Axially chiral bidentate ligands in asymmetric catalysis

Nickel-Catalyzed Chemoselective Carbomagnesiation for Atroposelective Ring-Opening Difunctionalization

There is a pressing need for methods that can connect enantioenriched organic compounds with readily accessible building blocks via asymmetric functionalization of unreactive chemical bonds in organic synthesis and medicinal chemistry. Herein, the asymmetric chemoselective cleavage of two unactivated C(Ar)-O bonds in the same molecule is disclosed for the first time through an unusual nickel-catalyzed carbomagnesiation. This reaction facilitates the evolution of a novel atroposelective ring-opening difunctionalization. Utilizing readily available dibenzo bicyclic substrates, diverse valuable axially chiral biaryls are furnished with high efficiencies. Synthetic elaborations showcase the application potential of this method. The features of this method include good atom-economy, multiple roles of the nucleophile, and a simple catalytic system that enables the precise magnesiation of an α-C(Ar)-O bond and arylation of a β-C(Ar)-O bond.

Obtaining giant Rashba-Dresselhaus spin splitting in two-dimensional chiral metal-organic frameworks

Two-dimensional (2D) nonmagnetic semiconductors with large Rashba-Dresselhaus (R-D) spin splitting at valence or conduction bands are attractive for magnetic-field-free spintronic applications. However, so far, the number of 2D R-D inorganic semiconductors has been quite limited, and the factors that determine R-D spin splitting as well as rational design of giant spin splitting, remain unclear. For this purpose, by exploiting 2D chiral metal-organic frameworks (CMOFs) as a platform, we theoretically develop a three-step screening method to obtain a series of candidate 2D R-D semiconductors with valence band spin splitting up to 97.2 meV and corresponding R-D coupling constants up to 1.37 eV Å. Interestingly, the valence band spin texture is reversible by flipping the chirality of CMOFs. Furthermore, five keys for obtaining giant R-D spin splitting in 2D CMOFs are successfully identified: (i) chirality, (ii) large spin-orbit coupling, (iii) narrow band gap, (iv) valence and conduction bands having the same symmetry at the Γ point, and (v) strong ligand field.

Studies on the Configurational Stability of Tropolone-Ketone-, Ester-, and Aldehyde-Based Chiral Axes

Recent studies have revealed that tropolone-amide aryl C-C(O) rotational barriers are dramatically higher than those of analogous benzamide-based systems, and as a result, they have an increased likelihood of displaying high configurational stability. Studies on other tropolone-based chiral axes are important to assess the generality of this phenomenon. Herein, we describe a series of studies on the rotational barriers of tropolone-ketone, tropolone-ester, and tropolone-aldehyde chiral axes. These studies are complemented with computational modeling of the dynamics of these and analogous benzenoid variants to illuminate the impact that tropolone may have on aryl-C(O) configurational stability.

Chiral-at-Metal Racemization Unraveled for MX2 (a-chel)2 by means of a Computational Analysis of MoO2 (acnac)2

Octahedral chiral-at-metal complexes MX2 (a-chel)2 (a-chel=asymmetric chelate) can rearrange their ligands by four mechanisms known as the Bailar (B), Ray-Dutt (RD), Conte-Hippler (CH), and Dhimba-Muller-Lammertsma (DML) twists. Racemization involves their interconnections, which were computed for MoO2 (acnac)2 (acnac=β-ketoiminate) using density functional theory at ωB97x-D with the 6-31G(d,p) and 6-311G(2d,p) basis sets and LANL2DZ for molybdenum. Racemizing the cis(NN) isomer, being the global energy minimum with trans oriented imine groups, is a three step process (DML-CH-DML) that requires 17.4 kcal/mol, while all three cis isomers (cis(NN), cis(NO), and cis(OO)) interconvert at ≤17.9 kcal/mol. The B and RD twists are energetically not competitive and neither are the trans isomers. The interconnection of all enantiomeric minima and transition structures is summarized in a graph that also visualizes valley ridge inflection points for two of the three CH twists. Geometrical features of the minima and twists are given. Lastly, the influence of N-substitution on the favored racemization pathway is evaluated. The present comprehensive study serves as a template for designing chiral-at-metal MX2 (a-chel)2 catalysts that may retain their chiral integrity.

Cross-Dehydrogenative Coupling Polymerization via C-H Activation for the Synthesis of Conjugated Polymers

Owing to their versatile (opto)electronic properties, conjugated polymers have found application in several organic electronic devices. Cross-coupling reactions such as Stille, Suzuki, Kumada couplings, and direct arylation reactions have proved to be effective for their synthesis. More atom-efficient oxidative direct arylation polymerization has also been reported for making homopolymers. However, growing interest toward donor-acceptor polymers has led to the recent emergence of cross-dehydrogenative coupling (CDC) polymerization to synthesize alternating copolymers without any prefunctionalization of monomers. Metal-catalyzed cross-coupling of two simple arenes via double C-H activation, or of an arene with an alkene via oxidative Heck-type reaction have been used so far for CDC polymerization. In this article, we discuss the development of CDC polymerization protocols along with the relevant small molecule CDC reactions for an improved understanding of these reactions.

Advances in the Asymmetric Synthesis of BINOL Derivatives

BINOL derivatives have shown relevant biological activities and are important chiral ligands and catalysts. Due to these properties, their asymmetric synthesis has attracted the interest of the scientific community. In this work, we present an overview of the most efficient methods to obtain chiral BINOLs, highlighting the use of metal complexes and organocatalysts as well as kinetic resolution. Further derivatizations of BINOLs are also discussed.

Palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage

Herein we report a novel palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage under mild conditions.

Rational Design of Axially Chiral Styrene-Based Organocatalysts and Their Application in Catalytic Asymmetric (2+4) Cyclizations

A new class of axially chiral styrene-based thiourea tertiary amine catalysts, which have unique characteristics such as an efficient synthetic route, multiple chiral elements, and multiple activating groups, has been rationally designed. These new chiral catalysts have proven to be efficient organocatalysts, enabling the chemo-, diastereo-, and enantioselective (2+4) cyclization of 2-benzothiazolimines with homophthalic anhydrides in good yields (up to 96 %) with excellent stereoselectivities (all >95:5 dr, up to 98 % ee). More importantly, theoretical calculations elucidated the important role of an axially chiral styrene moiety in controlling both the reactivity and enantioselectivity. This work not only represents the first design of styrene-based chiral thiourea tertiary amine catalysts and the first catalytic asymmetric (2+4) cyclization of 2-benzothiazolimines, but also gives an in-depth understanding of axially chiral styrene-based organocatalysts.

Enantiodivergent One-Pot Synthesis of Axially Chiral Biaryls Using Organocatalyst-Mediated Enantioselective Domino Reaction and Central-to-Axial Chirality Conversion

Enantiodivergent one-pot synthesis of biaryls was developed using a catalytic amount of a single chiral source. A domino organocatalyst-mediated enantioselective Michael reaction and aldol condensation provided centrally chiral dihydronaphthalenes with excellent enantioselectivity, from which an enantiodivergent chirality conversion from central-to-axial chirality was achieved. Both enantiomers of biaryls were obtained with excellent enantioselectivity. All transformations can be conducted in a single reaction vessel. A plausible reaction mechanism for the enantiodivergence is proposed.

Synthesis and chiral separation of atropisomers of 4,5-Di methyl ∆4 N-phenyl N-aryl imidazoline-2-thione derivatives

Synthesis of three chiral 4,5-Di methyl ∆⁴ N-phenyl N-aryl imidazole-2-thione derivatives was obtained by the condensation reaction of thiourea derivatives with α-hydroxy ketone. The structure of these compounds has been characterized by using spectroscopic methods (UV, IR, ¹ H NMR, and ¹³ C NMR). The 4,5-Di methyl ∆⁴ N-phenyl N-aryl imidazole-2-thiones display a chiral axis around the N-C bond linking between the nitrogen of the heterocyclic framework and the carbon of the aryl group. Screening on chiral analysis of the atropisomers of these derivatives was performed by high-performance liquid chromatography method on seven chiral selectors based on polysaccharides consisting of amylose and cellulose, namely, Chiralpak®AD, Chiralcel® OD, Chiralcel® OD-H, Chiralcel® OJ, Chiralcel® OD-3R, Chiralcel® OZ-3, and Chiralpak® AS-3R. The impact of ortho-substituent in the resolution of 4,5-Di methyl ∆⁴ N-phenyl N-aryl imidazole-2-thione derivatives was also studied in this work.

Evidence for Atropisomerism in Polycyclic γ-Butenolides: Synthesis, Scope, and Spectroscopic Studies

Design and development of a domino cyclative approach for the synthesis of new polycyclic γ-butenolides from β-aryl-Z-enoate propargylic alcohols, through the interception of an intermediate of the Z-enoate-assisted Meyer-Schuster rearrangement, has been reported. A systematic NMR analysis of various derivatives of this class revealed and supported the potential atropisomerism associated with them. These molecules represent first examples of butenolide ring-based atropisomeric compounds in organic chemistry. The synthetic process involves a synchronous construction of both rings with concurrent creation of the potential stereogenic rotational axis.

Nickel-catalyzed C–P cross-coupling of (het)aryl tosylates with secondary phosphine oxides

A novel and convenient approach to the synthesis of various tertiary phosphine oxides via nickel-catalyzed cross-coupling of (het)aromatic tosylates with secondary phosphine oxides is developed. The reaction employs cheap nickel as the catalyst, 1-(2-(di-tert-butylphosphanyl)phenyl)-4-methoxypiperidine (L3) as the ligand, and pyridine as the base. This reaction produces the corresponding (het)aromatic phosphorus compounds in good to high yields. Moreover, four new tertiary phosphine oxides are reported in this process.

Chiral bis(benzo[1,2-b:4,3-b′]dithiophene) atropisomers: experimental and theoretical investigations of the stereochemical and chiroptical properties

Conformational chirality is a feature that may arise from the presence of a hindered rotation around a single bond that corresponds to a stereogenic axis.

Application of the Povarov Reaction in Biaryls under Iron Catalysis for the General Synthesis of Dibenzo[a,c]Acridines

A modified Povarov reaction involving 2'-alkynylbiaryl-2-carbaldehydes and aryl amines with tandem oxidation was performed using catalytic FeCl₃. The outcome was an efficient general synthesis of dibenzo[a,c]acridines with moderate to high yields. This method offers simplicity in the preparation of substrates, diverse substrate scope, and high atom economy. The generality of the protocol was verified by synthesizing a tribenzo[a,c,h]acridine derivative. Photophysical properties of the synthesized compounds were also studied. The compounds absorb UV light typically in the range 230-330 nm and emit in the visible range of 400-420 nm.

A Decade of Electrochemical Dehydrogenative C,C-Coupling of Aryls

The importance of sustainable and green synthetic protocols for the synthesis of fine chemicals has rapidly increased during the last decades in an effort to reduce the use of fossil fuels and other finite resources. The replacement of common reagents by electricity provides a cost- and atom-efficient, environmentally friendly, and inherently safe access to novel synthetic routes. The selective formation of carbon-carbon bonds between two distinct substrates is a crucial tool in organic chemistry. This fundamental transformation enables access to a broad variety of complex molecular architectures. In particular, the aryl-aryl bond formation has high significance for the preparation of organic materials, drugs, and natural products. Besides well-known and well-established reductive- and oxidative-reagent-mediated or transition-metal-catalyzed coupling reactions, novel synthetic protocols have arisen, which require fewer steps than conventional synthetic approaches. Electroorganic conversions can be categorized according to the nature of the electron transfer processes occurring. Direct transformations at inert electrode materials are environmentally benign and cost-effective, whereas catalytic processes at active electrodes and mediated electrosynthesis using an additional soluble reagent can have beneficial properties in terms of selectivity and reactivity. In general, these conversions require challenging optimization of the reaction parameters and the appropriate cell design. Galvanostatic reactions enable fast conversions with a rather simple setup, whereas potentiostatic electrolysis may enhance selectivity. This Account discusses the development of seminal carbon-carbon bond formations over the past two decades, focusing on phenols leading to precursors for ligands in, e.g., hydroformylation reaction. A key element in the success of these electrochemical transformations is the application of electrochemically inert, non-nucleophilic, highly fluorinated alcohols such as 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which exhibit a large potential window for transformations and enable selective cross-coupling reactions. This selectivity is based on the capability of HFIP to stabilize organic radicals. Inert, carbon-based and metal-free electrode materials like graphite or boron-doped diamond (BDD) open up novel electroorganic pathways. Furthermore, novel active electrode materials have been developed to enable intra- and intermolecular dehydrogenative coupling reactions of electron-rich aryls. The application of 2,2'-biphenol derivatives as ligand components for catalysts requires reactions to be carried out on larger scale. In order to achieve this, continuous flow transformations have been established to overcome the drawbacks of heat transfer, overconversion, and conductivity. Modular cell designs enable the transfer of a broad variety of electroorganic conversions into continuous processes. Recent results demonstrate the application of organic electrochemistry to natural product synthesis of the pharmaceutically relevant opiate alkaloids (-)-thebaine or (-)-oxycodone.

Non-biaryl atropisomerism at the C–B bond in sterically hindered aminoarylboranes

Sterically hindered aminoarylboranes with atropisomerism about the C–B bond were prepared and resolved by chiral stationary phase HPLC.

