Catalytic Asymmetric Reactions by Metal and Chiral Phosphoric Acid Sequential Catalysis

Enamine/Iminium-based Dual Organocatalytic Systems for Asymmetric Catalysis and Synthesis

The rational combination of two catalysts to expedite the construction of chiral complex biologically and pharmacologically relevant chiral compounds has widely gained momentum over the past decade. In particular, enamine or iminium catalysis ensuing from the activation of aldehyde or ketone by chiral amine catalysts in conjugation with other organocatalytic cycles has facilitated several asymmetric transformations to yield the enantioenriched products. Regardless of the considerable discussion on the various dual catalytic approaches, literature lacks a comprehensive review focusing on the enamine and iminium-based dual organocatalytic systems. Thus, this review article has discussed the noteworthy achievements in the field of asymmetric catalysis and synthesis catalyzed by the enamine and iminium-based dual organocatalytic systems.

Catalytic Asymmetric Benzylation of Azomethine Ylides Enabled by Synergistic Lewis Acid/Palladium Catalysis

The synergistic chiral Lewis acid/achiral Pd catalyst system was successfully applied in the enantioselective benzylation of various imine esters, giving a range of α-benzyl-substituted α-amino acid derivatives in satisfactory yield with excellent enantioselectivity. It is worth noting that this strategy exhibits good tolerance for bicyclic and monocyclic benzylic electrophiles. Furthermore, the utility of this synthetic protocol was demonstrated by the expedient preparation of enantioenriched antihypertensive drug α-methyl-l-dopa.

Ternary Catalysis Enabled Three-Component Asymmetric Allylic Alkylation as a Concise Track to Chiral α,α-Disubstituted Ketones

Multicomponent reactions that involve interception of onium ylides through Aldol, Mannich, and Michael addition with corresponding bench-stable acceptors have demonstrated broad applications in synthetic chemistry. However, because of the high reactivity and transient survival of these in situ generated intermediates, the substitution-type interception process, especially the asymmetric catalytic version, remains hitherto unknown. Herein, a three-component asymmetric allylation of α-diazo carbonyl compounds with alcohols and allyl carbonates is disclosed by employing a ternary cooperative catalysis of achiral Pd-complex, Rh₂(OAc)₄, and chiral phosphoric acid CPA. This method represents the first example of three-component asymmetric allylic alkylation through an S_N1-type trapping process, which involves a convergent assembly of two active intermediates, Pd-allyl species, and enol derived from onium ylides, providing an expeditious access to chiral α,α-disubstituted ketones in good to high yields with high to excellent enantioselectivity. Combined experimental and computational studies have shed light on the mechanism of this novel three-component reaction, including the critical role of Xantphos ligand and the origin of enantioselectivity.

Asymmetric synthesis of dihydrocarbazoles through a Friedel-Crafts alkylation/annulation sequential reaction of indoles

An enantioselective tandem Friedel-Crafts alkylation/annulation of indoles with diazoacetoacetate enones is realized in one pot. A series of dihydrocarbazoles were obtained in moderate yields with good to excellent ee values by using a Rh^II/Sc^III dual-metallic catalyst system. Control experiments revealed that Sc^III is critical to both the alkylation and annulation.

Synthesis of Chiral Spirolactams via Sequential C-H Olefination/Asymmetric [4+1] Spirocyclization under a Simple CoII /Chiral Spiro Phosphoric Acid Binary System

An unprecedented enantioselective synthesis of spiro-γ-lactams via a sequential C-H olefination/asymmetric [4+1] spirocyclization under a simple Co^II /chiral spiro phosphoric acid (SPA) binary system is reported. A range of biologically important spiro-γ-lactams are obtained with high levels of enantioselectivity (up to 98 % ee). The concise, asymmetric synthesis of an aldose reductase inhibitor was successfully achieved. Notably, contrast to previous reports that relied on the use of cyclopentadienyl or its derivatives (achiral Cp*, Cp^tBu , or chiral Cp^x ) ligated Co^III complexes requiring tedious steps to prepare, cheap and commercially available cobalt(II) acetate tetrahydrate was used as an efficient precatalyst.

