacture the (S)-Naproxen and the (S)-Propranolol. With matched specificities such as the Lewis acidity and steric bulk, B(C₆F₅)₃ has gained accelerating attention on its application in asymmetric catalysis of Diels-Alder cycloaddition reactions, carbonyl-ene cyclization, and other various reactions, which have been demonstrated by the elegant examples from the most recent literature. Some significant progress in the reaction of indirect activation of substrates through in situ generation of numerous supramolecular catalysts from B(C₆F₅)₃ based on Lewis-acid-assisted Lewis acid (LLA) or Lewis acid assisted Brønsted acid (LBA) strategies or the reaction promoted by cooperative actions of chiral co-catalysts and B(C₆F₅)₃ which played a direct role on the activation of substrates have been demonstrated in this review.

Multiselective Diels-Alder Reaction of α-Arylacroleins Catalyzed by Boron Tribromide-Assisted Chiral Phosphoric Acids

A multiselective Diels-Alder (DA) reaction of α-arylacroleins with cyclopentadiene using BBr₃-assisted chiral BINOL-derived phosphoric acid catalysts has been developed. This unusual exo- and enantioselective DA reaction can be multicontrolled by the chiral cavity of the in situ-formed acid-base cooperative catalysts, in particular, suppressing the competitive hetero Diels-Alder (HDA) reaction effectively.

Enantioselective Friedel-Crafts Reaction of 2-Alkynyphenols with Aromatic Ethers by Chiral Brønsted Acid Catalysis

Herein, we report chiral strong Brønsted acid-catalyzed enantioselective Friedel-Crafts reaction of 2-alkynyphenols with aromatic ethers. The reaction affords the corresponding axially chiral styrenes in up to 91% yield and 97% ee.

An overview of the applications of chiral phosphoric acid organocatalysts in enantioselective additions to CO and CN bonds

Since 2004, chiral phosphoric acids (CPAs) have emerged as highyl efficient organocatalysts, providing excellent results in a wide reaction scope. In this review, the applications of CPA for enantioselective additions to CO and CN bonds are covered.

Enantioselective access to tricyclic tetrahydropyran derivatives by a remote hydrogen bonding mediated intramolecular IEDHDA reaction

Inverse-electron-demand-hetero-Diels-Alder reactions of alkenes with α,β-unsaturated keto compounds allow rapid access to the tetrahydropyran ring found in numerous natural products and bioactive molecules. Despite its synthetic interest, catalytic asymmetric versions of this process remain underdeveloped, especially regarding the use of non-activated alkenes reacting with α,β-unsaturated ketone or aldehyde, for which no report can be found in the literature. Herein, we describe the catalytic inverse-electron-demand-hetero-Diels-Alder reactions between neutral alkenes and an α,β-unsaturated ketones or aldehydes to produce a variety of trans-fused [5,6,8] tricyclic structures containing a central, chiral tetrahydropyran ring. This complex transformation, which is achieved using a chiral phosphoric acid, allows for the formation of four stereogenic centers in a single step with high regio-, diastereo- and enantioselectivity (up to 99% ee). Such level of stereocontrol could be achieved by a key remote double hydrogen atom bonding interaction between the linear substrate and the catalyst.

Although the hetero-Diels–Alder reaction is a staple of organic chemistry, catalytic asymmetric versions of the inverse-electron demand variant often require specially engineered substrates for the reaction to work. Here the authors cyclize non-activated alkenes with α,β-unsaturated ketones or aldehydes to form chiral fused heterocycles using a chiral phosphoric acid catalyst.

Cooperative Hydrogen-Bond-Donor Catalysis with Hydrogen Chloride Enables Highly Enantioselective Prins Cyclization Reactions

Cooperative asymmetric catalysis with hydrogen chloride (HCl) and chiral dual-hydrogen-bond donors (HBDs) is applied successfully to highly enantioselective Prins cyclization reactions of a wide variety of simple alkenyl aldehydes. The optimal chiral catalysts were designed to withstand the strongly acidic reaction conditions and were found to induce rate accelerations of 2 orders of magnitude over reactions catalyzed by HCl alone. We propose that the combination of strong mineral acids and chiral hydrogen-bond-donor catalysts may represent a general strategy for inducing enantioselectivity in reactions that require highly acidic conditions.

Catalytic Asymmetric Aza-Diels-Alder Reaction of Ketimines and Unactivated Dienes

The enantioselective aza-Diels-Alder reaction is efficient for constructing chiral tetrahydropyridines, but the catalytic asymmetric aza-Diels-Alder reaction of ketimines with unactivated dienes is still a challenging topic. Herein, guided by computational screening, a highly enantioselective aza-Diels-Alder reaction of 2-aryl-3H-indol-3-ones with unactivated dienes was realized by using a B(C₆ F₅ )₃ /chiral phosphoric acid catalyst system under mild conditions. The reaction has a broad scope with respect to both aza-Diels-Alder reaction partners and hence offers rapid access to an array of tetrahydropyridine derivatives with pretty outcomes (up to 99 % yield, >20:1 dr and 98:2 er). The reaction is very efficient: lowering catalyst loadings for the model reaction to 0.1 mol %, enantioselectivity is still maintained. The synthetic utility was confirmed by transformations of the products. DFT calculations provide convincing evidence for the interpretation of stereoselection.