Understanding of the activity difference between nanogold and bulk gold by relativistic effects

It is a challenge to thoroughly understand the astonishing difference in catalytic activity between nanogold and bulk gold for some oxidation reactions. In this work,the Au–O interactions in various surroundings were investigated by DFT calculations and compared with the Ag–O interactions. We have found the three points.First,only Au–O bond can be significantly strengthened by the linear O–Au–O structure. Second,the Au–O bond is always stronger than the Ag–O bond when the bonds are embedded in common surroundings. Third,the Au–O bond becomes weaker than the Ag–O bond when the number of neighboring Au atoms becomes large,because the Au–O interactions are suppressed by the presence of neighboring gold atoms. The origin of these three points can be attributed to wider spatial extension of d orbitals of gold,induced by strong relativistic effects. The strong relativistic effects make nanogold with smaller coordinate numbers highly active due to the ease in forming strong Au–O bonds,especially for the O–Au–O bond,whereas gold atoms in bulk with larger coordination numbers chemically inert due to the strong suppression by neighboring gold atoms destabilizing the O–Au–O bond.     

Carbon–sulfur bond formation via photochemical strategies: An efficient method for the synthesis of sulfur-containing compounds

The development of green and convenient methods for C–S bond formation has received significant attention because C–S bond widely occurs in many important pharmaceutical and biological compounds.Recently, visible-light photoredox catalysis has been established as an efficient and general tool for the construction of C–C and C-heteroatom bonds. In this review, we have focused on the research on recent advances in C–S bond formation via visible-light photoredox catalysis, and the growing opportuni...     

Catalytic transformation of cellulose and its derived carbohydrates into chemicals involving C–C bond cleavage

The catalytic transformation of cellulose, the major component of abundant and renewable lignocellulosic biomass, into building-block chemicals is a key to establishing sustainable chemical processes. Cellulose is a polymer of glucose and a lot research effort has been devoted to the conversion of cellulose to six-carbon platform compounds such as glucose and glucose derivatives through C–O bond activation. There also exist considerable studies on the catalytic cleavage of C–C bonds in biomass for the production of high-value chemicals, in particular polyols and organic acids such as ethylene glycol and lactic acid. This review article highlights recent advances in the development of new catalytic systems and new strategies for the selective cleavage of C–C bonds in cellulose and its derived carbohydrates under inert, reductive and oxidative atmospheres to produce C1–C5polyols and organic acids. The key factors that influence the catalytic performance will be clarified to provide insights for the design of more efficient catalysts for the transformation of cellulose with precise cleavage of C–C bonds to high-value chemicals. The reaction mechanisms will also be discussed to understand deeply how the selective cleavage of C–C bonds can be achieved in biomass.     

A Quantum Chemical Investigation on the Antioxidant Activity of Licoisoflavone Derivatives

In this paper,21 substituents with various electron donating and electron withdrawing characters were placed in the available positions of Licoisoflavone in order to study their effect on the three O–H bond dissociation enthalpies and ionisation potential via DFT/B3LYP method.Results show that the B-ring is more important than the A-ring from the BDE and IP point of view because the radical structure formed after hydrogen atom abstraction from the 4-OH group can be stabilized by the resonance between the three rings.The results show that intramolecular hydrogen bonding effects are able to considerably stabilize the parents and radicals.The NBO analysis results also confirmed the intramolecular hydrogen bond stabilization.The formation of strong intramolecular hydrogen bonds and suitable spin density distributions in several radicals result in low BDE and IP values.The calculated BDE and IP values have been correlated with Hammett constants.The found dependencies are suitably linear,which can be important for the synthesis of novel antioxidants based on Licoisoflavone.     

