Naked d-orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3+2] cycloaddition

Chiral Covalent Organic Cages: Structural Isomerism and Enantioselective Catalysis

Covalent organic cages are a prominent class of discrete porous architectures; however, their structural isomerism remains relatively unexplored. Here, we demonstrate the structural isomerism of chiral covalent organic cages that renders distinct enantioselective catalytic properties. Imine condensations of tetra-topic 5,10-di(3,5-diformylphenyl)-5,10-dihydrophenazine and ditopic 1,2-cyclohexanediamine produce two chiral [4 + 8] organic cage isomers with totally different topologies and geometries that depend on the orientations of four tetraaldehyde units with respect to each other. One isomer (PN-1) has an unprecedented Johnson-type J26 structure, whereas another (PN-2) adopts a tetragonal prismatic structure. After the reduction of the imine linkages, the cages are transformed into two amine bond-linked isomers PN-1R and PN-2R. After binding to Ni(II) ions, both can serve as efficient catalysts for asymmetric Michael additions, whereas PN-2R affords obviously higher enantioselectivity and reactivity than PN-1R presumably because of its large cavity and open windows that can concentrate reactants for the reactions. Density-functional theory (DFT) calculations further confirm that the enantioselective catalytic performance varies depending on the isomer.

Chiral Inorganic Nanomaterials for Photo(electro)catalytic Conversion

Chiral inorganic nanomaterials due to their unique asymmetric nanostructures have gradually demonstrated intriguing chirality-dependent performance in photo(electro)catalytic conversion, such as water splitting. However, understanding the correlation between chiral inorganic characteristics and the photo(electro)catalytic process remains challenging. In this perspective, we first highlight the chirality source of inorganic nanomaterials and briefly introduce photo(electro)catalysis systems. Then, we delve into an in-depth discussion of chiral effects exerted by chiral nanostructures and their photo-electrochemistry properties, while emphasizing the emerging chiral inorganic nanomaterials for photo(electro)catalytic conversion. Finally, the challenges and opportunities of chiral inorganic nanomaterials for photo(electro)catalytic conversion are prospected. This perspective provides a comprehensive overview of chiral inorganic nanomaterials and their potential in photo(electro)catalytic conversion, which is beneficial for further research in this area.

Experimental and Computational Investigation of Facial Selectivity Switching in Nickel-Diamine-Acetate-Catalyzed Michael Reactions

Chiral Ni complexes have revolutionized both asymmetric acid-base and redox catalysis. However, the coordination isomerism of Ni complexes and their open-shell property still often hinder the elucidation of the origin of their observed stereoselectivity. Here, we report our experimental and computational investigations to clarify the mechanism of β-nitrostyrene facial selectivity switching in Ni(II)-diamine-(OAc)₂-catalyzed asymmetric Michael reactions. In the reaction with a dimethyl malonate, the Evans transition state (TS), in which the enolate binds in the same plane with the diamine ligand, is identified as the lowest-energy TS to promote C-C bond formation from the Si face in β-nitrostyrene. In contrast, a detailed survey of the multiple potential pathways in the reaction with α-keto esters points to a clear preference for our proposed C-C bond-forming TS, in which the enolate coordinates to the Ni(II) center in apical-equatorial positions relative to the diamine ligand, thereby promoting Re face addition in β-nitrostyrene. The N-H group plays a key orientational role in minimizing steric repulsion.

Difluorodiazoethane as a masked acetylene equivalent in formal [3 + 2] cycloadditions with ketones to access 2,3-functionalized furans

A transition-metal-free [3+2] cycloaddition of CF2HCHN2 with β-ketones is reported, which enables the synthesis of 2,3-functionalized furans. Sequential defluorination, nucleophilic addition, and cyclization are key elemental steps of the reaction.

Dynamics in Catalytic Asymmetric Diastereoconvergent (3 + 2) Cycloadditions with Isomerizable Nitrones and α-Keto Ester Enolates

Reaction design in asymmetric catalysis has traditionally been predicated on a structurally robust scaffold in both substrates and catalysts, to reduce the number of possible diastereomeric transition states. Herein, we present the stereochemical dynamics in the Ni(II)-catalyzed diastereoconvergent (3 + 2) cycloadditions of isomerizable nitrile-conjugated nitrones with α-keto ester enolates. Even in the presence of multiple equilibrating species, the catalytic protocol displays a wide substrate scope to access a range of CN-containing building blocks bearing adjacent stereocenters with high enantio- and diastereoselectivities. Our computational investigations suggest that the enantioselectivity is governed in the deprotonation process to form (Z)-Ni-enolates, while the unique syn addition is mainly controlled by weak noncovalent bonding interactions between the nitrone and ligand.