Enantioselective Synthesis of Quaternary Carbon Stereocenters by Asymmetric Allylic Alkylation: A Review

Quaternary stereocenters are of great importance to the three-dimensionality and enhanced properties of new molecules, but the synthetic challenges in creating quaternary stereocenters greatly hinder their wide use in drug discovery, organic material design, and natural product synthesis. The asymmetric allylic alkylation (AAA) of allylic substrates has proven to be a powerful methodology for enantioselective formation of structure skeletons bearing single or more quaternary carbon centers in modern asymmetric organocatalysis. AAA has certain advantages in constructing the tetrasubstituted stereocenters, including but not limited to mild reactive conditions, effective reaction rates, new functional group introduction, and carbon chains length extension. This review outlines the key considerations in the application of AAA reactions and summarizes the recent progress of AAA reactions in the enantioselective synthesis of products containing quaternary stereocenters. Meanwhile, a detailed discussion of the AAA reactions such as ligands, scope of substrates, transformations and the general reaction mechanisms is also provided. We hope this review could stimulate further advances in much broader areas, including organic synthesis, asymmetric catalysis, C-H activation, and symmetrical pharmaceutical chemistry.

Tandem Catalytic Indolization/Enantioconvergent Substitution of Alcohols by Borrowing Hydrogen to Access Tricyclic Indoles

An efficient tandem catalysis method is achieved for the direct conversion of alcohol-containing alkynyl anilines to valuable chiral 2,3-fused tricyclic indoles. This method relies on a tandem indolization followed by enantioconvergent substitution of alcohols via borrowing hydrogen to construct two rings in one step, enabled by relay and cooperative catalysis of a chiral iridium complex with a chiral phosphoric acid. Highly diastereoselective transformations of the tricyclic indole products also provide efficient access to a diverse array of complex polycyclic indoline compounds.

Stereodivergent Synthesis of Enantioenriched 2,3-Disubstituted Dihydrobenzofurans via a One-Pot C-H Functionalization/Oxa-Michael Addition Cascade

A one-pot rhodium-catalyzed C-H functionalization/organocatalyzed oxa-Michael addition cascade reaction has been developed. This methodology enables the stereodivergent synthesis of diverse 2,3-disubstituted dihydrobenzofurans with broad functional group compatibility in good yields with high levels of stereoselectivity under exceptionally mild conditions. The full complement of stereoisomers of chiral 2,3-disubstituted dihydrobenzofurans and 3,4-disubstituted isochromans could be accessed at will by appropriate permutations of the two chiral catalysts. The current work provides a rare example of two chiral catalysts independently controlling two contiguous stereogenic centers subsequently via a two-step reaction in a single operation.

Enantioselective synthesis of indole derivatives by Rh/Pd relay catalysis and their anti-inflammatory evaluation

An efficient Rh/Pd relay catalyzed intermolecular and cascade intramolecular hydroamination for the synthesis of exclusive trans 1-indolyl dihydronaphthalenols (up to 88% yield, 99% ee) is described under mild conditions. Moreover, the in silico and in vitro screening showed that the novel 1-indolyl dihydronaphthalenol products are potent lead compounds for treating inflammation disease.

Catalytic asymmetric synthesis of monofluoroalkenes and gem-difluoroalkenes: advances and perspectives

The latest achievements in the catalytic asymmetric synthesis of both monofluoro- and gem-difluoroalkenes are discussed.

Enantioselective Synthesis of Pyrrolizidinone Scaffolds through Multiple-Relay Catalysis

A triple-tandem protocol for the synthesis of the pyrrolizidinone skeleton has been devised. It involves a cross metathesis-intramolecular aza-Michael reaction-intramolecular Michael addition tandem sequence, starting from N-pentenyl-4-oxo-2-alkenamides and conjugated ketones. In the presence of two cooperative catalysts, namely the second-generation Hoveyda-Grubbs catalyst and (R)-TRIP-derived BINOL phosphoric acid, this multiple-relay catalytic process takes place in good yields and outstanding levels of diastero- and enantioselectivity with the simultaneous generation of three contiguous stereocenters.

