Chiral Brønsted Acid Catalyzed Enantioselective Aza-Diels–Alder Reaction of Brassard's Diene with Imines

General, Modular Access toward Immobilized Chiral Phosphoric Acid Catalysts and Their Application in Flow Chemistry

Chiral phosphoric acids (CPAs) are among the most frequently used organocatalysts, with an ever-increasing number of applications. However, these catalysts are only obtained in a multistep synthesis and are poorly recyclable, which significantly deteriorates their environmental and economic performance. We herein report a conceptually different, general strategy for the direct immobilization of CPAs on a broad scope of solid supports including silica, polystyrene, and aluminum oxide. Solid-state catalysts were obtained in high yields and thoroughly characterized with elemental analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES), nitrogen sorption measurements, thermogravimetric analysis, scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX) images, and solid-state NMR spectroscopy. Further, the immobilized catalysts were applied to a variety of synthetically valuable, highly stereoselective transformations under batch and flow conditions including transfer hydrogenations, a Friedländer condensation/transfer hydrogenation sequence, and Mannich reactions under cryogenic flow conditions. Generally, high yields and stereoselectivities were observed along with robust catalyst stability and reusability. After being used for 10 runs under batch conditions, no loss of selectivity or catalytic activity was observed. Under continuous-flow conditions, the heterogeneous system was in operation for 19 h and the high enantioselectivity remained unchanged throughout the entire process. We expect our approach to extend the applicability of CPAs to a higher level, with a focus on flow chemistry and a more environmentally friendly and resource-efficient use of these powerful catalysts.

Bidentate substrate binding in Brønsted acid catalysis: structural space, hydrogen bonding and dimerization

BINOL derived chiral phosphoric acids (CPAs) are a prominent class of catalysts in the field of asymmetric organocatalysis, capable of transforming a wide selection of substrates with high stereoselectivities. Exploiting the Brønsted acidic and basic dual functionality of CPAs, substrates with both a hydrogen bond acceptor and donor functionality are frequently used as the resulting bidentate binding via two hydrogen bonds is expected to strongly confine the possible structural space and thus yield high stereoselectivities. Despite the huge success of CPAs and the popularity of a bidentate binding motif, experimental insights into their organization and origin of stereoinduction are scarce. Therefore, in this work the structural space and hydrogen bonding of CPAs and N-(ortho-hydroxyaryl) imines (19 CPA/imine combinations) was elucidated by low temperature NMR studies and corroborated by computations. The postulated bidentate binding of catalyst and substrate by two hydrogen bonds was experimentally validated by detection of trans-hydrogen bond scalar couplings. Counterintuitively, the resulting CPA/imine complexes showed a broad potential structural space and a strong preference towards the formation of [CPA/imine]₂ dimers. Molecular dynamics simulations showed that in these dimers, the imines form each one hydrogen bond to two CPA molecules, effectively bridging them. By finetuning steric repulsion and noncovalent interactions, rigid and well-defined CPA/imine monomers could be obtained. NOESY studies corroborated by theoretical calculations revealed the structure of that complex, in which the imine is located in between the 3,3'-substituents of the catalyst and one site of the substrate is shielded by the catalyst, pinpointing the origin or stereoselectivity for downstream transformations.

A Powerful Chiral Super Brønsted C-H Acid for Asymmetric Catalysis

A new type of chiral super Brønsted C-H acids, BINOL-derived phosphoryl bis((trifluoromethyl)sulfonyl) methanes (BPTMs), were developed. As compared to widely utilized BINOL-derived chiral phosphoric acids (BPAs) and N-triflyl phosphoramides (NTPAs), BPTMs displayed much higher Brønsted acidity, resulting in dramatically improved activity and excellent enantioselectivity as demonstrated in catalytic asymmetric Mukaiyama-Mannich reaction, allylic amination, three-component coupling of allyltrimethylsilane with 9-fluorenylmethyl carbamate and aldehydes, and protonation of silyl enol ether. These new strong Brønsted C-H acids have provided a platform for expanding the chemistry of asymmetric Brønsted acid catalysis.

Chiral phosphoric acid-catalyzed regio- and enantioselective reactions of functionalized propargylic alcohols

Chiral phosphoric acid has been utilized for covalent activation of propargylic alcohols to act as pre-catalyst. With this activation mode, a range of highly regio- and enantioenriched heterocyclic products could be generated efficiently from racemic propargylic alcohols.

When Anthracene and Quinone Avoid Cycloaddition: Acid-Catalyzed Redox Neutral Functionalization of Anthracene to Aryl Ethers

Benzoquinone and 9-phenylanthracene barely undergo anticipated cycloaddition under acid catalysis. Instead, 9-anthracenyl aryl ethers are obtained as unexpected products. Mechanistic studies indicate that the reaction likely undergoes an ionic mechanism between protonated anthracene species and nucleophilic oxygen of 1,4-benzoquinone or 1,4-hydroquinone. A variety of 9-anthracenyl aryl ethers are constructed with this method. Produced anthracenyl aryl ethers are potential scaffolds for new fluorescent molecules.

