ISSN:0947-6539    

DOI:10.1002/chem.19970030103

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractHelical complexes have been much studied as examples of self‐assembled supramolecular units. Their structural features are analysed in terms of the metal ion, the binding site on the ligand and the bridging group linking these sites. Understanding these elements allows the controlled assembly of multicomponent systems. Preliminary results show the complexes to have high stability, arising from the inertness of the self‐assembled spec

ISSN:0947-6539    

DOI:10.1002/chem.19970030104

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractAb initio calculations (up to MP4SDQ/6‐31+G*//MP2(FU)/6‐31+G*+ δZPVE) on several model nitroaldol (Henry) reactions have been performed. It is found that the free nitronate anions react with aldehydes via transition states in which the nitro and carbonyl dipoles are antiperiplanar to each other. This kind of reaction yieldsanti (erythro)nitroalcohols as major products. The Henry reaction between lithium nitronates and aldehydes is predicted to occur via cyclic transition structures yieldingsynnitroalcohols as major products. The stereocontrol in these model reactions is low. The factors affecting the stereoselectivity in the reaction between dilithiated nitronates and aldehydes are also discu

ISSN:0947-6539    

DOI:10.1002/chem.19970030105

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe acidity constants of H2‐(AMPS)±were determined by potentiometric pH titrations in aqueous solution at 25°C andI=0.1M (NaNO3). Titrations with a combined single‐junction glass electrode were hampered in the presence of AMPS by a “poisoning” effect; the problem could be avoided by use of two separated electrodes. The values of the acidity constants PKHH2(AMPS)= 3.72 ± 0.03 and pKHH2(AMPS)= 4.83 ± 0.02 are relatively close to each other; the buffer regions of the two equilibria overlap, and therefore a micro acidity constant scheme was developed and the constants for the various sites calculated. It is concluded that the thiophosphateprotonated species (AMPS⋅H)−dominates at about 75% occurrence, while the form (H⋅AMPS)−, with the proton at the N1 site of the adenine residue, occurs at about 25%. Semiempirical AM1 and PM3 calculations including water as a solvent locate the proton in (AMPS⋅H)−mainly on the terminal oxygen atoms rather than the sulfur. The acid‐base properties of H2(AMPS)±are considerably more complicated than those of the parent nucleotide, H2(AMP)±; for the latter the two (intrinsic) acidity constants are well separated and consequently practically all protons have left the N1 site before deprotonation at the monoprotonated phosphate group occurs. Finally, an estimate for the acidity const

ISSN:0947-6539    

DOI:10.1002/chem.19970030106

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractFunctional polyanions were prepared by copolymerization of sulfopropyl acrylate and sulfopropyl methacrylate with monomers bearing triaminopyrimidine or barbituric acid functionalities, respectively. Functionalized polyelectrolyte multilayers were assembled from these copolymers by stepwise alternating adsorption with poly(choline methacrylate). These multilayers are suited for molecular recognition of substrates that are complementary to the functional groups incorporated. Thus, multilayers containing triaminopyrimidine moieties selectively bind barbituric acid, and vice versa, when exposed to solutions of the 1:1 complex of barbituric acid and triaminopyrimidine. The molecular recognition process was monitored by UV/Vis spectroscopy, time‐of‐flight secondary ion mass spectroscopy (ToF‐SIMS), and photoelectron spectroscopy (XPS). Remarkably, after successful recognition and binding of the complementary substrates to the multilayers, the stepwise layering could be cont

ISSN:0947-6539    

DOI:10.1002/chem.19970030107

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe Kinetics and Thermodynamics of the Reversible Outer‐Sphere Electron‐Transfer Reactions between Horse Heart CytochromecII/IIIand [Co(phen)3]3+/2+and [Co(bpy)3]3+/2+were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one a