Synthesis of unnatural amino acids through palladium-catalyzed C(sp~3)-H functionalization

Unnatural a-amino acids have been extensively used in the modern drug discovery and protein engineering studies. They have also found applications in the development of chiral molecular catalysts and the total synthesis of diverse natural products. Accordingly the development of cost-effective approaches for the preparation of unnatural a-amino acids has received increasing attentions. Among all the available methods for this purpose, direct C–H functionalization of simple amino acids represents one of the most attractive approaches because it exhibits good atom-economy and step-efficiency. In particular, selective functionalization of either the primary or secondary C(sp~3)–H bonds in the amino acids has been explored to make versatile C–C, C–N, C–O, C–B and C–F bonds to modify the side chain of amino acids and even peptides. The present review surveys the recent advances of synthesis of chiral unnatural a-amino acids and peptides through palladium-catalyzed functionalization of un-activated C(sp~3)–H bonds.     

Putting aniline radical cations in a bottle

Salts containing aniline radical cations have been isolated and characterized by electron paramagnetic resonance(EPR)spectroscopy, UV-Vis absorption spectroscopy and single crystal X-ray diffraction. The EPR spectra and theoretical calculations indicate the unpaired electron is delocalized on phenyl rings and nitrogen atoms. Both radical cations feature a quinoidal geometry with a partially double C–N bond, but are distinct in that the C–N bond is coplanar to the phenyl plane in one cation while deviates from the plane in the other due to steric crowding. The work provides the first unequivocal examples of stable aniline radical cations.     

A DFT Study of Thiophene Adsorption on the Rh（111） Surfaces

Thiophene adsorption on the Rh(111) surfaces has been investigated by density functional theory.The results show that the adsorption at the hollow and bridge sites is the most stable.The molecular plane of the thiophene ring is distorted,the C=C bond is stretched to 1.448  and the C-C bond is shortened to 1.390.The C-H bonds tilt 22～42oaway from the surface.The calculated adsorption geometries are in reasonable agreement with population analysis and density of states.The thiophene molecule obtains 0.74 electrons,reflecting the interaction between the lone pair of sulfur and the d-orbitals of metal.The reaction paths and transition states for desulfurization of the molecule have been investigated.The bridge adsorption structure of thiophene leads to a thiol via an activated reaction with an energetic barrier of 0.30 eV.This second step is slightly difficult,and dissociation into a C4H4 fragment and a sulfur atom is possible,with an energetic barrier of 0.40 eV.     

N-phosphoryl amino acid models for P–N bonds in prebiotic chemical evolution

Post-translational modification of proteins by N-phosphorylation of the basic amino acid residues plays important roles in biological processes. The high-energy P–N bond might have contributed to the evolution of prebiotic chemistry. N-phosphoryl amino acids(PAAs) can serve as interesting small molecular models for the study of P–N bonds in prebiotic chemical evolution. PAAs are capable of simultaneously producing several important biomolecules such as polypeptides and oligonucleotides under mild reaction conditions. In this review, we describe the chemistry of PAAs, discusse their likely prebiotic origins and their reactivity and how they relate to biological P–N bond species. We also depict a possible prebiotic scenario mediated by PAAs in which PAAs may have acted as one of the essential forces driving prebiotic biomolecules to the first protocell.     

STUDY ON ELECTRONIC STRUCTURE AND BONDING OF CLUSTER COMPOUND Sc_7Cl_(10)C_2

<正> The electronic structure and bonding of the cluster compound Sc7Cl10C2 have been studied by INDO method. In contract to the weak interaction between metal-metal in the compound Gd10Cl18C4, the bonding between metal atoms (Sc-Sc) in Sc7Cl10C2 is rather strong. The contribution of the orbitals 4s and 4p is larger than that of 3d to the Sc-Sc bond. In the cluster compound, besides Sc-Sc bonding, there are Sc -C and Sc -Cl bonds. The contribution of 3d is larger than that of 4s and 4p to the bond Sc-C. The contribution of 3d is slightly less than that of 4p and 4s to the bond Sc-Cl. Among the three kinds of bonds, the Sc-Cl bond is the weakest, the bond order of the Sc-Sc is close to that of the Sc-C.     

Ni-catalyzed carbon–carbon bonds cleavage of mixed polyolefin plastics waste

The inert carbon–carbon(C–C) bonds cleavage is a main bottleneck in the chemical upcycling of recalcitrant polyolefin plastics waste. Here we develop an efficient strategy to catalyze the complete cleavage of C–C bonds in mixed polyolefin plastics over non-noble metal catalysts under mild conditions. The nickelbased catalyst involving Ni₂Al₃ phase enables the direct transformation of mixed polyolefin plastics into natural gas, and the gas carbon yield reaches up to 89.6%. R...     

C–H bond activation in the total syntheses of natural products

The transition metal-mediated C–H bond activation has emerged as a powerful and ideal method for the total syntheses of natural products and pharmaceuticals, and has had a significant impact on synthetic planning and strategy in complex natural products.In this review, we describe selected recent examples of the transition metal-mediated C–H bond activation strategies for the rapid syntheses of natural products.     

Radical cascade reactions of unsaturated C–C bonds involving migration

During the past few years, with the rapid development of mild methods for the generation of radical species, great progress in radical cascade reactions of unsaturated C–C bonds has been made. Many radical cascade reactions involve functional groups migration, which leads structurally much more diverse, complex and valuable compounds not easily obtained through other methods. In this review, the recent achievements in unsaturated C–C bonds radical cascade reactions involving migration are summarized.     

Transition metal carbides in electrocatalytic oxygen evolution reaction

Oxygen evolution reaction(OER) is admitted to an important half reaction in water splitting for sustainable hydrogen production.The sluggish four-electron process is known to be the bottleneck for enhancing the efficiency of OER.In this regard,tremendous efforts have been devoted to developing effective catalysts for OER.In addition to Ir-or Ru-based oxides taken as the benchmark,transition metal carbides have attracted ever-increasing interest due to the high activity and stability as low-cost OER electrocatalysts.In this review,the transition metal carbides for water oxidation electrocatalysis concerning design strategies and synthesis are briefly summarized.Some typical applications for various carbides are also highlighted.Besides,the development trends and outlook are also discussed.     

Photocatalytic decarboxylative alkylations of C(sp~3)–H and C(sp~2)–H bonds enabled by ammonium iodide in amide solvent

A simple ammonium iodide salt in amide solvent catalyzes regioselective decarboxylative alkylation of C(sp~3)-H bonds of Naryl glycine derivatives, of C(sp~2)-H bond of heteroarenes, and cascade radical addition to unsaturated bond followed by intramolecular addition to arene, with a broad scope of N-hydroxyphthalimide derived redox active esters under visible light irradiation. The reactions are suggested to proceed through photoactivation of a transiently assembled chromophore from electron-deficient phthalimide moiety and iodide anion through an anion-π interaction in solvent cage followed by diffusion to generate solvated free radical species to react with C-H substrates. The simplicity, practicality, and broad substrate scope of this method highlight the synthetic power of photocatalysis through transiently assembled chromophore, and will hopefully inspire further developments of low cost photocatalysis based on various non-covalent interactions, which are prevalent in supramolecular chemistry and biosystems, for sustainable organic synthesis.     

Unactivated C(sp3)–H functionalization via vinyl cations

Direct functionalization of inert C(sp³)–H bonds is a topic of immense contemporary interest and exceptional value in organic synthesis.The recent research has established a novel and practical protocol which features the engagement of vinyl cation species to functionalize C(sp³)–H bonds.The discussion of the topic is arranged by the strategies to generate the reactive intermediates,including ionization of vinyl triflates,addition of electrophiles to alkynes,tandem cyclization of enynes or diynes,and decomposition ofβ-hydroxy-α-diazo ketones.This review closes with a personal perspective on the dynamic research area of unactivated C(sp³)–H functionalization via vinyl cations.Hopefully,it will provide timely illumination and beneficial guidance for organic chemists who are interested in this area.Meanwhile continued development of the field is strongly anticipated in the future.     

Cocrystal Assembled by 1,2-Diiodotetrafluorobenzene and Acridine via C-I…N Halogen Bond and Tr-hole…F Bonds

The concepts on o-hole and ~-hole bonds are suggested. A cocrystal with repeated 8-F-atom unit as basic struc- tural motif is assembled based on bifurcated C-I…N…I-C halogen/σ-hole bond and antiparallel double π-hole… F bonds by 1,2-diiodotetrafluorobenzene and acridine and characterized well by XRD, powder XRD and solid 19F NMR, etc. Also the calculated interaction energies are -26.8 and -31.5 kJ/mol for bifurcated C-I…N sp……2 halogen bonds, and -14.3 kJ/mol for a pair of n-hole…F bonds. In this system C-I…N halogen bond has stronger competitive ability to C-I…π halogen bond due to stronger basicity of N than π-system in acridine. The combination of the halogen/σ-hole and π-hole bonds or together with other weak interactions could play a key role in assembling function materials, molecular recognition and design of drugs and so on.     

High pressure,a protocol to identify the weak dihydrogen bonds:experimental evidence of C–H···H–B interaction

Pressure, as a thermodynamic parameter, provides an appropriate method to detect weak intermolecular interactions. The C–H···H–B dihydrogen bond is so weak that the experimental evidence of this interaction is still limited. A combination of in situ high pressure Raman spectra and angle-dispersive X-ray diffraction(ADXRD) experiments was utilized to explore the dihydrogen bonds in dimethylamine borane(DMAB). Both Raman and ADXRD measurements suggested that the crystal structure of DMAB is stable in the pressure region from 1 atm(1 atm=1.01325×10~5 Pa) to 0.54 GPa. The red shift of CH stretching and CH_3 distortion modes gave strong evidence for the existence of C–H···H–B dihydrogen bonds. Further analysis of Raman spectra and Hirshfeld surface confirmed our proposal. This work provided a deeper understanding of dihydrogen bonds.And we wish that high pressure could be applied to identify other unconfirmed hydrogen or dihydrogen bond.     

Competition between C-C and C-H Activation in Reactions of Neutral Nickel Atom with Cycloalkanes （n = 3-7）

A theoretical investigation of the reaction mechanisms for C-H and C-C bond activation processes in the reaction of Ni with cycloalkanes C,,H2. （n = 3-7） is carried out. For the Ni ＋ CnH2, （n = 3, 4） reactions, the major and minor reaction channels involve C-C and C-H bond activations, respectively, whereas Ni atom prefers the attacking of C-H bond over the C-C bond in CnH2n （n = 5=7）. The results are in good agreement with the experimental study. In all cases, intermediates and transition states along the reaction paths of interest are characterized, It is found that both the C-H and C-C bond activation processes are proposed to proceed in a one-step manner via one transition state. The overall C-H and C-C bond activation processes are exothermic and involve low energy barriers, thus transition metal atom Ni is a good mediator for the activity of cycloalkanes CnH2n （n = 3 -7）.     

五羰基合铁分子化学键特性的研究

We have found from experimental data that, in the iron pentacarbonyl molecule, the length of Fe-C bond in axial is shorter than that in planar, but the bond is weaker; the bond distances of C-O of both planar and axial are equal, while the bond strengths are different. This is against the traditional view of “the shorter the bond is, the stronger the bond will be”. We have got through calculation and research, that the main cause of such difference is that the effects of the backdonor of Fe atom′s electrons to C-O bonds in axial and planar are different. The shorter the Fe-C bond is, the more effective the backdonor (or transfer) of electrons is. So that the population between Fe and C in axial are less than those in planar, so the bond is weaker, and the C-O bonds have gained more backdonor electrons than those in planar, thus the bond is stronger.     

Regioselective introduction of vinyl trifluoromethylthioether to remote unactivated C(sp~3)–H bonds via radical translocation cascade

Described herein is an efficient protocol for the regioselective introduction of a vinyl trifluoromethylthioether to remote unactivated C(sp~3)–H bonds. The cascade process involves the vinyl radical-mediated 1,5-hydrogen atom transfer(HAT) and remote vinyl migration. During the transformation, inert C–H and C–C bonds are consecutively cleaved under mild conditions.The reaction features good functional group tolerance, broad substrate scope, and high regio-/stereo-selectivity.