ISSN:0947-6539    

DOI:10.1002/chem.19970030803

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractDendrimers coated with carbohydrates on their exterior surfaces have been constructed by using both convergent and divergent synthetic routes. Alternatively, cluster glycosides in the form of highly branched oligosaccharides can serve as dendritic wedges in the subsequent elaboration of fully carbohydrate dendrimers. It is anticipated that these novel saccharide‐containing polymers, which are highly branched and water‐soluble, will find applications of a biological nature as well as in the context of new materi

ISSN:0947-6539    

DOI:10.1002/chem.19970030804

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractMetal colloids protected with chiral molecules can lead to a new type of enantioselective catalyst combining good selectivity control with an extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidine.

ISSN:0947-6539    

DOI:10.1002/chem.19970030805

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

ISSN:0947-6539    

DOI:10.1002/chem.19970030806

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThiacyclophane ligands1and2, containing ameta‐xylyldithiaether unit, an aromatic spacing unit and a polyether chain, were prepared in good yield in a three‐step synthesis. The macrocyclic organopalladium complexes [Pd(L)‐(MeCN)][BF4] (3: L = 1;4: L =2) were prepared through palladation of the respective thiacyclophane ligand by reaction with [Pd(MeCN)4][BF4]2. These complexes act asmetalloreceptorsto aromatic amines such asp‐aminopyridine (pap),m‐aminopyridine (map) and the DNA nucleobases adenine and guanine. Binding occurs through simultaneous first‐ and second‐sphere coordination. This involves three separate interactions: first‐sphere σ donation from an aromatic N atom to the Pd centre, second‐sphere hydrogen bonds between the NH2group and polyether O atoms, and π stacking between the electron‐poor aromatic rings of the substrate and the electron‐rich aromatic spacing units of the receptor.1H NMR spectra exhibit chemical shift changes indicative of the H‐bonding and π‐stacking interactions in solution. X‐ray structures for thiacyclophane1, metalloreceptor [Pd(1)(MeCN)][BF4] (3), metalloreceptor/model substrate complexes [Pd(1)(pap)][BF4(5) and [Pd(2)‐(pap)][BF4] (7), and metalloreceptor/nucleobase complexes [Pd(1)(adenine)][BF4] (13), [Pd(2)(adenine)][BF4] (14) and [Pd(1)(guanine‐BF3)][BF4] (15b) show details of the

ISSN:0947-6539    

DOI:10.1002/chem.19970030807

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe kinetics of the reactions of trialkylamine boranes and pyridine boranes with benzhydryl cations have been determined photometrically. Second‐order rate laws are obeyed, first‐order with respect to amine borane concentration and first‐order with respect to carbocation concentration. As for other reactions of carbocations with neutral nucleophiles, the rates of these reactions are only slightly affected by solvent polarity. The structure—reactivity relationships and kinetic isotope effects are in accord with a polar mechanism proceeding through a transition state where the migrating hydride is partly bound to the entering carbon and to the leaving boron atom. The rate constants correlate linearly with the electrophilicity parametersEof the carbenium ions. It is therefore possible to use the linear free enthalpy relationship logk2=s(E+N) for determining nucleophilicity parametersNfor the amine boranes and to compare their hydride‐donating abilities with those of other non‐charged hydride donors (silanes, germanes, stannanes, and dihydr

ISSN:0947-6539    

DOI:10.1002/chem.19970030808

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe twelve‐memberedO‐bridged cyclic ether obtusallene IV (1), a new isolate from the red seaweedLaurencia obtusafrom Kaş in the Turkish Mediterranean, revealed temperature‐dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180° flipping of thetransolefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10‐bromoobtusallene I (3), obtusallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic derivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for2–3and at low temperature for4. Marked cross‐saturation‐transfer effects between the couples of conformers confirmed these observations. Activation energies for processes involving 10‐membered subunit rings (2–3) are higher than for 11‐membered (4–5) analogues, where faster conformational motion occurs too resulting in mediated vicinalJcouplings. 1,3‐Dihydroxy substituents in5form an intramolecular hydrogen bond in low‐polarity, non‐H‐bonding solvents; this results in the existence of two further conformers, giving more complex NMR spectra. Descriptions in the literature of single conformers for obtusallenes2–5must have resulted from overlooking minor NMR signals

