Interaction of aromatic units of amino acids with guanidinium cation: The interplay of π···π, XH···π, and M+···π contacts

Complexes formed by guanidinium cation and a pair of aromatic molecules among benzene, phenol, or indole have been computationally studied to determine the characteristics of the cation···π interaction in ternary systems modeling amino acid side chains. Guanidinium coordinates to the aromatic units preferentially in the following order: indole, phenol, and benzene. Complexes containing two different aromatic units show an intermediate behavior between that observed for complexes with only one kind of aromatic unit. Most stable structures correspond to doubly‐T shaped arrangements with the two aromatic units coordinating guanidinium by its NH₂ groups. Other structures with only one aromatic unit coordinated to guanidinium, such as T‐shaped or parallel‐stacked ones, are less favorable but still showing significant stabilization. In indole and phenol complexes, the formation of hydrogen bonds between the aromatic molecules introduces extra stabilization in T‐shaped structures. Three body effects are small and repulsive in doubly T‐shaped minima. Only when hydrogen bonds involving the aromatic molecules are formed in T‐shaped structures a cooperative effect can be observed. In most complexes the interaction is controlled by electrostatics, with induction and dispersion also contributing significantly depending on the nature and orientation of the aromatic species forming the complex. Although the stability in these systems is mainly controlled by the intensity of the interaction between guanidinium and the aromatic molecules coordinated to it, interactions between aromatic molecules can modulate the characteristics of the complex, especially when hydrogen bonds are formed. © 2014 Wiley Periodicals, Inc.     

A Metal‐Bridged Tricyclic Aromatic System: Synthesis of Osmium Polycyclic Aromatic Complexes

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla‐aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal‐bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal‐bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five‐membered rings. These metalla‐aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five‐membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal‐bridged polycyclic aromatics.     

Dipole-induced structure in aromatic-terminated self-assembled monolayers: a study by near edge x-ray absorption fine structure spectroscopy

The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS.     

Aromatic Gain in a Supramolecular Polymer

The synergy of aromatic gain and hydrogen bonding in a supramolecular polymer is explored. Partially aromatic bis(squaramide) bolaamphiphiles were designed to self‐assemble through a combination of hydrophobic, hydrogen‐bonding, and aromatic effects into stiff, high‐aspect‐ratio fibers. UV and IR spectroscopy show electron delocalization and geometric changes within the squaramide ring indicative of strong hydrogen bonding and aromatic gain of the monomer units. The aromatic contribution to the interaction energy was further supported computationally by nucleus‐independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) indices, demonstrating greater aromatic character upon polymerization: at least 30 % in a pentamer. The aromatic gain–hydrogen bonding synergy results in a significant increase in thermodynamic stability and a striking difference in aggregate morphology of the bis(squaramide) bolamphiphile compared to isosteres that cannot engage in this effect.     

beta-Cyclodextrin as a stationary phase for the group separation of polycyclic aromatic compounds in normal-phase liquid chromatography

The separation of highly alkylated polycyclic aromatic compounds according to the size of their aromatic system is investigated using the polycyclic aromatic sulfur heterocycles in vacuum gas oil. A large number of reference compounds containing several parent ring systems and different alkylation patterns were first investigated to characterize the retention of polycyclic aromatic compounds likely to occur in high-boiling petroleum samples. A beta-cyclodextrin phase, Merck ChiraDex, was found to be more suitable than chemically bonded aminopropanosilane and tetrachlorophthalimide in normal-phase HPLC with respect to a combination of selectivity towards the number of aromatic double bonds and degree of influence of the alkyl groups of the aromatic compounds. Finally the preseparated polycyclic aromatic sulfur heterocycles from a vacuum gas oil were fractionated according to the number of condensed aromatic rings on the ChiraDex phase and were characterized by Fourier transform ion cyclotron resonance mass spectrometry.     

Selective Activation of Aromatic Aldehydes Promoted by Dispersion Interactions: Steric and Electronic Factors of a π-Pocket within Cage-Shaped Borates for Molecular Recognition

Selective bond formations are one of the most important reactions in organic synthesis. In the Lewis acid mediated electrophile reactions of carbonyls, the selective formation of a carbonyl–acid complex plays a critical role in determining selectivity, which is based on the difference in the coordinative interaction between the carbonyl and Lewis acid center. Although this strategy has attained progress in selective bond formations, the discrimination between similarly sized aromatic and aliphatic carbonyls that have no functional anchors to strongly interact with the metal center still remains a challenging issue. Herein, this work focuses on molecular recognition driven by dispersion interactions within some aromatic moieties. A Lewis acid catalyst with a π-space cavity, which is referred to as a π-pocket, as the recognition site for aromatic carbonyls is designed. Cage-shaped borates 1 B with various π-pockets demonstrated significant chemoselectivity for aromatic aldehydes 3 b – f over that of aliphatic 3 a in competitive hetero-Diels–Alder reactions. The effectiveness of our catalysts was also evidenced by intramolecular recognition of the aromatic carbonyl within a dicarbonyl substrate. Mechanistic and theoretical studies demonstrated that the selective activation of aromatic substrates was driven by the preorganization step with a larger dispersion interaction, rather than the rate-determining step of the C−C bond formation, and this was likely to contribute to the preferred activation of aromatic substrates over that of aliphatic ones.     

