N—H…X (X = Cl and Br) hydrogen bonds in three isomorphous 3,5‐dichloropyridinium salts

Specific short contacts are important in crystal engineering. Hydrogen bonds have been particularly successful and together with halogen bonds can be useful for assembling small molecules or ions into crystals. The ionic constituents in the isomorphous 3,5‐dichloropyridinium (3,5‐diClPy) tetrahalometallates 3,5‐dichloropyridinium tetrachloridozincate(II), (C₅H₄Cl₂N)₂[ZnCl₄] or (3,5‐diClPy)₂ZnCl₄, 3,5‐dichloropyridinium tetrabromidozincate(II), (C₅H₄Cl₂N)₂[ZnBr₄] or (3,5‐diClPy)₂ZnBr₄, and 3,5‐dichloropyridinium tetrabromidocobaltate(II), (C₅H₄Cl₂N)₂[CoBr₄] or (3,5‐diClPy)₂CoBr₄, arrange according to favourable electrostatic interactions. Cations are preferably surrounded by anions and vice versa ; rare cation–cation contacts are associated with an antiparallel dipole orientation. N—H…X (X = Cl and Br) hydrogen bonds and X …X halogen bonds compete as closest contacts between neighbouring residues. The former dominate in the title compounds; the four symmetrically independent pyridinium N—H groups in each compound act as donors in charge‐assisted hydrogen bonds, with halogen ligands and the tetrahedral metallate anions as acceptors. The M —X coordinative bonds in the latter are significantly longer if the halide ligand is engaged in a classical X …H—N hydrogen bond. In all three solids, triangular halogen‐bond interactions are observed. They might contribute to the stabilization of the structures, but even the shortest interhalogen contacts are only slightly shorter than the sum of the van der Waals radii.     

One-Electron Bonds in Frustrated Lewis Pair TPB Ligands: Boron Behaving as a Lewis Base

Owing to its versatility in synthetic chemistry, TPB (tris[2-diisopropylphospino)phenyl]borane) is a very important frustrated Lewis Pair. The unusual stability of the neutral radical (TPB)Cu has been related to the presence of a one-electron B−Cu bond. Herein we show, through the use of different computational chemistry methods, that the existence and nature of this kind of A⋅⋅⋅M bond (A=donor atom, M=transition metal) depends on the surrounding chemical structure, and can be genuine one-electron sigma bonds only if appropriate metal ligands (Y), able to trap the charge in the desired region, are chosen. This ability is modulated by the subtle balance between the electronegativity of the different atoms along the A⋅⋅⋅M⋅⋅⋅Y bond paths. Most importantly, contrary to many TPB complexes in which boron acts as a Lewis acid, in one-electron-bond-containing structures boron behaves as a Lewis base.     

氢键概念的形成和发展

1902年,瑞士化学家维尔纳对氯化铵结构的解释为氢键概念的形成奠定了基础。1920年,美国化学家拉提麦尔和罗德布什首先认识了水中的氢键：2个八隅体所持的氢核构成的“弱键(weak bond)”。1928年,美国化学家鲍林在解释[FHF]^-的结构时首次使用“氢键(hydrogen bond)”一词,但并未对氢键的概念进行明确定义。1939年,鲍林在《化学键的本质》中明确提出了氢键的概念,并解释了氢键的性质,自此,氢键的概念正式形成。近年来,科学家经过深入研究发现了氢键的新类型：π型氢键、双氢键、金属氢键和单电子氢键。随着科学思想和科学技术的发展,关于氢键的认识也会越来越深入。     

Computational Mechanism Investigation of C=C Bond Hydrogenation Catalyzed by Rhodium Hydride

The hydrogenation of unsaturated carbons is a commonly used synthetic tool in pharmaceutical and industrial production. Recently, the Norton group realized highly selective hydrogenation of C=C bonds catalyzed by a rhodium hydride. Despite the great efforts made by experimentalists, details regarding the mechanism remained unclear. In this work, detailed DFT calculations were carried out to elucidate the principal features of this transformation. For enones we find that two possible competing mechanisms proposed by the experimental groups are computationally excluded, our proposed alternative mechanism with a total barrier of 20.0 kcal mol⁻¹ is theoretically feasible, solvent methanol to also plays a crucial role in assisting β-hydrogenation in addition to being the hydrogen source for α-hydrogenation, and the cross-polarization of the substrate enone-conjugated system to result in an enhanced charge density of the α-carbon, which favors being hydrogenated first. For isolated alkenes, neither of the two possible competing mechanisms can be excluded computationally and which carbon atom is first hydrogenated depends on the electronic properties of the substrate itself. The combination of rhodium and C=C bonds changes the electronic properties of H on the rhodium hydride and enhances its hydrogenation activity.     

Development of Donor-π-Acceptor-Type Fluorinated Tolanes as Compact Condensed Phase Luminophores and Applications in Photoluminescent Liquid-Crystalline Molecules

Fluorinated tolanes, produced by introducing fluorine atoms into one of the aromatic rings of tolane, emitted almost no fluorescence in a solution state, but the fluorescence intensity increased dramatically in the crystalline state because of intermolecular H⋅⋅⋅F hydrogen bonds. The photoluminescent (PL) colors depend on the molecular orbitals, dipole moments, and molecular aggregated structures can be varied by controlling terminal substituents along the major molecular axis. The introduction of a long alkoxy or semifluoroalkoxy unit as a flexible chain into the terminal positions along the major molecular axis induced the formation of a liquid-crystalline (LC) phase; fluorinated tolanes act both as luminophores and as mesogens, leading to the molecular design of new photoluminescent LC molecules (PLLCs). The results also indicated that a fluorinated tolane dimer, which consists of two fluorinated tolanes linked by a flexible alkylene spacer, also becomes a novel PLLC.     

Organophosphine bearing multiple hydrogen-bond donors for asymmetric Michael addition reaction of 1-oxoindane-2-carboxylic acid ester via dual-reagent catalysis

Multiple hydrogen bonds containing nucleophilic phosphines derived from dipeptide dual-reagents catalyzed asymmetric Michael addition reactions between indene esters and activated olefins in high yields and good to excellent enantioselectivities under mild reaction conditions. The success of current highly selective reactions should provide inspiration for expansion to other reactions and would open up new paradigms for the synthesis of indanone derivatives bearing chiral quaternary carbon centers.     

Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)–NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)–N and C(acyl)–O bond cleavage reactions.     

A Versatile Solid-Phase Approach to the Synthesis of Oligonucleotide Conjugates with Biodegradable Hydrazone Linker

One of the ways to efficiently deliver various drugs, including therapeutic nucleic acids, into the cells is conjugating them with different transport ligands via labile or stable bonds. A convenient solid-phase approach for the synthesis of 5′-conjugates of oligonucleotides with biodegradable pH-sensitive hydrazone covalent bonds is proposed in this article. The approach relies on introducing a hydrazide of the ligand under aqueous/organic media to a fully protected support-bound oligonucleotide containing aldehyde function at the 5′-end. We demonstrated the proof-of-principle of this approach by synthesizing 5′-lipophilic (e.g., cholesterol and α-tocopherol) conjugates of modified siRNA and non-coding RNAs imported into mitochondria (antireplicative RNAs and guide RNAs for Mito-CRISPR/system). The developed method has the potential to be extended for the synthesis of pH-sensitive conjugates of oligonucleotides of different types (ribo-, deoxyribo-, 2′-O-methylribo-, and others) with ligands of different nature.