过渡金属催化的硅烷基C(sp3)–H键活化

有机硅化合物在有机合成、材料化学和药物化学中都有广泛应用.因此,其自身的合成方法学在近年来广受关注.从原子经济性的角度出发,选择性的C(sp³)–H键切断是一种高效经济的合成策略.硅烷基单元在有机化合物中广泛存在,通过对硅烷基中的C(sp³)–H键直接官能团化来合成新的有机硅化合物是一种十分有前景的合成方法.近年来,过渡金属催化的C(sp³)–H键活化成为有机合成研究的热点领域.与肟基、唑啉、吡啶基、酰胺基、羧酸酯基等官能团或是与氧、氮或硫等杂原子相连的C(sp³)–H键的活化研究已有许多报道,但是与硅相邻的C(sp³)–H键活化研究报道很少.本文综述了近年来过渡金属催化的硅烷基C(sp³)–H键切断的研究进展.     

手性配体在钯催化不对称碳氢键活化反应中的应用

目前,手性配体辅助钯催化的区域和对映选择性碳氢键活化是过渡金属催化的前沿领域.在过去的十年中,它作为一种越来越重要的合成工具,用于合成含有各种不对称元素(中心手性、轴手性和平面手性)的手性分子,也是快速构建各种碳碳键和碳杂原子键的最有效方法之一.本文介绍了一些典型手性配体在钯催化不对称sp²和sp³碳氢键活化/官能团化反应中的应用,如单齿配体亚磷酰胺、手性磷酸(CPAs)和双功能氮端单保护的氨基酸(MPAAs)配体;探讨了这些手性配体辅助催化反应的创新性和特点,在理清这一研究领域发展脉络的同时,展望了未来研究工作的方向.此外,本文还介绍了这一不对称合成策略在天然产物、药物分子全合成中的应用.     

Is planarity of pyridin-2-yl- and pyrazin-2-yl-formamide thiosemicarbazones related to their tuberculostatic activity? X-ray structures of two pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones

Crystal structures of two title compounds and several their relatives known earlier reveal conservative and characteristic features, which may be related to their tuberculostatic activity. The molecules are predominantly planar due to conjugation through five successive bonds in the zwitterionic fragment S⁻–C(sp²)–N–NH⁺–C(sp²)–NH₂ and intramolecular hydrogen bonds, which prevent rotation of the adjacent pyrazine (or pyridine) ring. It has been suggested that in spatial sense such planar molecules resemble acridines intercalating with nucleic acids and that similar process may be responsible for tuberculostatic activity of the title pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones.     

Ab initio computation of the potential energy surfaces of the water·hydrocarbon complexes H2O·C2H2, H2O·C2H4 and H2O·CH4: minimum energy structures, vibrational frequencies and hydrogen bond energies

On the basis of ab initio MP2/6–31 + + G(2d,2p) calculations, we examined the potential energy surfaces of the water·hydrocarbon complexes H₂O·CH₄, H₂O·C₂H₂ and H₂O·C₂H₂ to locate all the minimum energy structures and estimate the hydrogen bond energies and vibrational frequencies associated with the C(spⁿ)---H·O and the O---H·C(spⁿ) bonds (n = 1−3). Our calculations show that H₂O·C₂H₂, H₂O·C₂H₄ and H₂O·CH₄ have two minimum energy structures (i.e., the C---H·O and O---H·C hydrogen bond forms), but H₂O·C₂H₄ has only one when the vibrational motion is taken into account, the O---H·C hydrogen bond form. We have also computed the barrier for the interconversion from one minimum to the other. The fully optimized geometries of H₂O·CH₄, H₂O·C₂H₄ and H₂O·C₂H₂ as well as the vibrational shifts of the C---H stretching frequencies in their C---H·O hydrogen-bonded forms are in good agreement with the available experimental data. The calculated hydrogen bond energies show that the C(spⁿ---H·O bond strengths decrease in the order C(sp)---H·O>C(sp²)---H·O>C(sp³)---O>C(sp³---H·O, which is also consistent with the available experimental data.     