ISSN:0947-6539    

DOI:10.1002/chem.19970030809

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractProceeding from the α‐B‐SbW9O33fragment1, various novel large heteropolyanions ([Na2Sb8W36O132(H2O)4]22–(2), [Sb2W22O74(OH)2]12−(3) and [Sb2W20M2O70(H2O)6](14−2n)−(4); Mn+= Fe3+, Co2+, Mn2+, Ni2+) have been obtained in crystalline form by systematic synthesis. Their unexpected structures have been studied by X‐ray diffraction:2, 3and4contain β‐SbW9units connected to each other by Sb3+ions, WO2/WO2OH groups, and WO2/Mn+(H2O)3groups, respectively. Structural details and especially the systematic synthesis strategy for the formation of these interesting polytungstates are discussed in order to explain the unusual behaviour of1in aqueous solution under different re

ISSN:0947-6539    

DOI:10.1002/chem.19970030810

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractNew low molecular weight gelators based on the structure RNHCONHXNHCONHR have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds2(R =n‐dodecyl, X = ‐(CH2)9‐),3(R =n‐dodecyl, X = ‐(CH2)12‐),4(R =n‐dodecyl, X = 4, 4′‐biphenyl), and5(R = benzyl, X = ‐(CH2)9‐) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mgmL−1. Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane,p‐xylene, 1‐octanol,n‐butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100°C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds2–5spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2–10 μm wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three‐dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X‐ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for2and3, respectively. Scanning tunneling microscopic investigations of2absorbed on graphite showed that2forms long ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds. Based on these observations a model is proposed for the arrangement of the molecules in the fibers. In this model the bisurea molecules aggregate through hydrogen‐bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form long thin fibers. This mod

ISSN:0947-6539    

DOI:10.1002/chem.19970030811

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractDiazonium ions are reactive intermediates in deamination reactions pertinent to chemical carcinogenesis. While methyldiazonium ion has been shown to exist as a short‐lived intermediate in the gas phase and in solution, benzyldiazonium ions have never been observed, and the reaction intermediates in deaminations of benzyl systems remain a matter of debate. We therefore studied the benzylcation‐dinitrogen interaction by ab initio methods; several important conclusions resulted. Analysis of the potential energy surface at the level MP4(sdtq)/6‐31 G*/ MP 2(full)/6‐31 G* + ΔVZPE(MP 2(full)/6‐31 G*) revealed that a classical “benzyldiazonium ion” does not exist. The interaction of N2with benzyl cationM‐1results in an electrostatically bound complex2Cwith a long CN distance (2.935 Å) as the most stable structure. A covalently bound planar benzyldiazonium ion2Awith a “normal” CN bond length (1.514 Å) is the transition‐state structure for automerization of the electrostatic complex with concomitant rotation about the exocyclic CC bond. The potential energy surface characteristics result from the highly efficient π‐dative Ph → CH2bonding inM‐1; this is clearly demonstrated in its structure and that of its transition state for rotational automerizationTS‐1by the very high activation barrier for rotation (47.6 kcalmol−1!) and by the gradient vector fields of the total electron densities of conformers of1and2. The rotational barrier for1is reduced to 27.9 kcalmol−1in the N2complex2, and the potential energy surface characteristics of benzyldiazonium ion essentially facilitate the N2‐catalyzed rotational isomerization of benzyl cation. The benzyldiazonium ion system shows for the first time that the interaction of a donor molecule with a carbenium ion with a valence LUMO can lead to the formation of an electrostatic complex as opposed to dative bond formation. Dative σ‐bond formation between N2and the CH2carbon of1is energetically not c

ISSN:0947-6539    

DOI:10.1002/chem.19970030812

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY