Chiral Phosphoric Acid Research Articles

Organocatalytic Enantioselective 1,12-Addition of Alkynyl Biphenyl Quinone Methides Formed In Situ.

The chemistry of quinone methides formed in situ has been flourishing in recent years. In sharp contrast, the development and utilization of biphenyl quinone methides are rare. Herein, we achieved a remote stereocontrolled 1,12-conjugate addition of biphenyl quinone methides formed in situ for the first time. In the presence of a suitable chiral phosphoric acid, alkynyl biphenyl quinone methides were generated from α-[4-(4-hydroxyphenyl)phenyl]propargyl alcohols, followed by enantioselective 1,12-conjugate addition with indole-2-carboxylates. The strategy enabled the additional alcohols to serve as efficient allenylation reagents, providing a practical access to a broad range of axially chiral allenes bearing (1,1'-biphenyl)-4-ol unit that are previously less accessible. Combined with control experiments, density functional theory calculations shed light on the reaction mechanism, indicating that enantioselectivity originates from the nucleophilic addition of alkynyl biphenyl quinone methides. Notably, not only the presence of biphenyl quinone methides as versatile intermediates was confirmed but also organocatalytic enantioselective 1,12-addition was established for the first time. This work enriched the family of quinone methides and provided a new platform for the remote stereocontrolled transformation of such versatile intermediates.

Enantiospecific Analysis of Carboxylic Acids Using Cinchona Alkaloid Dimers as Chiral Solvating Agents.

Cinchona alkaloid derivatives as Brønsted base catalysts have attracted considerable attention in the field of asymmetric catalysis. However, their potential application as chiral solvating agents has not been described. In this research, we investigated the use of the Cinchona alkaloid dimer, namely, (DHQ)2PHAL, as a chiral solvating agent for discerning various mandelic acid derivatives through 1H NMR spectroscopy. The addition of catalytic amounts of DMAP facilitated this process. Our experimental results demonstrate that dimeric (DHQ)2PHAL exhibits remarkable chiral discrimination properties regarding the diagnostic split protons of 1H NMR signals (including 24 examples, up to 0.321 ppm). Furthermore, it serves as an excellent chiral discriminating agent and provides good resolution for racemic chiral phosphoric acid as determined by 31P NMR spectroscopy. The quality of enantiodifferentiation has also been evaluated by means of the parameter "resolution (Rs)". Significantly, this class of CSAs based on (alkaloid)2linker systems with an azaaromatic linker can be directly employed, which is commercially available in an enantiopure form at very low cost and exhibits promising potential in determining the enantiopurity of α-hydroxy acids by chemoselective and biocatalytic reactions.

Organocatalytic Enantioselective 1,12‐Addition of Alkynyl Biphenyl Quinone Methides Formed In Situ

The chemistry of quinone methides formed in situ has been flourishing in recent years. In sharp contrast, the development and utilization of biphenyl quinone methides are rare. Herein, we achieved a remote stereocontrolled 1,12‐conjugate addition of biphenyl quinone methides formed in situ for the first time. In the presence of a suitable chiral phosphoric acid, alkynyl biphenyl quinone methides were generated from α‐[4‐(4‐hydroxyphenyl)phenyl]propargyl alcohols, followed by enantioselective 1,12‐conjugate addition with indole‐2‐carboxylates. The strategy enabled the additional alcohols to serve as efficient allenylation reagents, providing a practical access to a broad range of axially chiral allenes bearing (1,1'‐biphenyl)‐4‐ol unit that are previously less accessible. Combined with control experiments, density functional theory calculations shed light on the reaction mechanism, indicating that enantioselectivity originates from the nucleophilic addition of alkynyl biphenyl quinone methides. Notably, not only the presence of biphenyl quinone methides as versatile intermediates was confirmed but also organocatalytic enantioselective 1,12‐addition was established for the first time. This work enriched the family of quinone methides and provided a new platform for the remote stereocontrolled transformation of such versatile intermediates.

Catalytic Asymmetric Cyclizative Rearrangement of Anilines and Vicinal Diketones to Access 2,2-Disubstituted Indolin-3-ones.

The efficient synthesis of chiral 2,2-disubstituted indolin-3-ones is of great importance due to its significant synthetic and biological applications. However, catalytic enantioselective methods for de novo synthesis of such heterocycles remain scarce. Herein, a novel cyclizative rearrangement of readily available anilines and vicinal diketones for the one-step construction of enantioenriched 2,2-disubstituted indolin-3-ones is presented. The reaction proceeds through a self-sorted [3+2] heteroannulation/regioselective dehydration/1,2-ester shift process. Only chiral phosphoric acid is employed to promote the entire sequence and simplify the manipulation of this protocol. Various common aniline derivatives are successfully applied to asymmetric synthesis as 1,3-binuclephiles for the first time. Remarkably, the observed stereoselectivity is proposed to originate from an amine-directed regio- and enantioselective ortho-Csp2-H addition of the anilines to the ketones. A range of synthetic transformations of the resulting products are demonstrated as well.

Switchable Enantioselectivity in Conjugate Alkyne Addition of β,γ-Unsaturated α-Keto Esters by Asymmetric Binary Acid Catalysis.

Switchable enantioselectivity was uncovered in the enantioselective catalytic conjugate addition of β,γ-unsaturated α-keto esters with terminal alkynes to the chiral Lewis acid complex of In(BF4)3 and chiral phosphoric acid.

Enantioselective Construction of Chiral THIQUINOL and Its Derivatives via Chiral Phosphoric Acid Catalysis.

The first catalytic enantioselective construction of chiral THIQUINOL and its derivatives has been accomplished through a chiral phosphoric-acid-catalyzed direct aza-Friedel-Crafts reaction of 3,4-dihydroisoquinolines with 2-naphthols/anthracen-2-ols/phenanthren-9-ol. This method offers a powerful and straightforward synthetic route toward chiral THIQUINOL derivatives with good to excellent yields and enantioselectivities. These structural motifs are crucial chiral components for further transformations into established or potential chiral ligands and catalysis.

Stereoselective reactions promoted by alkali metal salts of phosphoric acid organocatalysts

The catalytic application of chiral phosphoric acids (CPAs) from 2004 to date represents a true milestone for asymmetric organocatalysis. However, not only the free acids can be conveniently employed in numerous different asymmetric synthetic methodologies, thus being strictly compliant to the concept of “organocatalysis”, but also their metal salts. This review focuses on asymmetric reactions in which the catalyst is a chiral alkali or alkaline‐earth metal phosphate.

Atroposelective Construction of Naphthylpyrazoles by Chiral Phosphoric Acid Catalyzed Enantioselective Cross‐Coupling of Pyrazoles with Naphthoquinone Esters

The catalytic asymmetric reaction of 5‐aminopyrazoles with naphthoquinone esters has been established. A wide range of unprecedented axially chiral naphthyl‐pyrazole derivatives (29 examples) have been synthesized in moderate to excellent yields (up to 99% yield) with excellent enantioselectivities (up to 99% ee) by utilizing chiral phosphoric acid as a catalyst. This protocol features mild reaction conditions, a broad substrate scope, good scalability and facile derivatization. Moreover, preliminary mechanistic investigation was conducted to elucidate the mechanism.

Catalytic Asymmetric Dearomative Arylation of 2‐Naphthols Enabled by o‐Quinone Diimides

AbstractThe catalytic asymmetric dearomative arylation of 2‐naphthols enabled by the unconventional reactivity of o‐quinone diimides, i. e. the 1,4‐conjugate addition on quinone sp2 hybridized carbon, has been established. Under the catalysis of chiral phosphoric acid, various cyclohexaenones bearing an all‐carbon quaternary stereocenter have been prepared with excellent yields and enantioselectivities.

Asymmetric photoredox catalytic formal de Mayo reaction enabled by sensitization-initiated electron transfer.

Visible-light-driven photoredox catalysis is known to be a powerful tool for organic synthesis. Its occurrence critically depends on the twice exothermic single-electron transfer processes of photosensitizers, which are governed by the redox properties of the species involved. Hence, the inherently narrow range of redox potentials of photosensitizers inevitably constrains their further availability. Sensitization-initiated electron transfer has recently been found to effectively overcome this substantial challenge. However, feasible and practical strategies for designing such complicated catalytic systems are rather scarce. Herein we report an elaborate dual-catalyst platform, with dicyanopyrazine as a visible light photosensitizer and a pyrenyl-incorporated chiral phosphoric acid as a co-sensitizer, and we demonstrate the applicability of this sensitization-initiated electron transfer strategy in an asymmetric formal de Mayo-type reaction. The catalysis platform enables otherwise thermodynamically unfavourable electron transfer processes to close the redox cycle and allows for precise access to valuable enantioenriched 1,5-diketones with a wide substrate range.

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.

Manganese Pentacarbonyl Bromide as Regeneration Catalyst Enabled Biomimetic Asymmetric Reduction.

Using readily available manganese pentacarbonyl bromide as a regeneration catalyst, biomimetic asymmetric reduction of imines including quinoxalinones, benzoxazinones, and benzoxazine has been successfully developed in the presence of transfer catalyst chiral phosphoric acids, providing the chiral amines with high yields and enantioselectivities.

Organocatalytic Atroposelective Construction of Pentatomic Heterobiaryl Diamines through Arylation of 5-Aminoisoxazoles with Azonaphthalenes.

An efficient catalytic asymmetric Michael-type reaction of azonaphthalenes with 5-aminoisoxazoles has been developed. The reaction was based on the utilization of a chiral phosphoric acid as the catalyst, delivering a large panel of axially chiral heterobiaryl diamines in generally good yields with excellent enantioselectivities. The gram-scale reaction and postmodification of the chiral product demonstrated their potentials in the synthesis of chiral catalysts and ligands. This approach not only provides a useful method for the construction of pentatomic heterobiaryl scaffolds but also offers new members to the axially chiral diamine family with promising applications in synthetic and medicinal chemistry.

Enantioselective [2π + 2σ] Cycloadditions of Bicyclo[1.1.0]butanes with Vinylazaarenes through Asymmetric Photoredox Catalysis.

Here we present a highly enantioselective [2π + 2σ] photocycloaddition of bicyclo[1.1.0]butanes (BCBs). The reaction uses a variety of vinylazaarenes as partners and is catalyzed by a polycyclic aromatic hydrocarbon (PAH)-containing chiral phosphoric acid as a bifunctional chiral photosensitizer. A wide array of pharmaceutically important bicyclo[2.1.1]hexane (BCH) derivatives have been synthesized with high yields, enantioselectivity, and diastereoselectivity. In addition to the diverse 1-ketocarbonyl-3-substituted BCBs, α/β-substituted vinylazaarenes are compatible with such an unprecedented photoredox catalytic pathway, resulting in the successful assembly of an all-carbon quaternary stereocenter or two adjacent tertiary stereocenters on the product.

Mechanism and origin of enantioselectivity for asymmetric Passerini reaction in the synthesis of ɑ-acyloxyamide catalyzed by chiral phosphoric acid

The enantioselective control of the three-component Passerini reaction was successfully achieved using chiral phosphoric acid as a catalyst with a wide range of substrates and high stereoselectivity. Quantum mechanical calculations reveals the detailed reaction mechanism and enantioselective origin of the reaction. The calculations revealed that the chiral phosphoric acid first forms a stable heterodimer with the carboxylic acid substrate, followed by C-C and C-O bond formations, the generation of a five-membered cyclic intermediate, and finally acyl migration to yield the product. Four reaction paths for the overall process were identified and compared to determine the optimal major and by-product paths. The enantioselective origin of the reaction has been elucidated systematically using distortion/interaction theory and truncated model system. QTAIM and IGMH analyses demonstrated the type and strength of interaction between catalyst and the substrates. The calculated e.e. value was 93 %, which was in good agreement with the experimental value. It is expected that the results of this study will offer new insights into the field of asymmetric Passerini reactions.

Asymmetric cyclization catalyzed by a chiral phosphoric acid–gold(I) hybrid complex as a multifunctional catalyst

Abstract We herein report the development of a novel hybrid complex containing gold(I) and a chiral phosphoric acid moiety to generate a multifunctional catalyst. While the use of chiral phosphoric acid as a bifunctional catalyst is a common strategy in asymmetric organic synthesis, chiral phosphoric acid–transition metal hybrid complexes as multifunctional catalysts have not been investigated. Thus, we designed and synthesized a novel gold(I) hybrid complex as a multifunctional catalyst that promotes asymmetric catalytic reactions through multipoint nonclassical noncovalent interactions in substrates that lack classical hydrogen-bond donors. In addition, we demonstrate its usefulness as a multifunctional catalyst by successfully developing the first catalytic asymmetric synthesis of dihydrocyclohepta[b]indoles. Experimental and theoretical studies revealed that this asymmetric catalytic reaction involves the kinetic resolution of the reaction intermediate, and that the favored diastereomeric transition state for yielding enantiomeric products is formed through multipoint nonclassical noncovalent interactions originating from the acid–base nature of the chiral phosphoric acid moiety.

Enantioselective Cross-[4 + 2]-Cycloaddition/Decarboxylation of 2-Pyrones by Cooperative Catalysis of the Pd(0)/NHC Complex and Chiral Phosphoric Acid.

Here, we describe a cooperative Pd(0)/chiral phosphoric acid catalytic system that allows us to realize the first chemo-, regio-, and enantioselective sequential cross-[4 + 2]-cycloaddition/decarboxylation reaction between 2-pyrones and unactivated acyclic 1,3-dienes. The key to the success of this transformation is the utilization of an achiral N-heterocyclic carbene (NHC) as the ligand and a newly developed chiral phosphoric acid as the cocatalyst. Experimental investigations and computational studies support the idea that the Pd(0)/NHC complex acts as a π-Lewis base to increase the nucleophilicity of 1,3-dienes via η2 coordination, while the chiral phosphoric acid simultaneously increases the electrophilicity of 2-pyrones by hydrogen bonding. By this synergistic catalysis, the sequential cross-[4 + 2]-cycloaddition and decarboxylation reaction proceeds efficiently, enabling the preparation of a wide range of chiral vinyl-substituted 1,3-cyclohexadienes in good yields and enantioselectivities. The synthetic utility of this reaction is demonstrated by synthetic transformations of the product to various valuable chiral six-membered carbocycles.

Aromatic Amine and Chiral Phosphoric Acid Synergistic Catalyzed Cascade Reaction of Alkynylnaphthols with Aldehydes.

A novel approach using aromatic amines and chiral phosphoric acids in a synergistic catalytic cascade reaction of 2-alkynylnaphthols with aldehydes has been established. This method offers a direct route to preparing flavanone analogues with excellent stereoselectivity. Mechanistic studies reveal a sequential process involving addition, elimination, cyclization, and hydrolysis in which aromatic amines and chiral phosphoric acids play key roles via imine-enamine and hydrogen bonding models.

Asymmetric Transfer Hydrogenation of 2-Substituted Quinoxalines with Regenerable Dihydrophenanthridine.

The asymmetric hydrogenation of quinoxalines represents one of the most efficient approaches for the synthesis of optically active tetrahyroquinoxalines. In this paper, we demonstrate a metal-free asymmetric transfer hydrogenation of 2-substituted quinoxalines with regenerable dihydrophenanthridine under H2 using a combination of chiral phosphoric acid and achiral borane as catalysts. A wide range of optically active 2-substituted tetrahydroquinoxalines were produced in high yields with ≤98% ee.

Nickel and Chiral Phosphoric Acid Cocatalysis Enables Synthesis of C-Acyl Glycosides.

We disclosed a Ni/CPA cocatalyzed protocol to access diverse C-acyl glycosides under mild conditions with broad functional group compatibility through the coupling of readily available glycosyl bromides and carboxylic esters. The potential application of the methodology was demonstrated by the C-acyl glycosylation of bioactive molecules and the transformation of products to a variety of value-added molecules. Mechanistic studies revealed that CPA might serve as a bifunctional H-bond catalyst to activate carboxylic esters and nickel catalyst.