芳香反离子与离子表面活性剂胶团的相互作用

用紫外吸收光谱的方法研究了多种芳香反离子与离子表面活性剂胶团之间的相互作用。发现阳离子表面活性剂和阴离子表面活性剂胶团对芳香反离子的吸收光谱有着显著不同的影响,芳香反离子结构的细微变化可导致光谱特性显著不同。这被解释为阳离子表面活性剂胶团与芳香环之间的阳离子-π相互作用。阳离子-π相互作用需要阳离子表面活性剂分子与芳香反离子具有合适的相对位置与距离。通过测定各混合体系的吸附量证明,不同的芳香反离子在胶团表面有不同的排列方式。NMR的实验结果支持了上述解释。     

Electronic control of amide cis-trans isomerism via the aromatic-prolyl interaction

The cis-trans isomerization of prolyl amide bonds results in large structural and functional changes in proteins and is a rate-determining step in protein folding. We describe a novel electronic strategy to control cis-trans isomerization, based on the demonstration that interactions between aromatic residues and proline are tunable by aromatic electronics. A series of peptides of sequence TXPN, X = Trp, pyridylalanine, pentafluorophenylalanine, or 4-Z-phenylalanine derivatives (Z = electron-donating, electron-withdrawing, or electron-neutral substituents), was synthesized and Ktrans/cis analyzed by NMR. Electron-rich aromatic residues stabilized cis amide bond formation, while electron-poor aromatics relatively favored trans amide bond formation. A Hammett correlation between aromatic electronics and cis-trans isomerization was observed. These results indicate that the interaction between aromatic residues and proline, which is observed to stabilize cis amide bonds and is also a general stabilizing interaction ubiquitous in proteins and protein-protein complexes, is not stabilized exclusively by a classical hydrophobic effect. To a large extent, the aromatic-prolyl interaction is driven and controllable by an electronic effect between the aromatic ring pi-electrons and the proline ring, consistent with a C-H-pi interaction as the key stabilizing force. The aromatic-prolyl interaction is electronically tunable by 0.9 kcal/mol and is enthalpic in nature. In addition, by combining aromatic ring electronics and stereoelectronic effects using 4-fluoroprolines, we demonstrate broad tuning (2.0 kcal/mol) of cis-trans isomerism in tetrapeptides. We demonstrate a simple tetrapeptide, TWflpN, that exhibits 60% cis amide bond and adopts a type VIa1 beta-turn conformation.     

芳烃酰基化后还原制备芳香醇的工艺进展

芳烃制备高附加值精细化学品芳香醇(9-芴甲醇),一直以来存在产物选择性低以及合成成本高等问题。基于此,本文主要综述了芳烃酰基化后还原合成芳香醇的工艺,包括第一步采用Friedel-Crafts酰基化反应、Vilsmeier-Haack反应、Reimer-Tiemann反应、Duff反应等过程将芳烃酰基化合成芳香醛/酮;第二步通过金属氢化物还原、催化加氢还原、活泼金属还原、Cannizarro反应、Meerwein-Ponndorf-Verley还原反应等过程将芳香醛/酮还原合成芳香醇。在总结和归纳各种工艺过程优缺点的基础上,提出了合理的芳香醇制备工艺,为9-芴甲醇产业化制备技术的开发提供帮助。     

Theoretical investigation on regioselectivity of aromatic ketones in the addition with olefin catalyzed by RuH2(CO)(PPh3)3

Ab initio method is employed to study the structures of twelve aromatic ketones at HF/3-21G, HF/6-31G and HF/6-31G levels, respectively. A theoretical analysis is also carried out to study the regioselectivity and reactivity of aromatic ketones in the addition with olefin catalyzed by RuH2(CO)(PPh3)3. The results indicate that a U shape LUMO conjugation of aromatic ketones in a plane plays an important role in regioselectivity on the cleavage of p C-H bond and is a nec-essary factor to success of addition with olefin, and that sterle effect is an indispensable factor in forming additional ortho-product. Meanwhile, electronic effect may influence the rate of addition for the structures alike which only have different replacements in the same site of aromatic ring, such as furan, thiophene and pyrole. A possible catalytic reaction mechanism is proposed that the addition of C-H bond may be carried out by a coordination of aromatic ketones with Ru complex.     

Analysis for polynuclear aromatic hydrocarbons in working atmospheres by computerized gas chromatography-mass spectrometry

Polynuclear aromatic hydrocarbons in particulate matter from working atmospheres have been analyzed by a glass capillary gas chromatography—mass spectrometer—computer system which has high separation efficiency and is capable of separating and identifying these complex mixtures. More than one hundred polynuclear aromatic hydrocarbons have been identified in samples from a coke plant and an aluminum smelter, including both pure polycyclic aromatic hydrocarbons and compounds where a CH-group is substituted with the hetero atoms oxygen, nitrogen, or sulfur. The occurrence of polynuclear aromatic hydrocarbons in working atmospheres is compared to that in ambient air and emissions from other sources.     

Aromatic-aromatic interactions in crystal structures of helical peptide scaffolds containing projecting phenylalanine residues

Aromatic-aromatic interactions between phenylalanine side chains in peptides have been probed by the structure determination in crystals of three peptides: Boc-Val-Ala-Phe-Aib-Val-Ala-Phe-Aib-OMe, I; Boc-Val-Ala-Phe-Aib-Val-Ala-Phe-Aib-Val-Ala-Phe-Aib-OMe, II; Boc-Aib-Ala-Phe-Aib-Phe-Ala-Val-Aib-OMe, III. X-ray diffraction studies reveal that all three peptides adopt helical conformations in the solid state with the Phe side chains projecting outward. Interhelix association in the crystals is promoted by Phe-Phe interactions. A total of 15 unique aromatic pairs have been characterized in the three independent crystal structures. In peptides I and II, the aromatic side chains lie on the same face of the helix at i/i + 4 positions resulting in both intrahelix and interhelix aromatic interactions. In peptide III, the Phe side chains are placed on the opposite faces of the helix, resulting in exclusive intermolecular aromatic interactions. The distances between the centroids of aromatic pair ranges from 5.11 to 6.86 A, while the distance of closest approach of ring carbon atoms ranges from 3.27 to 4.59 A. Examples of T-shaped and parallel-displaced arrangements of aromatic pairs are observed, in addition to several examples of inclined arrangements. The results support the view that the interaction potential for a pair of aromatic rings is relatively broad and rugged with several minima of similar energies, separated by small activation barriers.     

Silver‐Catalyzed Arylation of (Hetero)arenes by Oxidative Decarboxylation of Aromatic Carboxylic Acids

A long‐standing challenge in Minisci reactions is achieving the arylation of heteroarenes by oxidative decarboxylation of aromatic carboxylic acids. To address this challenge, the silver‐catalyzed intermolecular Minisci reaction of aromatic carboxylic acids was developed. With an inexpensive silver salt as a catalyst, this new reaction enables a variety of aromatic carboxylic acids to undergo decarboxylative coupling with electron‐deficient arenes or heteroarenes regardless of the position of the substituents on the aromatic carboxylic acid, thus eliminating the need for ortho‐substituted aromatic carboxylic acids, which were a limitation of previously reported methods.     

Molecular fragment shape variation index applied to intramolecular interaction studies

The fragment shape variation index approach is applied to intramolecular interactions involving C6 aromatic molecular fragments in the special case where the shape-modifying interactions are also caused primarily by other C6 aromatic fragments of the same molecule. This report is a part of a series of studies aimed at the detailed modeling of various components of intramolecular interactions among molecular fragments, including aromatic ring interactions, aromatic ring and non-aromatic conjugated and non-conjugated system interactions, and more general through-space and through-bond interactions. The ultimate purpose of these studies is a better understanding of the electron density shape modifying effects of intramolecular interactions.     

β-Cyclodextrin epichlorohydrin copolymer as a solid-phase extraction adsorbent for aromatic compounds in water samples

A novel solid-phase extraction (SPE) procedure for trace aromatic compounds in water samples has been developed using 12 aromatic compounds as model compounds and GC-MS and UV spectrophotometry for detection. The method is based on the fact that β-cyclodextrin (β-CD) epichlorohydrin (ECH) copolymer (β-CDEP) can extract non-ionized aromatic compounds quantitatively from aqueous samples. The polymer used is a colorless, transparent and insoluble solid with a maximum capacity of 0.82 μmol aromatic compounds per gram. It was synthesized by co-polymerization of β-CD and ECH and characterized by FT-IR and UV. β-CDEP does not contain double bonds, and therefore it does not have appreciable absorbance in the UV region. The optimum pH range for the extraction of aromatic compounds is 2.5-5.0. The method has high extraction efficiency with the recoveries between 90 and 101% for aromatic compounds at 0.02-1.67 ppm levels, and the analytes can be easily eluted by methanol washing after preconcentration.     

