Fluorophores for Excited-State Intramolecular Proton Transfer by an Yttrium Triflate Catalyzed Reaction of Isocyanides with Thiocarboxylic Acids

Recent Advances of C-S Coupling Reaction of (Hetero)Arenes by C-H Functionalization

Organic sulfur compounds encompass a vast and diverse variety of species that possess unique biological activity due to the presence of sulfur atoms or sulfur-containing functional groups. These compounds are widely present in natural products, pharmaceuticals, agricultural chemicals, and functional materials. In recent years, numerous sulfur-containing compounds such as thiols, thioethers, disulfides, thiourea, dimethyl sulfoxide, sulfonates and their derivatives, as well as sulfur-containing inorganic compounds, have been utilized as coupling agents to synthesize (hetero)aryl sulfides via C-H Functionalization. These novel transformations provide effective methods for constructing C-S bond of (hetero)arenes, while also expanding the scope of (hetero)aryl sulfides with the potential biological activity. Therefore, the synthesis of aryl sulfides through C-H bond functionalization has attracted widespread attention. This review mainly focuses on the construction of (hetero)aryl sulfides via C-H bond functionalization since 2015. We hope this review offers a useful conceptual overview and inspires further advancements in the efficient construction of C-S bonds.

Insights into the Palladium(II)-Catalyzed Wacker-Type Oxidation of Styrene with Hydrogen Peroxide and tert-Butyl Hydroperoxide

Wacker oxidations are ubiquitous in the direct synthesis of carbonyl compounds from alkenes. While the reaction mechanism has been widely studied under aerobic conditions, much less is known about such processes promoted with peroxides. Here, we report an exhaustive mechanistic investigation of the Wacker oxidation of styrene using hydrogen peroxide (H2O2) and tert-butyl hydroperoxide (TBHP) as oxidants by combining density functional theory and microkinetic modeling. Our results with H2O2 uncover a previously unreported reaction pathway that involves an intermolecular proton transfer assisted by the counterion [OTf]- present in the reaction media. Furthermore, we show that when TBHP is used as an oxidant instead of H2O2, the reaction mechanism switches to an intramolecular protonation sourced by the HOtBu moiety generated in situ. Importantly, these two mechanisms are predicted to outcompete the 1,2-hydride shift pathway previously proposed in the literature and account for the level of D incorporation in the product observed in labeling experiments with α-d-styrene and D2O2. We envision that these insights will pave the way for the rational design of more efficient catalysts for the industrial production of chemical feedstocks and fine chemicals.

Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Triple-Consecutive Isocyanide Insertions with Aldehydes: Synthesis of 4-Cyanooxazoles

An efficient TMSOTf-promoted selective triple consecutive insertions of tert-butyl isocyanide into aldehydes has been developed, affording pharmacological interesting 4-cyanooxazoles in high yields in a one pot manner. The given method encompasses a wide range of substrates with tert-butyl isocyanide serving as sources of critical "CN" and "C-N═C" moieties. The versatile transformations of the resulting 4-cyanooxazoles were demonstrated. The key reaction intermediates for plausible mechanisms were determined.

Interplay of Dual-Proton Transfer Relay to Achieve Full-Color Panel Luminescence in Excited-State Intramolecular Proton Transfer (ESIPT) Fluorophores

A new design was applied for the facile synthesis of pure organic photoluminescent molecules with dual excited-state intramolecular proton transfer (ESIPT) sites. In this novel class of emitters, full-color panel emission from blue, green, and yellow to red, including white light, can be achieved in different solvents as modulated by the enol-keto(1st)-keto(2nd) tautomer emissions. A comprehensive transient photophysical study verifies that keto(1st) and keto(2nd) have a precursor (<0.8 ps)-successor (∼20 ps)-relayed absorbance relationship, and then a fast equilibrium between the two is established, resulting in dual emissions in the nanosecond scale (∼1900 ps). Through the research on copper ions' selective PL response, the dual-ESIPT mechanism was further verified; in addition, the study of solid-state PL changes upon the stimulus of organic vapor manifests the potential application sensitivity of the molecules as dual-ESIPT sensors. Theoretical results including reaction potential energy surface analyses manifest the fact that dual-proton transfer goes along a sequential route with a smaller energy barrier, firmly supporting the experimental results. An intrinsic system that undergoes intramolecular double proton relayed transfer is thus established for the achievement of much broadened optical responses and full-color display, providing reference for the design and application of advanced dual-ESIPT optical materials.

Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Δλ, 77-101 nm) with quantum yields (ϕ_FL ) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (ϕ_PL ) and lifetime (τ_p up to 251 μs). The presence of intramolecular H-bonds and negligible π-π stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, π-π and C-H^… π interactions could be responsible for the observed RTP of iodine containing phosphors.

The Dark Side of Isocyanides: Visible-Light Photocatalytic Activity in the Oxidative Functionalization of C(sp3)-H Bonds

The possibility to harness aromatic isocyanides as visible-light photocatalysts in the α-amino C(sp³)-H functionalization is herein presented. Actually, the three-component cross-dehydrogenative coupling of aromatic tertiary amines with isocyanides and water leads to amide products under very mild conditions in high yields and with a good substrate scope. While the reaction with aromatic isocyanides proceeds upon direct photoexcitation, aliphatic isocyanides are able to form a photoactive electron-donor-acceptor complex with aromatic amines. Moreover, the use of a catalytic loading of an aromatic isocyanide promotes the oxidative coupling of N-phenyl-1,2,3,4-tetrahydroisoquinoline with an array of different (pro)nucleophiles in good to excellent yields, thus providing the proof-of-concept for the development of a new highly tunable class of organic visible-light photocatalysts.

Double C-S bond formation via multiple Csp3-H bond cleavage: synthesis of 4-hydroxythiazoles from amides and elemental sulfur under metal-free conditions

A novel and efficient approach for the synthesis of 4-hydroxythiazoles from amides and elemental sulfur has been developed. In the presence of P₂O₅, DMSO and HMPA, this metal-free protocol proceeds smoothly and tolerates a spectrum of functional groups. Furthermore, this strategy involves the process of double Csp³-S bond formation through the cleavage of multiple Csp³-H bonds for the first time.

Alkylaminomaleimide fluorophores: synthesis via air oxidation and emission modulation by twisted intramolecular charge transfer

A novel strategy to synthesize 3-alkylaminomaleimide fluorophores via air oxidation is developed, and the structural features for the designed TICT fluorophores with bright emission are established.

Transition Metal and Inner Transition Metal Catalyzed Amide Derivatives Formation through Isocyanide Chemistry

The synthesis of amides is a substantial research area in organic chemistry because of their ubiquitous presence in natural products and bioactive molecules. The use of easily accessible isocyanides as amidoyl (carbamoyl) synthons in cross-coupling reactions using transition metal and inner transition metöal catalysts is a current trend in this area. Isocyanides, owing to their coordination ability as a ligand and inherent electronic properties for reactions with various partners, have expanded the potential application of these transformations for the preparation of novel synthetic molecules and pharmaceutical candidates. This review gives an overview of the achievements in isocyanide-based transition metal and inner transition metal catalyzed amide formation and discusses highlights of the proposed distinct mechanisms.

1 Introduction

2 Synthesis of Arenecarboxamides

3 Synthesis of Alkanamides

4 Synthesis of Cyclic Amides

5 Formation of Alkynamides

6 Formation of Acrylamide-like Molecules

7 Formation of Ureas and Carbamates

8 Conclusion

Restriction of Conformation Transformation in Excited State: An Aggregation-Induced Emission Building Block Based on Stable Exocyclic C=N Group

The development of aggregation-induced emission (AIE) building block and deciphering its luminescence mechanism are of great significance. Here a feasible strategy for the construction of AIE unit based on E-Z isomerization (EZI) of exocyclic C=N double bond is proposed. Taking [1,2,4]thiadiazole[4,3-a]pyridine (TZP) derivative as an example, its aryl-substituted derivative (TZPP) shows obvious AIE character. The analysis of spectral data and theoretical calculations indicates that fast structural relaxation of TZPP in the emissive state plays a key role in a low fluorescence quantum yield in dilute solution, which should be caused by the small energy gap between locally excited (LE) state and twisted intramolecular charge transfer state. When in solid state, the bright emission with LE state characteristic reappears due to the large shift barrier of geometry transformation. As a potential building block for AIEgens with special heterocyclic structure, these findings would open up opportunities for developing various functional materials.

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A new aggregation-induced emission building block

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A novel AIE mechanism with spectral measurements and theoretical calculations

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Available starting materials resulting in convenient synthesis and modification

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A stable exocyclic C=N double bond in heterocycles

Ce(OTf)3-Catalyzed Multicomponent Reaction of Alkynyl Carboxylic Acids, tert-Butyl Isocyanide, and Azides for the Assembly of Triazole-Oxazole Derivatives

Cerium(III) triflate-catalyzed multicomponent reactions between alkynyl carboxylic acids, tert-butyl isocyanide, and organic azides have been developed. In the presence of Ce(OTf)₃ (10 mol %), the cascade reaction of one molecule of alkynyl carboxylic acid with three molecules of tert-butyl isocyanides proceeds chemoselectively and regioselectively via a triple and ordered isocyanide insertion process at room temperature, and then the cesium-catalyzed [3 + 2] cycloaddtion reaction between the resulted alkynyl oxazole and organic azides was further initiated by the temperature elevation (100 °C), thereby leading to multisubstituted triazole-oxazole derivatives in practical, time-saving, one-pot operations. Furthermore, some of the synthesized target compounds showed potential anticancer activities against MGC803 (human gastric cancer cell) with IC₅₀ values below 20 μmol L^-1.

Facile synthesis of 2-alkynyl oxazoles via a Ce(OTf)3-catalyzed cascade reaction of alkynyl carboxylic acids with tert-butyl isocyanide

We developed an efficient and novel protocol to synthesize 2-alkynyl oxazoles from tert-butyl isocyanide and alkynyl carboxylic acids. This method allowed the synthesis of diversely functionalized oxazoles under mild reaction conditions, coupled with operational simplicity and these functionalized oxazoles showed a certain degree of biological activity. Moreover, compounds 2b, 2h, 2k, 2n, 2p and 2t exhibited good anticancer activities in human gastric cancer cells (MGC803) and human bladder tumor cells (T24), with IC₅₀ below 20.0 μM.

Low-Lying Excited States of hqxcH and Zn-hqxc Complex: Toward Understanding Intramolecular Proton Transfer Emission

Excited state intramolecular proton transfer (ESIPT) has been a topic of interest due to its potential to lead to multiple emissions. Although many organic molecules showing ESIPT emission are already known, studies on metal complexes showing ESIPT and their related theoretical understandings are very limited. In this study, we focus on [Zn(hqxc)₂(DMSO)₂] (Zn-hqxc: hqxc = 3-hydroxy-2-quinoxalinecarboxylate, DMSO = dimethyl sulfoxide), which shows ESIPT emission in the solid state, even though the hqxcH ligand does not show ESIPT emission. To gain insights into the role of the zinc atom and the emission mechanisms, we examined excited states of free hqxcH and the Zn-hqxc complex using time-dependent density functional theory calculations. From the results, it was shown that the zinc atom triggers a structural change of the hqxcH ligand from the lactam form (3,4-dihydro-3-oxo-2-quinoxalinecarboxylic acid) to the enol form (3-hydroxy-2-quinoxalinecarboxylic acid), where the latter form has several stable excited states. Several stable geometries were found for singlet and triplet excited states, suggesting that emissions for the Zn-hqxc complex can be both phosphorescence and fluorescence caused by the enol-enol, keto-keto, and keto-enol forms of the two hqcx ligands in the complex. We found that the photophysical properties of the Zn-hqxc complex are dominated by the ligand due to the filled d¹⁰ of Zn(II). The presented results suggest that, to design new ESIPT metal complexes, one possible approach is to combine a metal atom showing ligand centered emission and a ligand that has separate ESIPT and coordination sites.

Three-component bis-heterocycliation for synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles

A cooperative base system has been developed for the novel three-component synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles via bis-heterocyclization of methylketoxime acetates.

Retracted Article: Molybdenum-silver co-catalyzed cycloaddition of alkynes with N-isocyanoiminotriphenylphosphorane (NIITP): an efficient strategy for the synthesis of monosubstituted pyrazoles

A highly efficient synthesis of monosubstituted pyrazoles from alkynes and N-isocyanoiminotriphenylphosphorane (NITTP) is developed via molybdenum-silver co-catalyzed [3+2] cycloaddition.

Synthesis, Molecular Engineering, and Photophysical Properties of Fluorescent Thieno[3,2- b]pyridine-5(4 H)-ones

We describe a synthetic approach for a set of fluorescent thieno[3,2- b]pyridine-5(4 H)-one derivatives and their photophysical properties. These fluorophores are prepared by a series of reactions employing the Suzuki-Miyaura cross-coupling reaction and a regioselective aza-[3 + 3] cycloaddition of 3-aminothiophenes with α,β-unsaturated carboxylic acids. Our findings revealed that the photophysical properties are chemically tunable by an appropriate choice of functional group on the thieno[3,2- b]pyridine-5(4 H)-one scaffold.

Microcapsule-Containing Self-Reporting Polymers

Self-reporting polymers, which can indicate damage or exposure to excessive stress with a clearly perceptible optical signal, are potentially useful for several technological applications, including stress-sensitive sensors that enable in situ monitoring of mechanical events and structural health monitoring systems. A versatile and simple concept to realize this function is the exploitation of microcapsules that are filled with solutions of dyes that are released and chemically or physically activated when the protective shell is damaged. Such microcapsules can readily be incorporated into polymers and the composites thus made can be processed into films, coatings, or other objects. Mechanochromic effects can be realized with different types of dyes and activation schemes. In this concept article, a selection of recent key studies is presented to provide an overview of the state of the field. Different architectures and operating principles and their advantages and drawbacks are reviewed. The parameters that influence the design of microcapsule-based mechanochromic systems are considered and unexplored chromophore systems that might be useful to design future self-reporting polymers are discussed. Finally, specific aspects of capsule design, fabrication, and integration into polymers are presented. Throughout the article, challenges and opportunities of the concept are highlighted and possible future directions are discussed.

Still Unconquered: Enantioselective Passerini and Ugi Multicomponent Reactions

The Passerini three-component (P-3CR) and the Ugi four-component (U-4CR) are two of the most prominent isocyanide-based multicomponent reactions (IMCRs). The P-3CR transforms isocyanides, aldehydes (ketones), and carboxylic acids to α-acyloxy carboxamides, while the U-4CR converts isocyanides, aldehydes (ketones), amines, and carboxylic acids to α-acetamido carboxamides. Conversion of the high energy formal divalent isocyano carbon into a tetravalent amide carbonyl carbon provides the driving force for these reactions. While the prototypical P-3CR and U-4CR provide linear adducts, many heterocycles and macrocycles are now readily synthesized by modifying these truly versatile reactions. As one stereocenter is generated by the nucleophilic addition of the isocyanide to the carbonyl and imine functions, the search for enantioselective versions of these reactions has become a much sought after goal among synthetic chemists. This seemingly trivial endeavor turns out to be extremely difficult to achieve, in sharp contrast to the remarkable progress documented in the field of asymmetric synthesis in general and catalytic enantioselective nucleophilic addition to C═X bond in particular. Since Denmark's first report in 2003 on the catalytic enantioselective Passerini two-component reaction of isocyanides with aldehydes, several Lewis acid (LA) and Brønsted acid-catalyzed enantioselective protocols have been developed. However, it is fair to say that truly catalytic enantioselective P-3CR and U-4CR with wide application scope remain elusive. In this Account, we summarize the progress recorded in this field over the past 15 years. We entered the field by investigating the enantioselective reaction of α-isocyanoacetamides with aldehydes and imines, which was previously developed in our lab for the synthesis of functionalized 5-aminooxazoles. Our initial experimental results, in conjunction with Dömling's and Schreiber's earlier findings, prompted us to assume that the low turnover number in LA-catalyzed asymmetric IMCRs is a main hurdle for enantioselectivity. We speculated that the LA incapable of forming chelates would be the catalyst of choice for enantioselectivity, the rational being that the P-3CR and the U-4CR afforded bidentate intermediates (α-hydroxy imidates, α-amino imidates) and products (α-acyloxy carboxamides, α-acetamido carboxamides) from nonchelating inputs. Therefore, the transfer of catalyst from these chelating intermediates or products to the monocoordinating starting materials would be difficult, hence the problem with catalyst turnover. This working hypothesis turned out to be a valuable guide that allowed us to develop Al-salen and Al-phosphate-catalyzed enantioselective P-3CR and enantioselective construction of chiral heterocycles such as oxazoles and tetrazoles. Nevertheless, all our attempts to apply these LA catalysts to the Ugi reaction failed. Indeed, to date, no reports on the successful LA-catalyzed asymmetric Ugi-type reactions exist in the literature. However, significant progress has been made in recent years employing organocatalysts. We developed a chiral phosphoric acid (CPA)-catalyzed enantioselective three-component synthesis of 2-(1-aminoalkyl)-5-aminooxazoles, a four-component synthesis of epoxy-tetrahydropyrrolo[3,4- b]pyridin-5-ones and a Ugi four-center, three-component reaction of isocyanides, anilines, and 2-formylbenzoic acids for the synthesis of isoindolinones. Other groups have found that chiral dicarboxylic acid and BOROX are effective catalysts for truncated Ugi three-component reactions.