Elastic Moduli: a Tool for Understanding Chemical Bonding and Thermal Transport in Thermoelectric Materials

The elastic behavior of a material can be a powerful tool to decipher thermal transport. In thermoelectrics, measuring the elastic moduli—directly tied to sound velocity—is critical to understand trends in lattice thermal conductivity, as well as study bond anharmonicity and phase transitions, given the sensitivity of elastic moduli to the chemical bonding. In this review, we introduce the basics of elasticity and explain the origin of high-temperature lattice softening from a bonding perspective. We then review elasticity data throughout classes of thermoelectrics, and explore trends in sound velocity, anharmonicity, and thermal conductivity. We reveal how experimental sound velocities can improve the accuracy of common thermal conductivity models and present a critical discussion of Grüneisen parameter estimates from elastic moduli. Readers will be equipped with tools to leverage elasticity measurements or calculations to accurately interpret thermal transport trends.     

Trifluoromethylation of Carbonyl and Unactivated Olefin Derivatives by C(sp3)−C Bond Cleavage

Herein, we report a Cu-mediated trifluoromethylation of carbonyl-type compounds and unactivated olefins enabled by visible-light irradiation via σ C(sp³)−C bond-functionalization. The reaction is distinguished by its modularity, mild conditions and wide scope—even in the context of late-stage functionalization—thus offering a complementary approach en route to valuable C(sp³)−CF₃ architectures from easily accessible precursors.     

Visible-Light-Induced N-Heterocyclic Carbene-Catalyzed Single Electron Reduction of Mono-Fluoroarenes

Fluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono-fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light-induced N-heterocyclic carbene (NHC)-catalyzed single electron reduction of mono-fluoroarenes for biaryl cross-couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for C_aryl−F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce C_aryl−O, C_aryl−N, and C_aryl−S bonds for biaryl cross-couplings.     

C−H Activation of Unbiased C(sp3)−H Bonds: Gold(I)-Catalyzed Cycloisomerization of 1-Bromoalkynes**

Selective functionalization of non-activated C(sp³)−H bonds is a major challenge in chemistry, so functional groups are often used to enhance reactivity. Here, we present a gold(I)-catalyzed C(sp³)−H activation of 1-bromoalkynes without any sort of electronic, or conformational bias. The reaction proceeds regiospecifically and stereospecifically to the corresponding bromocyclopentene derivatives. The latter can be readily modified, comprising an excellent library of diverse 3D scaffolds for medicinal chemistry. In addition, a mechanistic study has shown that the reaction proceeds via a so far unknown mechanism: a concerted [1,5]-H shift / C−C bond formation involving a gold-stabilized vinylcation-like transition state.     

Gold(I)-Catalyzed Benzylic C(sp3)−H Functionalizations: Divergent Synthesis of Indole[a]- and [b]-Fused Polycycles**

Phenyl azides substituted by an (alkylphenyl)ethynyl group facilitate benzylic sp³(C−H) functionalization in the presence of a JohnPhosAu catalyst, resulting in indole-fused tetra- and pentacycles via divergent N- or C-cyclization. The chemoselectivity is influenced depending on the counter-anion, the electron density of the α-imino gold(I) carbene, and the alkyl groups stabilizing the benzylic carbocation originating from a 1,5-hydride shift. An isotopic labeling experiment demonstrates the involvement of an indolylgold(I) species resulting from a tautomerization that is much faster than the deauration. The formation of a benzylic sp³(C−H) functionalization leading to an indole-fused seven-membered ring is also demonstrated.     

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C−F and S−F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅, which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C−F and S−F bond activation.     

Practical Access to meta-Substituted Anilines by Amination of Quinone Imine Ketals Derived from Anisidines: Efficient Synthesis of Anti-Psychotic Drugs

Reversing the conventional site-selectivity of C−H activation provides efficient retrosynthetic disconnections to otherwise unreactive bonds. Described herein is the Brønsted acid-catalyzed reaction that selectively performs meta-amination of anisidines with aliphatic-, heterocyclic- and aromatic amines in a one-pot procedure. In addition to dramatically simplifying the synthesis of meta-substituted anilines, our approach has the advantage of the scalability and remarkable functional group tolerance, including late-stage functionalization of pharmaceutical compounds and natural products. The control experiments and detailed computational analysis provide insight into the reaction mechanism and the origin of meta-selectivity. The protocol extended to the synthesis of challenging tri-aminated arenes and successfully applied for the efficient synthesis of 5-HT6 receptor antagonists, the anti-psychotic drugs viz.. SB-214111, SB-258510, and specifically, anti-schizophrenic drug SB-271046.     

Room Temperature Anhydrous Suzuki–Miyaura Polymerization Enabled by C−S Bond Activation

The Suzuki–Miyaura cross-coupling is one of the most important and powerful methods for constructing C−C bonds. However, the protodeboronation of arylboronic acids hinder the development of Suzuki–Miyaura coupling in the precise synthesis of conjugated polymers (CPs). Here, an anhydrous room temperature Suzuki–Miyaura cross-coupling reaction between (hetero)aryl boronic esters and aryl sulfides was explored, of which universality was exemplified by thirty small molecules and twelve CPs. Meanwhile, the mechanistic studies involving with capturing four coordinated borate intermediate revealed the direct transmetalation of boronic esters in the absence of H₂O suppressing the protodeboronation. Additionally, the room temperature reaction significantly reduced the homocoupling defects and enhanced the optoelectronic properties of the CPs. In all, this work provides a green protocol to synthesize alternating CPs.     

Single-Nitrogen Atom Incorporation into B=B Bond via the N=N Bond Splitting of Diazo Compound and Diazirine

Triboraazabutenyne 3 is synthesized by the reaction of diboraazabutenyne 1 with aryl boron dibromide followed by the reduction. The ligand exchange to replace phosphine on the terminal sp² B atom with carbene furnishes 4 . ¹¹B NMR, solid-state structures, and computational studies disclose that 3 and 4 feature the extremely polarized B=B bond. 4 readily splits the N=N bond of both diazo compound and diazirine under ambient conditions, whereby one nitrogen atom is incorporated into the B=B moiety leading to a neutral diboraazaallene 6 . The mechanism of the reaction between 4 and diazo compound is extensively investigated by density functional theory (DFT) calculations, as well as the isolation of an intermediate.     

Janus-Type ESIPT Chromophores with Distinctive Intramolecular Hydrogen-bonding Selectivity

Excited-state intramolecular proton transfer (ESIPT)-based solid luminescent materials with multiple hydrogen bond acceptors (HBAs) remain unexplored. Herein, we introduced a family of Janus-type ESIPT chromophores featuring distinctive hydrogen bond (H-bond) selectivity between competitive HBAs in a single molecule. Our investigations showed that the central hydroxyl group preferentially forms intramolecular H-bonds with imines in imine-modified 2-hydroxyphenyl benzothiazole (HBT) chromophores but tethers the benzothiazole moiety in hydrazone-modified HBT chromophores. Imine-derived HBTs generally exhibit higher fluorescence efficiency, while hydrazone-derived HBTs show a reduced overlap between the absorption and fluorescence bands. Quantum chemical calculations unveiled the molecular origins of the biased intramolecular H-bonds and their impact on the ESIPT process. This Janus-type ESIPT chromophore skeleton provides new opportunities for the design of solid luminescent materials.