Synthesis and properties of 1,3,5-benzene periodic mesoporous organosilica (PMO): novel aromatic PMO with three point attachments and unique thermal transformations

A new aromatic periodic mesoporous organosilica material containing benzene functional groups that are symmetrically integrated with three silicon atoms in an organosilica mesoporous framework is reported. The material has a high surface area, well-ordered mesoporous structure and thermally stable framework aromatic groups. The functional aromatic moieties were observed to undergo sequential thermal transformation from a three to two and then to a one point attachment within the framework upon continuous thermolysis under air before eventually being converted to periodic mesoporous silica devoid of aromatic groups at high temperatures and longer pyrolysis times. The mesoporosity of the material was characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and nitrogen porosimetry, whereas the presence and transformation of the aromatic groups in the walls of the materials were characterized by solid-state NMR spectroscopy, mass spectrometry, and thermogravimetric analysis. The attachment of a benzene ring symmetrically onto three siloxanes of the framework was used advantageously as a cross-linker to enhance the thermal stability of the organic group. Some of these properties are investigated in comparison with other aromatic PMOs that have only two point attachments and an amorphous phenylsilica gel that has only one point attachment. The successful synthesis of the first aromatic PMO with its organic group attached within the framework through more than two points is an important step toward the synthesis of PMOs having organic groups with more complex and multiple attachments within the framework.     

芳香烃双加氧酶的结构与功能研究

芳香烃双加氧酶是细菌来源的参与生物降解芳香烃类化合物的主要酶类,它们能将空气中的氧分子活化,并使芳香烃类化合物在邻位碳上双氧化.这是一类具有重要生物功能和潜在应用前景的酶.本文从生物学与化学相结合的角度,介绍了以邻苯二酚双加氧酶、萘双加氧酶、联苯双加氧酶及栎精双加氧酶为代表的一系列能够催化氧化芳香烃类化合物的双加氧酶,重点讨论了它们的结构与功能,综述了这一领域的研究进展。     

Aromatic Radical Anions in Neat Aromatic Hydrocarbons as Solvents. Direct evidence of through space spin-density transfer to the ligand of the counter ion

Contact ion pairs of aromatic radical anions, with a crown ether complex of potassium as cation in a neat aromatic hydrocarbon, can be obtained by reducing the aromatic hydrocarbon in which a small amount of crown ether is dissolved. The unpaired electron stays attached to one aromatic molecule during a time interval which is long on the ESR. time scale. The radicals are stabilized by ion-pair formation in the low polarity solvent. As a consequence of this stabilization, radicals of compounds with low electron affinities, e.g. mesitylene, can be prepared. Mesitylene, m-xylene, and toluene show additional hyperfine splitting in the ESR. spectra of their anion radical pairs of the order of 18 μT. The proton ENDOR. spectra have signals at the corresponding frequencies, indicating a hyperfine coupling with protons of the crown ether ligand. Using mixtures of two aromatic compounds, their relative electron affinities can be determined by studying the temperature dependence of the radical concentrations.     

Liquid chromatographic properties of aromatic sulfur heterocycles on a Pd(II)-containing stationary phase for petroleum analysis

Polycyclic aromatic sulfur heterocycles (PASHs) can show very poor reactivities in catalytic hydrodesulfurization processes in refineries, especially those in high-boiling fractions and distillation residues. An insight into the structural features of the most recalcitrant PASHs is essential for developing more efficient catalysts and improving refinery processes. The very high complexities of such mixtures necessitate fractionation of the samples into smaller subsets according to defined criteria. A stationary phase containing a palladium(II)-complex was previously shown to be efficient for separating PASHs in lighter petroleum fractions. Here we characterize this ligand exchange chromatographic phase using a large number of sulfur aromatic model compounds that were synthesized for the purpose. In general, compounds containing thiophene rings that are not condensed with other aromatic systems are weakly retained and elute in a first fraction with the polycyclic aromatic hydrocarbons. Thiophene rings condensed with other aromatic rings are more strongly retained and elute in a later fraction with a more polar eluent. If the sulfur is in a non-aromatic ring, the compound is irreversibly retained by the Pd(II) ions. Some steric effects are seen in compounds with alkyl or aryl substituents close to the sulfur atom but in general they do not interfere strongly with the complexation. Thus it seems possible to separate groups of aromatic sulfur compounds according to their complexation properties. For instance, such fractionated samples can be studied much more easily by mass spectrometric techniques.