Catalytic Enantioselective Diels-Alder Reactions of Benzoquinones and Vinylindoles with Chiral Magnesium Phosphate Complexes

Tetrahydrocarbazole and its derivatives have received much attention due to the prevalence of this scaffold in natural products and their use in organic synthesis. We have developed a Diels-Alder reaction of benzoquinones and 3-vinylindoles catalyzed by chiral magnesium phosphate complexes to provide tetrahydrocarbazole derivatives in excellent yields and enantioselectivities (up to >99% yield, 99% ee). This transformation features a wide substrate scope, excellent enantioselectivities, and mild conditions.

Asymmetric aerobic decarboxylative Povarov reactions of N-aryl α-amino acids with methylenephthalimidines via cooperative photoredox and chiral Brønsted acid catalysis

This work realizes a new synthetic utility of methylenephthalimidines as a 1,2-synthon and the synthesis of valuable chiral isoindolin-1-ones featuring a 3,3-spiro-tetrahydroquinoline-based stereocenter.

Enantioselective Radical Cyclization of Tryptamines by Visible Light-Excited Nitroxides

Nitroxides can absorb both ultraviolet (UV) and visible light, and their electron can be excited from the π-bonding orbital to the antibonding π* orbital or the n-bonding orbital to the antibonding π* orbital, respectively. Despite the reported UV-induced hydrogen atom transfer (HAT) process, the potential of nitroxides for visible light-excited photosynthesis is underexplored. Here we demonstrate that nitroxide can convert indole to its radical through a visible light-induced HAT process. A chiral phosphoric acid-catalyzed cyclization of the in situ-formed imine radical, followed by trapping by another molecule of nitroxide, provides the product in high yield and enantioselectivity. To highlight the novelty and efficiency of this strategy, an asymmetric total synthesis of natural product (-)-verrupyrroloindoline was accomplished in 5 steps.

Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates

It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Advances in covalent organic frameworks in separation science

Covalent organic frameworks (COFs) are a new class of multifunctional crystalline organic polymer constructed with organic monomers via robust covalent bonds. The unique properties such as convenient modification, low densities, large specific surface areas, good stability and permanent porosity make COFs great potential in separation science. This review shows the state-of-the art for the application of COFs and their composites in analytical separation science. COFs and their composites have been explored as promising sorbents for solid phase extraction, potential coatings for solid phase microextraction, and novel stationary phases for gas chromatography, high-performance liquid chromatography and capillary electrochromatography. The prospects of COFs for separation science are also presented, which can offer an outlook and reference for further study on the applications of COFs.

Synthetic versatility of 2-substituted-6-methyl 2,3-dihydropyridinones in the synthesis of polyfunctional piperidine-based compounds and related β amino acid derivatives

Chiral 2-substituted-6-methyl 2,3-dihydropyidinones 9, which can be facilely obtained from an asymmetric vinylogous Mannich reaction (VMR) with 1,3-bis-trimethysily enol ether, were used as versatile intermediates in constructing chiral polyfunctional piperidine-based compounds. The 6-methyl group of such compounds can be conveniently functionalized via alkylation and acylation reactions to provide efficient entries to the synthesis of a variety of chiral multi-substituted piperidine-based compounds. Further elaboration of the corresponding intermediates also provided access to polyfunctional indolizidine-based compounds. These methods were showcased in an asymmetric synthesis of 2,6-di-substituted piperidine compound 13, reported as the key intermediate in the synthesis of (+)-calvine and a natural alkaloid (-)-indolizidine 209D. Furthermore, selective C5 iodination of compound 9 enabled the installation of additional functional groups at this position. Finally, we demonstrated that the oxidative cleavage of 2-substituted-6-methyl-2,3-dihydropyidinones is a practical and efficient method for the enantioselective synthesis of β-amino acids, which can undergo further intra-molecular cyclization to give the corresponding chiral four-membered β-lactam derivatives.

The Brønsted Acid-Catalyzed, Enantioselective Aza-Diels-Alder Reaction for the Direct Synthesis of Chiral Piperidones

We disclose herein the first enantioselective aza-Diels-Alder reaction of β-alkyl-substituted vinylketene silyl-O,O-acetals and imines furnishing a broad range of optically highly enriched 4-alkyl-substituted 2-piperidones. As a catalyst for this one-pot reaction we employed a chiral phosphoric acid which effects a vinylogous Mannich reaction directly followed by ring-closure to the lactam. Subsequent fully diastereoselective transformations including hydrogenation, enolate alkylation, and lactam alkylation/reduction processes converted the cycloadducts into various highly substituted piperidines of great utility for the synthesis of natural products and medicinally active compounds.

Chiral dendrigraft polymer for asymmetric synthesis of isoquinuclidines

A copper complex of chiral modified dendrigraft amidoamine polymer with a pentaerythritol initiated polyepichlorohydrin core, PEN-G₂, on a solid resin support is employed in the synthesis of isoquinuclidines via aza Diels–Alder reaction between cyclohexenone and imines.

N-Heterocyclic carbene-catalyzed [4 + 2] annulation of α,β-unsaturated carboxylic acids: enantioselective synthesis of dihydropyridinones and spirocyclic oxindolodihydropyridinones

The N-heterocyclic carbene-catalyzed [4 + 2] annulation of α,β-unsaturated carboxylic acid with hydrazones and isatin-derived imines was developed, affording the corresponding dihydropyridinones, respectively, in good yields with high enantioselectivities.

Enantioselective Cycloaddition Reactions Catalyzed by BINOL-Derived Phosphoric Acids and N-Triflyl Phosphoramides: Recent Advances

Over the last several years there has been a huge increase in the development and applications of new efficient organocatalysts for enantioselective pericyclic reactions, which represent one of the most powerful types of organic transformations. Among these processes are cycloaddition reactions (e.g., [3+2]; formal [3+3]; [4+2]; vinylogous [4+2] and 1,3-dipolar cycloadditions), which belong to the most utilized reactions in organic synthesis of complex nitrogen- and oxygen-containing heterocyclic molecules. This review presents the breakthrough realized in this field using chiral BINOL-derived phosphoric acids and N-triflyl phosphoramide organocatalysts.

Heteropoly acid-catalyzed three-component aza-Diels–Alder reaction in a continuous micro-flow system

The first direct aza-Diels–Alder reaction catalyzed using phosphotungstic acid combined with a water absorption device in a micro-flow system was reported.

Recyclable organocatalyst-promoted one-pot Michael/aza-Henry/lactamization reactions for fluorinated 2-piperidinones bearing four stereogenic centres

A highly efficient method for asymmetric synthesis of fluorinated 2-piperidinones bearing four stereogenic centres is introduced.

Theoretical study of the BINOL–zinc complex-catalyzed asymmetric inverse-electron-demand imino Diels–Alder reaction: mechanism and stereochemistry

The enantioselectivity originates from the hindrance of a BINOL ligand, and the exo-selectivity is achieved by the favored electrophilic/nucleophilic interaction.

N-Heterocyclic carbene-catalyzed [3+3] cyclocondensation of bromoenals with aldimines: highly enantioselective synthesis of dihydropyridinones

The N-heterocyclic carbene-catalyzed [3+3] cyclocondensation of bromoenals and aldimines was developed, giving the corresponding dihydropyridinones in good yields with excellent enantioselectivities.

Catalytic asymmetric Diels-Alder reactions involving aryl vinyl ketones

A catalytic asymmetric Diels-Alder reaction of an aryl vinyl ketone with 1,3-dienylcarbamate has been developed. Cyclohexenes bearing vicinal amino and aroyl groups in a cis-configuration were prepared in excellent ee (>99%) and endo (single diastereomer) selectivity. The absolute configuration of one DA product was unambiguously confirmed using XRD analysis. The transition state structure was proposed on the basis of DFT calculations.

Asymmetric N-Heterocyclic Carbene Catalyzed Annulation of 2-Alkenylbenzothiazoles with α-Chloro Aldehydes

Diastereo- and enantioselective N-heterocyclic carbene catalyzed 1-azadiene Diels-Alder reactions of (E)-2-styrylbenzothiazoles with α-chloro aldehydes are reported. This annulation strategy provides an efficient access to medicinally important dihydrobenzothiazolopyridin-1-ones in good to excellent yields (44-97%) with very good to excellent stereoselectivities (up to 9:1 dr, 98% ee) and tolerates quite a range of substituents.

An enantioselective inverse-electron-demand imino Diels-Alder reaction

The imino Diels-Alder reaction is an efficient method for the synthesis of aza-heterocycles. While different stereo- and enantioselective inverse-electron-demand imino Diels-Alder (IEDIDA) reactions have been reported before, IEDIDA reactions including electron-deficient dienes are unprecedented. The first enantioselective IEDIDA reaction between electron-poor chromone dienes and cyclic imines, catalyzed by zinc/binol complexes is described. The novel reaction provides a facile entry to a natural product inspired collection of ring-fused quinolizines including a potent modulator of mitosis.

Formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide: one-pot access to polysubstituted pyridin-2(1H)-ones

One-pot synthesis of polysubstituted pyridin-2(1H)-ones by formal [4+2] annulation of enaminones and cyanomethyl sulfur ylide is described involving sequential nucleophilic vinylic substitution, intramolecular nucleophilic cyclization and dealkylation reactions.