C-H键选择性直接电氧化研究

C-H键是有机化合物中最基本的化学键,C-H键的活化和直接转化避免了反应物的预先官能化,是最终实现烷烃类化合物转化为不同种类有机化合物最直接、高效的转换方式,通过C-H键构建C-X键（X=O、C、N）是非常重要和具有挑战性的研究. C-H键直接电氧化活化过程中以“电子”参与反应,不需要加入额外的催化剂,并可通过选择合适的电极材料、支持电解质、溶剂和反应温度,通过恒电流或者恒电位电解,进行具有特定的反应选择性和区域选择性的C-H键电氧化活化,从而获得含其他活性基团的目标产物.     

Structural and elemental investigations of isolated aquatic humic substances using X-ray photoelectron spectroscopy

Structural and elemental investigations of aquatic humic substances (HS) by means of X-ray photoelectron spectroscopy (XPS) are described. For that purpose small amounts (10-50 microg) of dissolved reference HS, which were characterized within the German research program DFG-ROSIG, were dried as thin films on small pieces of a high-purity silicon wafer. The photoelectrons from such HS layers exhibited characteristic signals of carbon C1s, nitrogen N1s, oxygen O1s and sulfur S2s, which could be fitted by Gaussian curves and used for the quantification of various moieties of HS: carbon (C-C, C-O, C=O, O=C-O), oxygen (C-O, C=O), nitrogen (C-N, C-N+) and sulfur. Moreover, by adding up the element signals of the HS samples their elemental composition of C, O, N and S was assessed. A comparison of the data based on solution state NMR and conventional elementary analysis revealed a satisfactory accuracy with those obtained by XPS.     

木质素模化物键离解能的理论研究

采用密度泛函理论B3P86方法,在6-31G(d,p)基组水平上,对木质素结构中的6种连接方式(β-O-4、α-O-4、4-O-5、β-1、α-1、5-5)的63个木质素模化物的醚键(C-O)和C-C键的键离解能E_B进行了理论计算研究。分析了不同取代基对键离解能的影响以及键长与键离解能的相关性。计算结果表明,C-O键的键离解能通常比C-C键的小,在各种醚键中C_α-O键的平均键离解能最小,为182.7 kJ/mol;其次是β-O-4连接中的C_β-O键,苯环和烷烃基上的取代基对醚键的键离解能有较强的弱化作用,C-O键的键长和键离解能的相关性较差。与C-O键相比,C-C键的键离解能受苯环上取代基的影响很小,而烷烃基上的取代基对C-C键的键离解能有较大的影响,C-C键的键离解能和键长之间存在较强的线性关系,C-C键的键长越长,其键离解能越小。     

Understanding glycine conformation through molecular orbitals

The four most stable C(s) conformers of glycine have been investigated using a variety of quantum-mechanical methods based on Hartree-Fock theory, density-functional theory (B3LYP and statistical average of orbital potential), and electron propagation (OVGF) treatments. Information obtained from these models were analyzed in coordinate and momentum spaces using dual space analysis to provide insight based on orbitals into the bonding mechanisms of glycine conformers, which are generated by rotation of C-O(H) (II), C-C (III), and C-N (IV) bonds from the global minimum structure (I). Wave functions generated from the B3LYP/TZVP model revealed that each rotation produced a unique set of fingerprint orbitals that correspond to a specific group of outer valence orbitals, generally of a' symmetry. Orbitals 14a', 13a', 12a', and 11a' are identified as the fingerprint orbitals for the C-O(H) (II) rotation, whereas fingerprint orbitals for the C-C (III) bond rotation are located as 16a' [highest occupied molecular orbital (HOMO)], 15a' [next highest molecular occupied molecular orbital (NHOMO)], 14a', and 12a' orbitals. Fingerprint orbitals for IV generated by the combined rotations around the C-C, C-O(H), and C-N bonds are found as 16a', 15a', 14a', 13a', and 11a', as well as in orbitals 2a" and 1a". Orbital 14a' is identified as the fingerprint orbital for all three conformational processes, as it is the only orbital in the outer valence region which is significantly affected by the conformational processes regardless rotation of which bond. Binding energies, molecular geometries, and other molecular properties such as dipole moments calculated based on the specified treatments agree well with available experimental measurements and with previous theoretical calculation.     

Photoemission and near-edge X-ray absorption fine structure studies of the bacterial surface protein layer of Bacillus sphaericus NCTC 9602

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.     