Recent applications of chiral phosphoric acids in palladium catalysis

Through the combined action of palladium catalysts and chiral phosphoric acids (CPAs) a variety of catalytic asymmetric reactions have been realized during the past decade, including allylation, alkene functionalization, and C-H activation. This review surveys key examples across these various reaction types and examines the different mechanisms by which CPAs can affect stereoinduction in these reaction systems.

Harnessing structurally unbiased ortho-benzoquinone monoimine for biomimetic oxidative [4+2] cycloaddition with enamines

Reported herein is the first catalytic oxidative [4+2] cycloaddition of 2-aminophenols with cyclic enamines.

Recent Advances in First-Row Transition Metal/Chiral Phosphoric Acid Combined Catalysis

Since the pioneering independent reports of Akiyama and Terada, the use of chiral phosphoric acids (CPAs) and derivatives as a versatile tool for asymmetric synthesis with good reactivity, regioselectivity, diastereoselectivity and enantioselectivity has emerged, forming an important part of the implementation of asymmetric counteranion-directed catalysis reported to date. In these achievements, the combination of metals with CPAs has enabled various catalytic modes beyond the scope of typical acid catalysis, such as relay catalysis, ion-pairing catalysis, and binary acid catalysis. The first-row transition metals (Sc-Zn) are considered to be sustainable transition metals and have received a great deal of attention. These naturally abundant metals display excellent Lewis acidity and function as powerful redox catalysts in synthesis involving both one and two-electron transfers. Hence, in this chapter, we summarize recent advances in the development of asymmetric reactions using a combination of first-row transition metals and CPAs. Furthermore, we provide a detailed discussion of the mechanisms involved in order to understand the interaction of the metal/phosphate and the origins of the asymmetric control of the transformations.

Outer-Sphere Control for Divergent Multicatalysis with Common Catalytic Moieties

We herein report two examples of one-pot, simultaneous reactions, mediated by multiple, orthogonal catalysts with the same catalytic motif. First, BINOL-derived chiral phosphoric acids (CPA) and phosphothreonine (pThr)-embedded peptides were found to be matched for two different steps in double reductions of bisquinolines. Next, two π-methylhistidine (Pmh)-containing peptides catalyzed enantio- and chemoselective acylations and phosphorylations of multiple substrates in one pot. The selectivity exhibited by common reactive moieties is adjusted solely by the appended chiral scaffold through outer-sphere interactions.

Enantioselective [3 + 2] Cycloaddition Reaction of Ethynylethylene Carbonates with Malononitrile Enabled by Organo/Metal Cooperative Catalysis

The first catalytic asymmetric decarboxylative [3 + 2] cycloaddition reaction of ethynylethylene carbonates with malononitrile has been developed successfully by an organo/copper cooperative system. This strategy led to a series of optically active polysubstituted dihydrofurans in good yields with high levels of enantioselectivities (up to 99% yield, 97% ee). The presence of the terminal alkynyl and the cyano group in the dihydrofuran products provides a wide scope for further structural transformations. More importantly, this organo/metal cooperative catalytic system will broaden the substrate scope and enable fundamentally new reactions.

Multifunctionalization of Unactivated Cyclic Ketones via Synergistic Catalysis of Copper and Diarylamine: Access to Cyclic α-Enaminone

A multifunctionalization of unactivated cyclic ketones via synergistic catalysis of copper and diarylamine for the direct synthesis of cyclic α-enaminone is reported for the first time. This reaction goes through oxidative α-amination, followed by a desaturation, and features mild reaction conditions, a broad substrate scope, and great functional group tolerance.

Pd/Cu dual catalysis: highly enantioselective access to α-substituted α-amino acids and α-amino amides

In this work, we have developed a synergistic Pd/Cu catalyst system for the asymmetric allylation of glycine iminoesters/amides, affording a range of α-substituted α-amino acids/amides in high yields and with excellent enantioselectivities (88 → 99% ee).

Relay tricyclic Pd(ii)/Ag(i) catalysis: design of a four-component reaction driven by nitrene-transfer on isocyanide yields inhibitors of EGFR

A four-component reactions promoted by Pd(ii)/Ag(i) relay catalysis paved the way for the development of a new EGFR inhibitor.