Hydrogen-Bonding-Assisted Cationic Aqua Palladium(II) Complex Enables Highly Efficient Asymmetric Reactions in Water

Metal-bound water molecules have recently been recognized as a new facet of soft Lewis acid catalysis. Herein, a chiral palladium aqua complex was constructed that enables carbon-hydrogen bonds of indoles to be functionalized efficiently. We embraced a chiral 2,2'-bipyridine as both ligand and hydrogen-bond donor to configure a robust, yet highly Lewis acidic, chiral aqua complex in water. Whereas the enantioselectivity could not be controlled in organic solvents or under solvent-free conditions, the use of aqueous environments allowed the σ-indolylpalladium intermediates to react efficiently in a highly enantioselective manner. This work thus describes a potentially powerful new approach to the transformation of organometallic intermediates in a highly enantioselective manner under mild reaction conditions.

Asymmetric Synthesis of C1-Chiral THIQs with Imines in Isoquinoline Rings

Tetrahydroisoquinoline (THIQ) scaffolds are important structural units that widely exist in a variety of natural alkaloids and synthetic analogues. Asymmetric synthesis of C1-chiral THIQ is of particular importance due to its significant pharmaceutical, agrochemical, and other biological activities, and the usually distinct bioactivities exhibited by the two enantiomers. In this review, we highlight the significant advances achieved in this field, present recent asymmetric synthesis with imines in isoquinoline rings ordered according to the sequence of various substrate types. New strategies could be inspired and more types of substrates need further development.

1 Introduction

2 Catalytic Asymmetric Reaction of Dihydroisoquinolines

2.1 Asymmetric Reactions of 3,4-Dihydroisoquinolines

2.2 Asymmetric Reactions of Dihydroisoquinolinium Salts

2.3 Asymmetric Reactions of C,N-Cyclic N′-Acyl Azomethine Imines

2.3.1 NED [3+2] Cycloaddition of C,N-Cyclic N′-Acyl Azomethine Imines

2.3.2 IED [3+2] Cycloaddition of C,N-Cyclic N′-Acyl Azomethine Imines

2.3.3 [3+3] Cycloaddition of C,N-Cyclic N′-Acyl Azomethine Imines

2.3.4 [4+3] Cycloaddition of C,N-Cyclic N′-Acyl Azomethine Imines

2.3.5 Asymmetric Addition Reactions to C,N-Cyclic N′-Acyl Azomethine Imines

2.4 Asymmetric Reactions of C,N-Cyclic Nitrones

3 Catalytic Asymmetric Mannich Reactions of Isoquinolines

4 Conclusions and Perspectives

Single atom catalysts: a surface heterocompound perspective

The concept of single atom catalysts (SACs) originated from reducing the amount of noble metals used, by steadily refining the particle size loaded on a substrate surface. It has been rapidly moving to non-noble elements and their compounds in recent years, notably transition metals and even non-metals. They are of heterogeneous types, where the active species are refined to atomic dispersion scales on the surfaces/sub-surfaces of the solid support. The catalytic performance is governed by both the type and population of accessible active sites, and their bond and coordination environment, largely as a result of the interactions with the substrate surface. Unlike the internal structure within a crystalline solid, there is a large spatial variation in the bond and coordination environment of different atoms on the solid surface across different length scales, and in particular with the unsaturated surface, where there are various defects. They can also be dramatically altered during both the catalyst synthesis and actual catalysis process. In a way, they form a "surface heterocompound", where the local bonds for each metal atom are of a compound type, while there can be a large variation from one to another. Herein, we will look into the evolution from traditional heterogeneous catalysts to SACs, from the surface heterocompound perspective. Discussion will then be made on the on-going strategies and challenges in manipulating and identifying the local bond and coordination environment on the hetero-surfaces, in an attempt to develop efficient catalysts for the targeted applications, where both synthesis techniques and analytical tools are critically important, and computational studies can provide the key guiding principles. With selected paradigm studies, we will briefly examine the future perspectives for this newly emerging catalysis frontier.

Catalytic Asymmetric Formal [3+2] Cycloaddition of Azoalkenes with 3-Vinylindoles: Synthesis of 2,3-Dihydropyrroles

Chiral phosphoric acid-catalyzed highly enantioselective formal [3 + 2] cycloaddition reaction of azoalkenes with 3-vinylindoles has been established. Under mild conditions, the projected cycloaddition proceeded smoothly, affording a variety of 2,3-dihydropyrroles in high yields and excellent enantioselectivities, and also in a diastereospecific manner. As opposed to the common 4-atom synthons in the previous literature reports, azoalkenes served as 3-atom synthons. Besides, the observed selectivity was supported by primary theoretical calculation. The unique chemistry of azoalkenes disclosed herein will empower asymmetric synthesis of nitrogen-containing ring structural motifs in a broader context.

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Chiral phosphoric acid catalyzed formal [3 + 2] cycloaddition reaction

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2,3-Dihydropyrroles were enantioselectively synthesized

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Azoalkenes served as 3-atom synthons

New Insights into the Catalytic Activity of Cobalt Orthophosphate Co3 (PO4 )2 from Charge Density Analysis

An extensive characterization of Co3 (PO4 )2 was performed by topological analysis according to Bader's Quantum Theory of Atoms in Molecules from the experimentally and theoretically determined electron density. This study sheds light on the reactivity of cobalt orthophosphate as a solid-state heterogeneous oxidative-dehydration and -dehydrogenation catalyst. Various faces of the bulk catalyst were identified as possible reactive sites given their topological properties. The charge accumulations and depletions around the two independent five- and sixfold-coordinated cobalt atoms, found in the topological analysis, are correlated to the orientation and population of the d-orbitals. It is shown that the (011) face has the best structural features for catalysis. Fivefold-coordinated ions in close proximity to advantageously oriented vacant coordination sites and electron depletions suit the oxygen lone pairs of the reactant, mainly for chemisorption. This is confirmed both from the multipole refinement as well as from density functional theory calculations. Nearby basic phosphate ions are readily available for C-H activation.

Catalytic asymmetric cycloaddition of unsymmetrical EWG-activated alkenes to fully substituted pyrrolidines bearing three different carbonyl groups

A unique 1,3-dipolar [3+2] cycloaddition of alkyl 4-oxo-4-arylbut-2-enoates bearing two different electron-withdrawing groups was completed by using the silver/(R)-DTBM-Segphos catalyst system, which gives the corresponding fully substituted pyrrolidines with four stereogenic centers in good yields and with excellent enantioselectivities (up to 98% ee).

Selective Monoacylation of Diols and Asymmetric Desymmetrization of Dialkyl meso-Tartrates Using 2-Pyridyl Esters as Acylating Agents and Metal Carboxylates as Catalysts

With 2-pyridyl benzoates as acylating agents and Zn(OAc)₂ as a catalyst, 1,2-diols, 1,3-diols, and catechol were selectively monoacylated. Furthermore, the highly enantioselective desymmetrization of meso-tartrates was achieved for the first time, utilizing 2-pyridyl esters and NiBr₂/AgOPiv/Ph-BOX in CH₃CN or CuCl₂/AgOPiv/Ph-BOX in EtOAc catalyst systems (up to 96% ee). The latter catalyst system was also effective for the kinetic resolution of dibenzyl dl-tartrate.

Mechanistic investigations of the Au catalysed C-H bond activations in on-surface synthesis

Recently, Au-based nanostructures have attracted extensive interest due to their excellent activities in heterogeneous catalysis. The reaction mechanisms have been interpreted qualitatively by the quantum confinement effect due to the low-coordination of Au atoms in nanostructures. In this work, systematic first-principles calculations were carried out to obtain an in-depth understanding of the origin of C-H bond activations with Au-based catalysts in on-surface synthesis. Combining density functional theory (DFT) calculations and scanning tunneling microscopy (STM) studies, we reveal that the d-band centre and the d-band width of the Au-5dz2 orbital in an energy window of -6.80 to 0.00 eV may serve as theoretical descriptors for the prediction of the activity of Au catalysts in C-H bond activations. This work may therefore inspire further investigations on the design of new catalysts.

Highly efficient desymmetrization of cyclopropenes to azabicyclo[3.1.0]hexanes with five continuous stereogenic centers by copper-catalyzed [3 + 2] cycloadditions

An unprecedented copper-catalyzed desymmetrization/cycloaddition reaction of 1,1-disubstituted cyclopropenes provides an efficient access to azabicyclo[3.1.0]hexanes bearing five continuous carbon-stereogenic centers.

Nickel-Mediated Asymmetric Allylic Alkylation between Nitroallylic Acetates and Acyl Imidazoles

A nickel-mediated asymmetric allylic alkylation reaction between imidazole-modified ketones and nitroallylic acetates is presented. This reaction is catalyzed by a simple chiral diamine-nickel catalyst under mild conditions and leads to a series of novel enantioenriched α-allylic adducts in moderate to good yields with excellent enantioselectivities. Furthermore, transformation of the allylic adducts could smoothly lead to chiral γ-nitro-esters containing three continuous stereocenters in good yields.

Stereogenic-Only-at-Metal Asymmetric Catalysts

Chirality is an essential feature of asymmetric catalysts. This review summarizes asymmetric catalysts that derive their chirality exclusively from stereogenic metal centers. Reported chiral-at-metal catalysts can be divided into two classes, namely, inert metal complexes, in which the metal fulfills a purely structural role, so catalysis is mediated entirely through the ligand sphere, and reactive metal complexes. The latter are particularly appealing because structural simplicity (only achiral ligands) is combined with the prospect of particularly effective asymmetric induction (direct contact of the substrate with the chiral metal center). Challenges and solutions for the design of such reactive stereogenic-only-at-metal asymmetric catalysts are discussed.

Catalytic Enantioselective [3 + 2] Cycloaddition of α-Keto Ester Enolates and Nitrile Oxides

An enantioselective [3 + 2] cycloaddition reaction between nitrile oxides and transiently generated enolates of α-keto esters has been developed. The catalyst system was found to be compatible with in situ nitrile oxide-generation conditions. A versatile array of nitrile oxides and α-keto esters could participate in the cycloaddition, providing novel 5-hydroxy-2-isoxazolines in high chemical yield with high levels of diastereo- and enantioselectivity. Notably, the optimal reaction conditions circumvented concurrent reactions via O-imidoylation and hetero-[3 + 2] pathways.