Recent advances in sulfenylation of C(sp3)-H bond under transition metal-free conditions

In recent years,the transition metal-free sulfenylation of C-H bond for C-S formation has been rapidly advanced and has become an eco-friendly synthetic tool for pharmacists and organic chemists.Various natural or bioactive molecules such as(hetero)arenes,olefins,carbonyl compounds,alkanes,have been employed for sulfenylating reactions.This review will focus on the recent five-year advances in C-S bond formation via direct sulfenylation of C(sp^3)-H bonds under metal-free conditions and elaborate their mechanisms from a new perspective.     

Stereoselective Palladium-catalyzed Cross-coupling of (2-Amido-1-phenylpropyl)zinc Compounds with Aryl Bromides

A controllable diastereoselective C(sp²)―C(sp³) Negishi coupling reaction of secondary benzylic zinc reagents with aryl bromides to form medicinally important 2-arylphenylethylamines was demonstrated. In the presence of Pd(OAc)₂ and 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl(S-phos), open-chain(2-amido-1-phenylethyl)zinc reagent bearing a β-NHAc or NHCHO group undergoes coupling reaction to give syn-1-arylphenylethylamine mainly, whereas the zinc reagent bearing a sterically hindered β-amido group, for example NHCOC(CH₃)₂OTBS undergoes the coupling reaction to yield anti-1-arylphenylethylamines with a configuration inversion. In addition, a working mechanism for the stereoselective Negishi cross-coupling was also proposed.     

Amorphous Structure and Bonding Chemistry of Aluminium Antimonide(AlSb) Alloy for Phase-change Memory Device

With the help of first-principles molecular dynamics calculations, we obtained the atomic picture of amorphous AlSb(a-AlSb) for phase-change memory application. Generally, a-AlSb shows sp³ bonding network, which is the intrinsic characteristic for its good thermal stability. Significant wrong(homogenous) Al-Al bonds can also be observed from the pair correlation function. This hints the amorphous phase may consist of Al cluster and Sb-rich Al-Sb alloy. Recent experiment has observed the Sb-rich region of AlSb alloy can be switched to crystal, on the basis of which, combined with our calculations, we thus propose that on the one hand such a Sb-rich region in a-AlSb can retain the rapid crystallization like pure Sb solid and on the other hand some Al atoms play the important role of stabilizing Sb rich network with sp³ bonding. The present study offers a microscopic view to understand the phase change mechanism of AlSb alloy for information storage device.     

The gas phase structure of tetrakis(trifluoromethylthio)ethene, CF3S)2C=C(SCF3)2

The geometric structure of (CF₃S)₂C=C(SCF₃)₂ in the vapour phase was determined by electron diffraction. The molecule possesses D₂ symmetry with the S---CF₃ bonds oriented perpendicular to the ethene plane, in alternating directions up-down-up-down. The following skeletal geometric parameters were obtained (r_a distances and angles, experimental uncertainties are 3σ values): C=C = 1.34Å (ass.), C(sp²---S = 1.761(5)Å, S---C(sp³) = 1.832(5)Å, S---C---C = 119.6(4)°, C---S---C = 100.6(13)°, and ø(C=C---S---C) = 90.9(11)°. The gas phase conformation differs considerably from the crystal structure, where the molecule possesses C_i symmetry and the CF₃ groups, which are bonded to cis-standing sulfur atoms, lie on the same side of the ethene plane with dihedral angles ø(C=C---S---C) of 117° and 127°.     

Theoretical Studies on the Dihydrogen Bonding Between Shortchain Hydrocarbon and Magnesium Hydride

The C--H…H dihydrogen-bonded complexes of methane, ethylene, acetylene, and their derivatives with magnesium hydride were systematically investigated at MP2/aug-cc-PVTZ level. The results confirm that the strength of dihydrogen bonding increases in the following order of proton donors： C（sp3）-H〈C（sp2）-H〈C（sp）-H and chlorine substituents enhance the C-H…H interaction. In the majority of the complexes with a cyclic structure, the Mg-H proton-accepting bond is more sensitive to the surroundings than C-H proton-donating bond. The nature of the electrostatic interaction in these C-H…H dihydrogen bonds was also unveiled by means of the atoms in mo- lecules（AIM） analysis. The natural bond orbital（NBO） analysis suggests that the charge transfer in the cyclic com- plexes is characteristic of dual-channel. The direction of the net charge transfer in the cyclic complexes is contrary to that previously found in dihydrogen bonded systems.     

Pd/Fe3O4-C催化剂对甲醇、乙醇和丙醇氧化的电催化活性

制备对醇氧化反应具有优异电活性的钯催化剂是醇燃料电池研究的重要内容。本文用硼氢化钠还原法制备了钯纳米颗粒, 然后沉积在Fe3O4/C复合物表面, 得到了不同Fe₃O₄负载量的Pd/Fe₃O₄-C催化剂. 透射电镜（TEM）图显示钯纳米颗粒均匀地分散在Fe₃O₄/C表面. 对制备好的Pd/Fe₃O₄-C催化剂进行了循环伏安法（CV）、计时电流（CA）和电化学阻抗谱（EIS）的测试, 研究了其在碱性介质中对C1-C3醇类（甲醇、乙醇和丙醇）氧化的电催化活性. 结果表明, 所制备的不同Fe₃O₄负载量的Pd/Fe₃O₄(2%)-C,Pd/Fe₃O₄(5%)-C, Pd/Fe₃O₄(10%)-C和Pd/C催化剂中, Pd/Fe₃O₄(5%)-C催化剂表现出最高的醇氧化电流密度. 依据循环伏安（CV）数据,Pd/Fe₃O₄(5%)-C催化剂对甲醇、乙醇、正丙醇和异丙醇氧化的阳极峰电流密度分别是Pd/C催化剂的1.7、1.4、1.7和1.3倍. Pd/Fe₃O₄(5%)-C催化剂对乙醇氧化的电荷传递电阻也远低于Pd/C催化剂. 制备的所有催化剂对C1-C3醇类电氧化的电流密度大小排序如下: 正丙醇﹥乙醇﹥甲醇﹥异丙醇. 此外, 碳粉中Fe₃O₄纳米颗粒的存在提高了钯纳米颗粒的电化学稳定性.     

Palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylates via direct C(sp2)-H or C(sp3)-H bond activation

A palladium-catalyzed multi-acetoxylation of 1,3-disubstituted 1H-pyrazole-5-carboxylate derivatives containing multiple potential reactive sites is described. Therein, the sequence of this process has been appropriately investigated. The protocol mainly provides the di- and tri-acetoxylated products for 1,3-diarylpyrazoles. Besides, it is found that the acetoxylation of C(sp³)-H bond is prior to that of C(sp²)-H bond under structurally competitive conditions.     

Synthesis and Characteristics of Fe_3C Nanoparticles Embedded in Amorphous Carbon Matrix

We proposed a new way to synthesize a nanocomposite consisted of cementite Fe3C nanoparticles and amorphous carbon by radio frequency plasma-enhanced chemical vapor deposition. Transmission electron microscope images show the existence of nanometric dark grains(Fe3C) embedded in a light matrix(amorphous carbon) in the samples. X-ray photoelectron spectroscopy experiment exhibit that the chemical bonding state in the films corresponded to sp3/sp2 amorphous carbon, sp3 C―N(287.3 eV) and C1s in Fe3C(283.5 eV). With increasing deposition time, the ratio of amorphous carbon increased. The magnetic measurements show that the value of in-plane coercivity increased with increasing carbon matrix concentration(from about 6.56×103 A/m for film without carbon structures to approximately 2.77×104 and 5.81×104 A/m for nanocomposite films at room temperature and 10 K, respectively). The values of saturation magnetization for the synthesized nanocomposites were lower than that of the bulk Fe3C (140 Am2/kg).     

VO1-4+阳离子与n-CmH2m+2 (m = 3, 5, 7)烷烃的反应性研究：氧含量和碳链长度的影响

Vanadium oxides are one of the most important heterogeneous catalysts that are widely used to oxidize hydrocarbon molecules into value-added chemicals. In order to reveal the mechanisms and the nature of active sites, numerous experimental and theoretical studies have been reported on the reactivities of gas-phase vanadium oxide clusters toward small molecules. However, there has been very limited research on the chemical reactivity changes associated with the oxygen contents of vanadium oxides and the carbon chain lengths of alkanes. In this work, the reactions of vanadium oxide ions VO₁₋₄⁺ with alkanes (n-C_mH_2m+2, m = 3, 5, 7) were systematically investigated by time-of-flight mass spectrometry and the reactions of VO₁₋₃⁺ with pentane were further studied by density functional theory calculations. Experimental results show that in the reactions of VO⁺, VO₃⁺, and VO₄⁺ with n-C₅H₁₂, in addition to the major adsorption processes, the activation of the C―H and C―C bonds of n-C₅H₁₂ was observed. The activation of both the bonds was observed experimentally during the reaction of VO₂⁺ with n-C₅H₁₂ with large branching ratios. Among the vanadium oxide cations studied, VO₂⁺ shows the strongest oxidizability and the generation of lighter alkanes and alkenes dominates the reactions; VO⁺ is more reactive than VO₃⁺. VO₄⁺ pocesses only one η²-O₂ unit. Due to the weak bond between VO₂⁺ and η²-O₂, the η²-O₂ unit is released in VO₄⁺/n-C₅H₁₂ system leading to the formation of VO₂⁺; thus VO₄⁺ cations reflect some reactivity of VO₂⁺. Although the oxidation states in the vanadium oxide clusters increase from +Ⅲ in VO⁺ to +Ⅴ in VO₂⁺ and +Ⅳ in VO₃⁺, the reactivity does not gradually increase. Moreover, the reactivity of the mononuclear vanadium oxide cations also does not exhibit a gradually increasing trend with the increase in oxygen content. Based on the observed reactivity trend, the adsorption channels gradually become weak as the carbon chain lengths increase; meanwhile, the dehydrogenation and C―C bond activation channels gradually become obvious and some oxygen transfer products appear. Therefore, much lighter fragments of alkanes/alkenes will be obtained if linear alkanes with more carbon atoms were reacted with VO₁₋₄⁺. The theoretical results are generally consistent with those obtained from the experiments. The various reaction channels and versatile reactivity of the mononuclear vanadium oxide cations investigated in this study not only offer new insights into gas-phase reactions but also shed light on the processes occurring on the surfaces of the corresponding condensed-phase catalysts.     

Chuan-Cai Fan;Li-Jin Xu;Han-Yuan Gong;%T Neutral C–H bond vs. electron pair of N（sp~2）： A binding site effect study of macrocycle anion receptor

To evaluate the effect of neutral C–H bond or electron pair of nitrogen atom with sp2hybridization（N（sp2）） involving into the same chemical environment for anion binding, two analogous tetracationic imidazolium macrocycles, namely cyclo[2]（2,6-bis-（1H-imidazol-1-yl）pyridine） [2]（1,3-dimethylenebenzene）（14＋）, and cyclo[2]（2,6-bis-（1H-imidazol-1-yl）pyridine）[2]（2,6-di methylenepyridine）（24＋）were studied in detail as small inorganic anion receptors. The guest anions with different shapes are Cl,N3, NO3, and H2PO4. The host–guest interactions were characterized via1 H NMR spectroscopy,electrospray ionization mass spectrometry（ESI-MS） and single crystal X-ray crystallography. The results implied that macrocyclic hosts with similar backbone but two distinct binding sites（14＋with neutral C–H vs. 24＋with N（sp2）） vary markedly in their response to anions, including the binding modes and association constants. The finding will serve to the construction of new anion receptors, even improve insights into the anion binding process in biology.     

NiWS/γ-Al2O3催化剂上吲哚加氢脱氮反应:H2S和喹啉的影响

在连续固定床微反装置上考察了吲哚（IND）和1,2-二氢吲哚（HIN）在NiWS/γ-Al₂O₃催化剂上加氢脱氮（HDN）的反应以及 H₂S和喹啉（Q）对其加氢脱氮反应的影响。结果表明,碱性含氮化合物HIN较吲哚对其自身的加氢脱氮反应抑制作用更为明显。H₂S能够促进HIN的C（sp³）－N断裂,但抑制了邻乙基苯胺（OEA）的 C（sp²）－N断裂;同时吲哚加氢反应途径也受到了抑制。喹啉的添加严重降低了吲哚加氢脱氮反应的转化率和脱氮率;喹啉对吲哚加氢反应和C－N键断裂反应均产生明显的抑制作用。喹啉的抑制作用主要源于喹啉及其中间产物1,2,3,4-四氢喹啉（THQ1）和5,6,7,8 -四氢喹啉（THQ5）与吲哚及其中间产物的竞争吸附。     

光催化甲苯衍生物、环烷烃与无机亚磺酸盐的位点选择性C(sp3)-H砜基化

烃类是最基础的有机分子之一,具有品种丰富、来源广泛、价格低廉等优势,其多样性、选择性转化一直备受合成化学家关注.然而,这些化合物中通常存在多个键能高、极性低、相互间差异性小的C(sp3)-H键,因此,实现高效、选择性C-H官能化极具挑战性.近年来,可见光催化广泛应用于有机合成中,与氢转移催化相结合所发展的可见光氢转移光...     

Nano palladium catalyzed C(sp3)-H bonds arylation by a transient directing strategy

Reported herein is the first example of heterogeneous palladium catalyzed C(sp³)-H bonds arylation by a transient-ligand-directed strategy.Using supported palladium(metallic state) na nopariticles as catalyst,a wide range of aryl iodides undergo the coupling with various o-methylbenzaldehyde derivatives to assemble a library of highly selective and functionalized o-benzylbenzaldehydes.The stability of the catalyst was easily recovered four runs without significant loss of activity.The XPS analysis of the catalyst before and after reaction indicated that the reaction might be carried out by a catalytic cycle starting with Pd~0.     

Comparison of calculated DFT/B3LYP and experimental C and O NMR chemical shifts, ab initio HF/6-31G* optimised structures, and single crystal X-ray structures of some substituted methyl 5β-cholan-24-oates

Single crystal X-ray structures (monoclinic space group P2₁) for methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate have been solved and compared with HF/6-31G* optimised structures. In the crystalline packings the side chains are connected with weak OC(sp³)HO-type of interactions between C25–H and C24–O–C25 and the keto ends with weak C(sp³)HO=C-type of interactions between C4–H and O=C3. The orientations of the side chains, which steric configurations are of great importance to the biological activity of the molecules, are compared with the experimental structure of methyl 3-hydroxy-5β-cholan-24-oate. Probable reasons for the observed differences are discussed. In addition, ¹³C and ¹⁷O NMR chemical shifts of methyl 3-oxo-5β-cholan-24-oate and methyl 3,12-dioxo-5β-cholan-24-oate as well as the epimeric methyl 3-hydroxy-5β-cholan-24-oate and methyl 3β-hydroxy-5β-cholan-24-oate have been calculated (DFT/B3LYP/6-311G*) and compared with the experimental values by linear regression analyses. In general, the correspondence between the theoretical and experimental parameters is good or excellent.     

Control factors for hydrogen-atom abstraction from organic molecules in cryogenic solids

Hydrogen-atom transfer from organic molecules to free deuterium atoms and to methyl radicals in cryogenic organic solids due to quantum-mechanical tunneling of C–H hydrogen was studied for elucidating the control factors for the reaction. The following differences were found in comparison with regular thermal reactions: (1) The rate of hydrogen-atom tunneling decreases with increasing number and length of alkyl chains attaching to C–H carbon to be hydrogen-abstracted. This is due to the increase of steric hindrance to the deformation of the chemical bonds of the C–H carbon. The abstraction accompanies the change of the chemical bonds for the C–H carbon from sp³ to sp², so that prevention of the deformation by the alkyl chains causes the increase of the thickness of the potential barrier for the tunneling. The tunneling rate therefore decreases. (2) The rate of abstraction increases with the energy released by the reaction. The energy is used to excite the vibrational states of product molecules, which accelerates the tunneling from the initial reactant state to the final product state.     

1,4-Diynes from alkynyl–propargyl coupling reactions

This review deals with state-of-the-art propargylation of metal-alkynyls. After outlining the importance of the ---CC---Csp³---CC--- sequence in organic and organometallic molecules, processes for obtaining these ‘skipped diynes’ from metal-alkynyls (or metal acetylides ---CC---[M]) and propargyl electrophiles (X---CR₁R₂---CC--- or X=CR---CC---) are classified according to the nature of the metal (M=Li, Na, K, Mg, Cu, Zn, Al, Si, ---CMe₂OAl(OR)₂, …) and to the hybridization state of the electrophilic carbon (sp³, sp²). The allenic/propargylic regioselectivity being crucial, emphasis is placed on methods resorting to pre-complexation of the propargylic moiety with transition-metals: η²/η³ coordination with cobalt, and η¹ coordination with ruthenium are specifically discussed. Whenever such processes have been used for definite synthetic purposes, the ultimate target is mentioned. For comparison, alternative routes to skipped diynes are illustrated by a few recent examples.