Atroposelective synthesis of configurationally stable nonbiaryl N–C atropisomers through direct asymmetric aminations of 1,3-benzenediamines

An atroposelective synthesis of configurationally stable nonbiaryl N–C atropisomers through direct asymmetric aminations of 1,3-benzenediamines has been developed.

Atroposelective Synthesis of Biaryl Diamines and Amino Alcohols via Chiral Phosphoric Acid Catalyzed para-Aminations of Anilines and Phenols

A versatile method for atroposelective synthesis of chiral biaryl diamines and amino alcohols has been developed via para-amination of anilines and phenols with azodicarboxylates enabled by chiral phosphoric acid catalysis. Meanwhile, highly efficient kinetic resolution of the racemic biaryl anilines has also been realized through these reactions, giving selectivity factor up to 246. The gram-scale reaction and facile derivatizations of the chiral products well demonstrate the potential of these reactions in the development of novel chiral ligands and catalysts.

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Versatile methods for asymmetric synthesis of biaryl diamines and amino alcohols

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Atroposelective para-aminations of biaryl anilines and phenols

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Kinetic resolution of racemic biaryl anilines

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Facile transformations of chiral products

Synthesis and Characterisation of Mixed Ligand Complexes of Cu(II) Containing Optically Active R-(+)- & S-(–)-1,1′-bi-2-Naphthol (BINOL)

The novel mixed ligand complexes of Cu(II) with optically active R-(+)- / S-(–)-1,1′-bi-2-naphthol and N, N′ – donor bidentate ligands have been synthesised and characterised by elemental analysis and various spectral studies.

Selective Reduction of Carbon–carbon Double Bonds of Aryl Substituted Chalcones with Zn/CH3COONH4/C2H5OH/H2O

In this paper, 12 aryl substituted chalcones are easily reduced to the corresponding saturated carbonyl compounds using Zn/CH3COONH4/C2H5OH/H2O under mild condition with excellent yields for the first time.

Towards the Design of Optically Active Thiophene S-Oxides using Quantum Chemistry

The importance of axially chiral biaryls has risen steeply in the recent decades. This structural motif proved to be successful in catalytic asymmetric synthesis and the configuration of the biaryl axis is decisive for the biological activity. A new approach for the atroposelective synthesis of biaryls would be through a cycloaddition between an enantiopure phenyl-substituted thiophene S-oxide and an alkyne. Importantly, the chiral center of the thiophene S-oxide needs to be stable enough to avoid pyramidal inversion during the cycloaddition. Considering that the racemization of thiophene monoxides has been scarcely investigated so far, we perform a thorough quantum chemical study on the inversion barriers of a large number of chiral thiophene S-oxide derivatives. Our main goal is to identify substitution patterns leading to stable atropisomers at room temperature. Appealingly, the role of stereoelectronic effects and the position of the substituents as well as the importance of aromaticity on the pyramidal inversion barrier are elucidated for the first time.

Enantioselective bromination of axially chiral cyanoarenes in the presence of bifunctional organocatalysts

Enantioselective bromination of axially chiral cyanoarenes bearing high intrinsic rotational barriers via dynamic kinetic resolution using bifunctional organocatalysts is reported. Sequential addition of a brominating reagent in several portions at an optimized temperature was effective in accomplishing high enantioselectivities.

Enantioselective bromination of axially chiral cyanoarenes bearing high intrinsic rotational barriers via dynamic kinetic resolution using bifunctional organocatalysts is reported.

Transoid-to-Cisoid Conformation Changes of Single Molecules on Surfaces Triggered by Metal Coordination

Conformational isomers are stereoisomers that can interconvert over low potential barriers by rotation around a single bond. However, such bond rotation is hampered by geometrical constraints when molecules are adsorbed on surfaces. Here, we show that the adsorption of 4,4'-bis(4-carboxyphenyl)-6,6'-dimethyl-2,2'-bipyridine molecules on surfaces leads to the appearance of prochiral single molecules on NiO(001) and to enantiopure supramolecular domains on Au(111) surfaces containing the transoid-molecule conformation. Upon additional Fe adatom deposition, molecules undergo a controlled interconversion from a transoid-to-cisoid conformation as a result of coordination of the Fe atoms to the 2,2'-bipyridine moieties. As confirmed by atomic force microscopy images and X-ray photoelectron spectroscopy measurements, the resulting molecular structures become irreversibly achiral.

Arene-Limited Nondirected C-H Activation of Arenes

The nondirected C(sp² )-H activation of simple arenes has advanced significantly in recent years through the discovery of new catalyst systems that are able to perform transformations with the arene as the limiting reagent. Important developments in catalyst and ligand design that have improved reactivity and selectivity are reviewed.