ISSN:0947-6539    

DOI:10.1002/chem.19970030108

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe reaction products and intermediates from the interaction of acetonitrile with olefins (oct‐1‐ene) or alcohols (tert‐butyl alcohol) in zeolite H‐ZSM‐5 at 296 K have been characterized with13C and15N solid‐state NMR. It has been shown that coadsorption of acetonitrile and olefin on H‐ZSM‐5 gives rise to the intermediateN‐alkylnitrilium cation represents a persistent species inside a zeolite under anhydrous conditions. Upon admittance of water to the pores of the zeolite, theN‐alkylnitrilium cation slowly converts intoN‐alkylamide in accordance with the classic Ritter reaction. In the case of acetonitrile and alcohol, just after coadsorption both the intermediateN‐alkylnitrilium cation and the finalN‐alkyl‐amide are identified simultaneously; the former slowly disappears over a few days. Thus, 1) it has been shown that the Ritter reaction can occur not only in liquid acidic media but also on a solid acid catalyst, zeolite H‐ZSM‐5; 2)N‐alkylnitrilium cations have been detected and characterized with solid‐state NMR as persistent intermediates in the Ritter reaction for the fir

ISSN:0947-6539    

DOI:10.1002/chem.19970030109

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe reaction of [Cp(*)2Ru2‐(CO)4] [a: Cp(*)= Cp*(η5‐C5Me5);b: Cp(*)= Cpx(η5‐C5Me4Et)] with β‐realgar in boilingn‐decane gives [Cp(*)2Ru2‐As4S4] (1a,b). in good yields along with some [Cp(*)2Ru2‐As4S2] (2a,b). According to X‐ray diffraction analyses of1band2a, their structures belong to two different types of inorganic clusters. Whereas1bhas a cage structure in which the [Cp(*)2Ru2unit is inserted into the As4S4] cradle,2ahas a sandwich structure in which two coplanar η2‐As4S ligands and the Cp*ligands are oriented parallel to one another. Analysis of the As‐S connectivity in the core of1 breveals a structural relationship to the extremely rare form of arsenic sulfide As4S4(II), and not to the metastable high‐temperature form β‐As4S4used in the synthesis. At lower temperatures (60, 115°C) formation of CO‐

ISSN:0947-6539    

DOI:10.1002/chem.19970030110

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe first rotational isomers of thiobenzaldehydes, TbtCHS (2aand2b; Tbt = 2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl), were synthesized and isolated as stable crystalline compounds by the desulfurization of the corresponding overcrowded cyclic polysulfides TbtCHSn(n= 5 or 8) with phosphine reagents. The molecular structures of2aand2bin the solid state, determined by X‐ray crystallographic analysis, differed in their conformations, which were essentially identical with those in solution as revealed by1H{1H} nuclear Overhauser effect (NOE) experiments. The isomeric thiobenzaldehydes2aand2bwere found to undergo thermal interconversion. A kinetic study of this process gave reasonable kinetic and thermodynamic parameters for conformational isomerizations of this type. Interesting differences in reactivity among the two isomers2a,band 2,4,6‐tri‐tert‐butylthiobenzaldehyde (1) were shown in the reactions with hydrazine andm‐chloroperox

ISSN:0947-6539    

DOI:10.1002/chem.19970030111

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractArcyriacyanin A (1) has been synthesized by three different routes. In the first synthesis the bisbromomagnesium salt of 2,4′‐biindole (5) was treated with dibromomaleimide (6) to yield arcyriacyanin A (1). The second approach used an intramolecular Heck reaction for the cyclization of a 4‐(triflyloxy)arcyriarubin8toN‐methylarcyriacyanin A (2). Thirdly, compound2was obtained by a domino Heck reaction between 3‐bromo‐4‐[1‐(tert‐butoxycarbonyl)indol‐3‐yl]‐1‐methylmaleimide (9) and 4‐bromoindole (10). TheN‐methyl derivative2could be transformed into arcyriacya

ISSN:0947-6539    

DOI:10.1002/chem.19970030112

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY