Classical‐Reaction‐Driven Stereo‐ and Regioselective C(sp3)–H Functionalization of Aliphatic Amines

A large variety of synthetic methods have been developed for the synthesis of functionalized aliphatic amines because of their broad spectrum of application. Metallic reagents/catalysts and/or toxic oxidants are involved in most of the cases. Direct C? H functionalization of aliphatic amines via their classical condensation reactions with suitable carbonyl compounds is advantageous because this method avoids hazardous metallic reagents, toxic oxidants and pre‐activation/pre‐functionalization step(s). In this account, the concept of direct C? H functionalization of aliphatic amines based on the classical condensation–isomerization–addition (CIA) strategy followed by recent contributions from our ongoing research in the field along with relevant examples from other groups are described. Successes in stereo‐ and regioselective C? C and C? O bond formation via direct α‐ as well as β‐C(sp³)–H functionalization are discussed.

     

Development of C3‐Symmetric Tris‐Urea Low‐Molecular‐Weight Gelators

This article describes recent developments in C₃‐symmetric tris‐urea low‐molecular‐weight gelators and their applications. The C₃‐symmetric tris‐ureas are excellent frameworks to form supramolecular polymers through noncovalent interactions. In organic solvents, hydrophobic tris‐ureas form supramolecular gels. Amphiphilic tris‐ureas form supramolecular gels in aqueous media. Functional supramolecular gels were prepared by introducing appropriate functional groups into the outer sphere of tris‐ureas. Supramolecular hydrogels obtained from amphiphilic tris‐ureas were used in the electrophoresis of proteins. These electrophoreses results showed several unique characteristics compared to typical electrophoreses results obtained using polyacrylamide matrices.

     

Metal‐Free Sensitizers for Dye‐Sensitized Solar Cells

This review focuses on our work on metal‐free sensitizers for dye‐sensitized solar cells (DSSCs). Sensitizers based on D?A′?π?A architecture (D is a donor, A is an acceptor, A′ is an electron‐deficient entity) exhibit better light harvesting than D?π?A‐type sensitizers. However, appropriate molecular design is needed to avoid excessive aggregation of negative charge at the electron‐deficient entity upon photoexcitation. Rigidified aromatics, including aromatic segments comprising fused electron‐excessive and ‐deficient units in the spacer, allow effective electronic communication, and good photoinduced charge transfer leads to excellent cell performance. Sensitizers with two anchors/acceptors, D(–π–A)₂, can more efficiently harvest light, inject electrons, and suppress dark current compared with congeners with a single anchor. Appropriate incorporation of heteroaromatic units in the spacer is beneficial to DSSC performance. High‐performance, aqueous‐based DSSCs can be achieved with a dual redox couple comprising imidazolium iodide and 2,2,6,6‐tetramethylpiperidin‐N‐oxyl, and/or using dyes of improved wettability through the incorporation of a triethylene oxide methyl ether chain.

     

Organo‐ and Organometallic‐Catalytic Intramolecular [1,5]‐Hydride Transfer/Cyclization Process through C(sp3)–H Bond Activation

The direct functionalization of C(sp³)–H bonds is one of the most synthetically powerful research areas in current organic synthesis. Organocatalytic C(sp³)–H bond activation reactions have recently been developed in addition to the traditional metal‐catalyzed C(sp³)–H activation reactions. In this review, we aim to give a brief overview of organo‐ and organometallic internal redox cascade reactions with respect to the mechanism, the reactivity of hydrogen donors and acceptors, and the migration modes of hydrogen.

     

Switching of Single‐Molecule Magnetic Properties of TbIII–Porphyrin Double‐Decker Complexes and Observation of Their Supramolecular Structures on a Carbon Surface

Double‐decker complexes based on single‐molecule magnets (SMMs) are a class of highly promising molecules for applications in molecular spintronics, wherein control of both the ligand oxidative states and the 2D supramolecular structure on carbon materials is of great importance. This study focuses on the synthesis and study of 2,3,7,8,12,13,17,18‐octaethylporphyrin (OEP)–Tb^III double‐decker complexes with different electronic structures comprising protonated, anionic, and radical forms. Magnetic susceptibility measurements revealed that only the anionic and radical forms of the OEP–Tb^III double‐decker complexes exhibited SMM properties. The barrier heights for magnetic moment reversal were estimated to be 207 and 215 cm^?1 for the anionic and radical forms, respectively. Scanning tunneling microscopy (STM) investigations revealed that these OEP–Tb^III complexes form well‐ordered monolayers upon simple dropcasting from dilute dichloromethane solutions. All three complexes form an isomorphic pseudo‐hexagonal 2D pattern, regardless of the differences in the electronic structures of their porphyrin–Tb cores. This finding is of interest for SMM technology as ultrathin films of these materials undergoing chemical transformations will not require any detrimental reorganization. Finally, we demonstrate self‐assembly of the protonated 5,15‐bisdodecylporphyrin (BDP)–Tb^III double‐decker complex as an example of successful supramolecular design to achieve controlled alignment of SMM‐active sites.     

From QCA (Quantum Cellular Automata) to Organocatalytic Reactions with Stabilized Carbenium Ions

What do quantum cellular automata (QCA), “on water” reactions, and S_N1‐type organocatalytic transformations have in common? The link between these distant arguments is the practical access to useful intermediates and key products through the use of stabilized carbenium ions. Over 10 years, starting with a carbenium ion bearing a ferrocenyl group, to the 1,3‐benzodithiolylium carbenium ion, our group has exploited the use of these intermediates in useful and practical synthetic transformations. In particular, we have applied the use of carbenium ions to stereoselective organocatalytic alkylation reactions, showing a possible solution for the “holy grail of organocatalysis”. Examples of the use of these quite stabilized intermediates are now also considered in organometallic chemistry. On the other hand, the stable carbenium ions are also applied to tailored molecules adapted to quantum cellular automata, a new possible paradigm for computation. Carbenium ions are not a problem, they can be a/the solution!

     

The Application of Copper/Iron Cocatalysis in Cross‐Coupling Reactions

For conventional cross‐couplings in organic chemistry, precious metal (such as Pd or Rh) complexes have been the preferable choices as catalysts. However, their high cost, toxicity, and potential contamination of products limit their massive applications on some occasions, particularly in the pharmaceutical industry, where close monitoring of the metal contamination of products is required. Therefore, the use of metals that are less expensive and less toxic than Pd or Rh can be greatly advantageous and earth abundant metal (such Fe or Cu) catalysts have shown promise for replacing the precious metals. Interestingly, a certain copper catalyst combined with an iron catalyst displays higher catalytic efficiency than itself in various coupling reactions. Notably, ligand‐free conditions make such protocols more useful and practical in many cases. In this account, we summarize the recent progress made in this increasingly attractive topic by describing successful examples, including our own work in the literature, regarding effective copper/iron cocatalysis. In addition, a few examples involving a magnetic and readily recyclable CuFe₂O₄ nanoparticle cocatalyst are also included.

     

Domino Reactions Based on Combinatorial Bond Transformations in Electron‐Deficient Tertiary Enamines

Electron‐deficient enamines such as enaminones and enaminoesters are moieties showing widespread application in organic synthesis. Among the various available electron‐deficient enamines, the N,N‐disubstituted amino‐functionalized ones (tertiary enamines) represent a class of special enamines with distinct properties and important applications. Based on our longstanding interest in exploring novel synthetic methods using electron‐deficient tertiary enamines, we present herein the research advances in organic synthesis via domino reactions making use of the combinatorial C–N, C=C, C–H, and other bond transformations of electron‐deficient tertiary enamines.

     

Advances in Transition‐Metal‐Catalysed Alkyne Hydroarylations

We present herein a personal account of our achievements in the development of novel catalytic systems based on late‐transition‐metal complexes for the hydroarylation of alkynes. In particular, our targets were intermolecular hydroarylation reactions with arene or heteroarene substrates devoid of directing groups. We have shown that complexes of palladium, platinum or gold with N‐heterocyclic carbene (NHC) ligands can be particularly useful catalysts for this reaction; the NHC ligand imparts greater stability to the complex and renders the catalytic system more productive. Furthermore, we have identified promoters and reaction media that allow to significantly improve the catalytic activity under mild conditions, to control the reaction chemoselectivity and to steer it towards more complex products; thus making this reaction considerably more attractive for the synthetic chemist.

     

Transition‐Metal‐Catalyzed Direct Addition of Aryl C–H Bonds to Unsaturated Electrophiles

The direct addition of Csp²–H bonds onto polar C=C, C=O, and C=N bonds is both synthetically and mechanistically important, because using aromatic C–H substrates in place of organometallic reagents provides a more direct and atom‐economical alternative to many important compounds without the pre‐generation of organometallic compounds from stoichiometric halides and the unavoidable generation of stoichiometric metal halide waste. In this account, we summarize our contributions to the transition‐metal‐catalyzed addition of aromatic C–H bonds to polar C=C, C=O, and C=N bonds via directing‐group‐assisted regiospecific reactions. These synthetic methods provide efficient access to benzylic alcohols, alkylbenzenes, 3‐substituted phthalides, N‐substituted phthalimides, N‐aryl benzamides, and indene derivatives from commercially available reagents. It is worth noting that valuable heterocycles such as 3‐substituted phthalides and N‐substituted phthalimides can be obtained in one step by this approach.

     

Synthesis of Multisubstituted Olefins through Regio‐ and Stereoselective Addition of Interelement Compounds Having B–Si,B–B,and Cl–S Bonds to Alkynes,and Subsequent Cross‐Couplings

Multisubstituted olefins are fundamental motifs in organic compounds. In this account, we describe the synthesis of organic molecules bearing an olefinic moiety by the transition‐metal‐catalyzed regio‐ and stereoselective addition of a variety of interelement compounds to alkynes. Regio‐ and stereoselective silaboration, diborylation, and chlorothiolation have been achieved by using the transition‐metal catalysts. The subsequent cross‐coupling reactions of the boron‐containing alkenes to install various aryl groups afforded the corresponding tri‐ and tetraarylated olefins. This account describes our research on the highly regio‐ and stereoselective synthesis of multifunctionalized olefins such as tetraarylethenes with four different aryl groups.

     

Enantioselective 1,3‐dipolar cycloaddition reactions using complexes of (−)‐sparteine

Regio‐ and stereoselective 1,3‐dipolar cycloaddition of nitrile oxides to internal 2‐pentenols and α,β‐unsaturated esters catalyzed by (?) sparteine‐lanthanide complexes affords corresponding 3‐aryl‐2‐isoxazolines with enantioselectivities up to 68% ee.     

Design,Synthesis, and Functionalization of Imidazoheterocycles

Over the years, various strategies have been reported for the synthesis of imidazo[1,2‐a]pyridines due to their importance in different fields. In this account, we represent the methods developed by our group for the synthesis and functionalization of imidazo[1,2‐a]pyridines. Different synthetic strategies have been developed using easily accessible reactants for this purpose. We envisage that these newly developed protocols will be very useful for the synthesis of functionalized molecules bearing imidazo[1,2‐a]pyridine scaffolds. These strategies will also be attractive for the construction of other pharmaceutically important heterocycles.

     

Synthesis of Polystyrene and Poly(4‐vinylpyridine) Mixed Grafted Silica Nanoparticles via a Combination of ATRP and CuI‐Catalyzed Azide‐Alkyne Click Chemistry

Binary polystyrene and poly(4‐vinylpyridine) mixed grafted silica nanoparticles (PSt/P4VP‐g‐SNPs) are fabricated using Cu^I‐catalyzed azide‐alkyne Huisgen cycloaddition (CuAAC) via grafting‐to method. Azide‐terminated PSt and P4VP are synthesized via post‐ and pre‐atom transfer radical polymerization modification, respectively. Then, the polymers are simultaneously anchored onto alkyne‐modified SNPs by CuAAC yielding mixed brushes as shown by Raman spectroscopy, dynamic light scattering, and thermogravimetric analysis. To the best of our knowledge, this is the first report of simultaneously grafting two distinct polymer chains to synthesize mixed grafted silica nanoparticles using CuAAC technique via grafting‐to method.