Calculations of Cotton Effects in the Vacuum UV Region in the Non-Anomeric Part of Pyranoses: Correlations With the Absolute Stereochemistries

ABSTRACT

A graphic method of Drude equations was applied to calculate the Cotton effects below 180 nm for a series of 1, 5-anhydro-D-alditols I-IX. The calculations, taking the vacuum CD data of Johnson et al. into account, gave a Cotton effect at ca. 140 nm region (Band 2) and a popular band (Band 1) due to the n-σ transition at ca.180 nm. Band 2, probably due to the σ-electron transition of the C-O or C-C bond, could be correlated with the helical structures of vicinal C-O and C-O bonds (C-O/C-O) and C-O and C-C bonds (C-O/C-C) providing a new additivity rule to calculate intensity (K2) using the vicinal C—O/C-O and C-O/C-C units.     

Structural and elemental investigations of isolated aquatic humic substances using X-ray photoelectron spectroscopy

Structural and elemental investigations of aquatic humic substances (HS) by means of X-ray photoelectron spectroscopy (XPS) are described. For that purpose small amounts (10–50 μg) of dissolved reference HS, which were characterized within the German research program DFG-ROSIG, were dried as thin films on small pieces of a high-purity silicon wafer. The photoelectrons from such HS layers exhibited characteristic signals of carbon C1s, nitrogen N1s, oxygen O1s and sulfur S2s, which could be fitted by Gaussian curves and used for the quantification of various moieties of HS: carbon (C-C, C-O, C=O, O=C-O), oxygen (C-O, C=O), nitrogen (C-N, C-N⁺) and sulfur. Moreover, by adding up the element signals of the HS samples their elemental composition of C, O, N and S was assessed. A comparison of the data based on solution state NMR and conventional elementary analysis revealed a satisfactory accuracy with those obtained by XPS.     

C-C, C-O and C-N bond formation via rhodium(III)-catalyzed oxidative C-H activation

Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references).     

An efficient three-component protocol for the synthesis of novel spiro-oxazinobarbiturates

Some novel spiro-oxazinobarbiturate derivatives have been successfully synthesized in a one-pot, three-component cascade reaction from various azines (pyridine, isoquinoline, quinoline and phenanthridine), 1,3-dimethylalloxan, and several activated acetylenes (alkyl propiolates, dialkyl acetylenedicarboxylates, and butyne-2-one). The high bond forming efficiency (formation of new C-N, C-C, and C-O bonds) of this reaction makes it attractive for the synthesis of spiro-oxazinobarbiturates in a single operation.     

Multi-component anion relay cascade of 1-acetylcyclopropanecarboxamides, aldehydes and acrylonitrile: access to biscyanoethylated furo[3,2-c]pyridinones

A highly efficient multi-component anion relay cascade reaction based on 1-acetylcyclopropanecarboxamides, aldehydes and acrylonitrile has been developed, which provides strategically novel and atom-economic access to biologically important biscyanoethylated furo[3,2-c]pyridinones. In this one-pot transformation, up to five bonds (one C-N, one C-O and three C-C bonds) were constructed.     

C–O,C–S,C–N,and C–C Bond Formation at the Periphery of the Macrocycle during Chemical Modification of Phytochlorins: Key Methods and Synthetic Applications

Russian Journal of General Chemistry - The main methods of C-O, C-S, C-N, and C-C bond formation at the periphery of the macocrocycle during chemical modification of phytochlorins in the synthesis...     

Catalytic C-H functionalization by metalloporphyrins: recent developments and future directions

Metalloporphyrins are a class of versatile catalysts with the capability to functionalize saturated C-H bonds via several well-defined atom/group transfer processes, including oxene, nitrene, and carbene C-H insertions. The corresponding hydroxylation, amination, and alkylation reactions provide direct approaches for the catalytic conversion of abundant hydrocarbons into value-added functional molecules through C-O, C-N, and C-C bond formations, respectively. This tutorial review describes metalloporphyrin-based catalytic systems for the functionalization of different types of sp(3) C-H bonds, both inter- and intramolecularly, including challenging primary C-H bonds. Additional features of metalloporphyrin-catalyzed C-H functionalization include unusual selectivities and high turnover numbers.     

Tandem reactions initiated by copper-catalyzed cross-coupling: a new strategy towards heterocycle synthesis

Copper-catalyzed cross-coupling reactions which lead to the formation of C-N, C-O, C-S and C-C bonds have been recognized as one of the most useful strategies in synthetic organic chemistry. During past decades, important breakthroughs in the study of Cu-catalyzed coupling processes demonstrated that Cu-catalyzed reactions are broadly applicable to a variety of research fields related to organic synthesis. Representatively, employing these coupling transformations as key steps, a large number of tandem reactions have been developed for the construction of various heterocyclic compounds. These tactics share the advantages of high atom economics of tandem reactions as well as the broad tolerance of Cu-catalyst systems. Therefore, Cu-catalyzed C-X (X = N, O, S, C) coupling transformation-initiated tandem reactions were quickly recognized as a strategy with great potential for synthesizing heterocyclic compounds and gained worldwide attention. In this review, recent research progress in heterocycle syntheses using tandem reactions initiated by copper-catalyzed coupling transformations, including C-N, C-O, C-S as well as C-C coupling processes are summarized.     

Dynamics of the interaction of vapor-deposited copper with alkanethiolate monolayers: bond insertion, complexation, and penetration pathways

We have investigated the interaction of vapor-deposited copper with -CH3, -OH, -OCH3, -COOH, and -CO2CH3 terminated alkanethiolate self-assembled monolayers (SAMs) adsorbed on polycrystalline Au using time-of-flight secondary ion mass spectrometry and density functional theory calculations. For -OH, -COOH, and -CO2CH3 terminated SAMs measurements indicate that for all copper coverages there is a competition between Cu atom bond insertion into C-O bonds, stabilization at the SAM/vacuum interface, and penetration to the Au/S interface. In contrast, on a -OCH3 terminated SAM Cu only weakly interacts with the methoxy group and penetrates to the Au substrate, while for a -CH3 terminated SAM deposited copper only penetrates to the Au/S interface. The insertion of copper into C-O terminal group bonds is an activated process. We estimate that the barriers for Cu insertion are 55 +/- 5 kJ mol(-1) for the ester, 50 +/- 5 kJ mol(-1) for the acid, and 55 +/- 5 kJ mol(-1) for the hydroxyl terminated SAMs. The activation barrier for the copper insertion is much higher for the -OCH3 SAM. Copper atoms with energies lower than the activation barrier partition between complexation (weak interaction) with the terminal groups and penetration through the monolayer to the Au/S interface. Weakly stabilized copper atoms at the SAM/vacuum interface slowly penetrate through the monolayer. In contrast to the case of Al deposition, C-O bond insertion is favored over C=O, C-H, and C-C bond insertion.     

Methyl 3-methyl-2H-azirine-2-carboxylate photochemistry studied by matrix-isolation FTIR and DFT calculations

The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C-O bond but differing in the arrangement around the C-C(alpha) bond. The two conformers were photoreactive upon in situ broadband UV excitation (lambda > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C-C or C-N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C-N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C-C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T(1) triplet state and, in this way, favors the C-N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed.     

Metal promoted synthesis of heterocycles from heteroacyclic compounds

New synthetic methods, based upon metal mediated or catalyzed C-C/C-O/C-N bond formation, for heterocycles and their application to natural product syntheis are described.     

Local orbital study of bonding in small rings: Cyclopropane,thiirane, oxirane,and aziridine

The nature of the bonding in the three-membered ring molecules cyclopropane, thiirane, oxirane, and aziridine has been investigated throughab initio FSGO calculations. The direct correspondence between floating spherical Gaussian orbitals and specific chemical bonds has been used to study the degree of “bond bending”. In accord with chemical intuition, it is demonstrated that the C-C ring bond becomes progressively more bent as the bond length is reduced. C-C bonds are found to be more flexible than C-N (O, S) bonds. The sizes and locations of carbon-heteroatom bond orbitals and C-H bond orbitals are also discussed.     

Take the Right Catalyst: Palladium-Catalyzed C-C, C-N, and C-O Bond Formation on Chloroarenes

Unreactive chloroarenes have been outwitted: The key to successful C-C, C-N, and C-O bond formation on chloroarenes is the development and optimization of suitable catalysts. Electron-rich alkylphosphanes are mandatory as ligands in these palladium-catalyzed reactions.     

Bifunctional transition metal-based molecular catalysts for asymmetric syntheses

The discovery and development of conceptually new chiral bifunctional transition metal-based catalysts for asymmetric reactions is described. The chiral bifunctional Ru catalyst was originally developed for asymmetric transfer hydrogenation of ketones and imines and is now successfully applicable to enantioselective C-C bond formation reaction with a wide scope and high practicability. The deprotonation of 1,3-dicarbonyl compounds with the chiral amido Ru complexes leading to the amine Ru complexes bearing C- or O-bonded enolates, followed by further reactions with electrophlies gives C-C bond formation products. The present bifunctional Ru catalyst offers a great opportunity to open up new fundamentals for stereoselective molecular transformation including enantioselective C-H and C-C as well as C-O, C-N bond formation.     

Ring-opening reactions of aromatic N-heterocycles by scandium and yttrium alkyl complexes

The C-N bond in aromatic N-heterocycles is a strong bond, its cleaving involving mostly examples of metal-element multiple bonds. We report on the C-C coupling of two molecules of an aromatic N-heterocycle mediated by scandium and yttrium benzyl complexes supported by a ferrocene 1,1'-diamide ligand. The reaction with 1-methylimidazole leads, ultimately, to the formation of a ring-opened imidazole coupled to a 1-methylimidazole fragment, a structure showing extended conjugation of double bonds. The experimental evidence agrees with involvement of only sigma bonds in these transformations.