     

Bio‐/Chemosensors and Imaging with Aggregation‐Induced Emission Luminogens

Aggregation‐induced emission (AIE) luminogens show abnormal fluorescent behavior; they are non‐emissive in solution, but they become strongly emissive after aggregation. Sensing and imaging are the major applications of AIE luminogens. By properly manipulating the aggregation and deaggregation of AIE molecules, various bio‐/chemosensors have been developed. Moreover, AIE molecules with targeting groups have been devised for imaging of organelles and cancer cells. In this account, we report our recent work on the application of AIE luminogens for the construction of bio‐/chemosensors and imaging.

     

Preparation and evaluation of N‐sulfinyl dienophiles for asymmetric hetero‐diels‐alder reactions

A series of N‐sulfinyl dienophiles 1c‐i has been screened in asymmetric hetero‐Diels‐Alder reactions using chiral bis(oxazoline)copper(II) and ‐zinc(II) triflates. The survey pointed out N‐sulfine 1c (R ? P(?O)(OPh)₂) as the most promising N‐sulfine regarding yield and stereoselectivity (up to 97% ee). The relative configurations, and in one case the absolute configuration, of the HDA adducts were established by X‐ray analysis.     

Extending the Substrate Scope for the Asymmetric Iridium‐Catalyzed Hydrogenation of Minimally Functionalized Olefins by Using Biaryl Phosphite‐Based Modular Ligand Libraries

Asymmetric hydrogenation is one of the most efficient and atom‐economical tools to prepare chiral molecules. However, the enantiodiscrimination of simple, minimally functionalized olefins is still challenging and requires more sophisticated ligand design. Herein, we discuss our progress in the successful development of ligand design for the iridium‐catalyzed asymmetric hydrogenation of minimally functionalized olefins.

     

Molecular Spintronics Based on Single‐Molecule Magnets Composed of Multiple‐Decker Phthalocyaninato Terbium(III) Complex

Unlike electronics, which is based on the freedom of the charge of an electron whose memory is volatile, spintronics is based on the freedom of the charge, spin, and orbital of an electron whose memory is non‐volatile. Although in most GMR, TMR, and CMR systems, bulk or classical magnets that are composed of transition metals are used, this Focus Review considers the growing use of single‐molecule magnets (SMMs) that are composed of multinuclear metal complexes and nanosized magnets, which exhibit slow magnetic‐relaxation processes and quantum tunneling. Molecular spintronics, which combines spintronics and molecular electronics, is an emerging field of research. Using molecules is advantageous because their electronic and magnetic properties can be manipulated under specific conditions. Herein, recent developments in [LnPc]‐based multiple‐decker SMMs on surfaces for molecular spintronic devices are presented. First, we discuss the strategies for preparing single‐molecular‐memory devices by using SMMs. Next, we focus on the switching of the Kondo signal of [LnPc]‐based multiple‐decker SMMs that are adsorbed onto surfaces, their characterization by using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of [TbPc₂]. Finally, the field‐effect‐transistor (FET) properties of surface‐adsorbed [LnPc₂] and [Ln₂Pc₃] cast films are reported, which is the first step towards controlling SMMs through their spins for applications in single‐molecular memory and spintronics devices.     

Oxygen‐Promoted Suzuki–Miyaura Reaction for Efficient Construction of Biaryls

As one of the most powerful and versatile methods for the construction of carbon–carbon bonds, the Suzuki–Miyaura cross‐coupling reaction has attracted great attention over the past three decades. In recent years, a huge amount of interest has been focused on the development of ligand‐free Suzuki–Miyaura reaction systems, which have the advantages of low cost, mild reaction conditions, and easy operation. So far, a number of ligand‐free Suzuki–Miyaura reaction systems have been developed by using simple palladium salts, nanopalladium, or supported palladium catalysts. In this account, we will review our recent research on the oxygen‐promoted ligand‐free Suzuki–Miyaura reaction. Interestingly, the oxygen‐promoting effect has been observed in different reaction media, including polyethylene glycol, organic/water mixed solvents and pure water. The oxygen‐promoted reaction systems demonstrate high efficiency for the construction of biaryls.