摘要:

AbstractThe syntheses, crystal structures, and physical properties of some recently discovered layered ternary and quaternary chalcogenides are reviewed. One component of these systems is an alkali metal or thallium; another is a d‐, f‐, or p‐ block

ISSN:0009-2940    

DOI:10.1002/cber.19971300102

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractTricarbonyl(η6‐1,3,5,7‐cyclooctatetraene)chromium (1) yields upon UV irradiation in ether at 233 K with 2,3‐dimethyl‐1,3‐butadiene (2) or 2‐methyl‐1,3‐butadiene (3) by [6+2] cycloadditions the correspondingly substituted tricarbonyl(η6‐bicyclo[4.2.2]deca‐2,4,7‐triene)chromium complexes4,5aand5b. With ammonium cerium(IV) nitrate the organic ligand of complex4, 9‐isopropenyl‐9‐methyl‐bicyclo[4.2.2]‐deca‐2,4,7‐triene (6) is liberated. The complexes4,5aand5bwere characterised by IR and NMR spectro

ISSN:0009-2940    

DOI:10.1002/cber.19971300103

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe palladium‐catalyzed reductive carbonylation oforthoandpara‐substituted nitrobenzenes has proven to be an attractive route for the synthesis of functionalized carbamates. For the Pd(1, 10‐phenanthroline)2(triflate)2catalyst system, the scope of the reaction has been studied. Substrates with electron‐donating substituents at theparaposition were found to decrease the catalytic activity, most probably as a result of their relatively low oxidizing capacity. the selectivity towards the desired carbamate, however, was increased for these substrates. Under the influence of electron‐withdrawing substituents the azoxybenzene and azobenzene derivatives became important side products. Introduction of large steric hindrance at theorthoposition of the nitro substrates gave rise to an interesting side reaction, viz. methoxylation of the aromatic ring. The methoxylation reaction appeared to occur on an intermediate species in the catalytic cycle. Several functionalities have shown to be resistant to the reaction conditions required for the conversion ot the nitro group. Especially with 4‐nitrobenzoic acid, an extremely high activity and selectivity was found, thus yielding a very convenient synthesis forN‐protected amines containing carboxylic

ISSN:0009-2940    

DOI:10.1002/cber.19971300104

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractNMR and Mössbauer spectroscopy show that a novel tetrathiol ligand, based on the cavitand diphenylglycoluril, encapsulates a 4Fe‐4S cluster and induces asymmetry in it. The cluster gives a weak electrochemical current response in DMF, with a half‐wave potential for the 2−/3−reduction vs. Fc+/Fc of ‐1.7 V. Ba2+ions are adsorbed, according to X‐ray analysis of the SEM image of the electrode, and act as modulator and promoter of the electrochemical response. On the basis of cyclic voltammograms it is proposed that this adsorption creates electroactive sites, changing the type of diffusion controlling the mass transport to the electrode from radial to linear, and that it helps the negatively charged complex, which contains a dipole, to approach the negative electrode in an orientation favourable for electron exchange. This feature makes the complex an important model for ferredoxins, in spite of a difference in red

ISSN:0009-2940    

DOI:10.1002/cber.19971300105

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe reaction of palladium acetate with two equivalents of di‐ and triethanolamines RN(CH2CH2OH)2in the presence of a base affords the new chelate‐stabilized alkoxo Pd(II) complexes [Pd(OCH2CH2N(R)CH2CH2OH)2] [R = Me (1), Et (2),n‐Bu (3), benzyl (4) or CH2CH2OH (5)]. TheseN,O‐ligated complexes are isolated in high yield as yellow, crystalline solids and are thermally stable despite the presence of several β‐hydrogen atoms in the ligand system. Both complexes possess a square‐planar palladium coordination geometry with the two oxygen atoms positioned mutuallytrans.The most notable difference in the molecular structures is that1forms a two dimensional network of intermolecular O–HO hydrogen bonds, whereas5forms intramolecular O–H ⃛O hydrogen bonds, which cage the palladium center. In solution1–4exist as a diastereoisomeric mixture (a racemic enantiomeric pair SNSN, RNRNand a mesomeric form RNSN) in a 1:1 molar ratio, and this ratio is independent of temperature in nonalcoholic solvents, When complexes1–4are dissolved in protic solvents (e.g. MeOH) the diastereomeric excess is temperature‐dependent due to an exchange process between themesodiastereoisomer and the (racemic) enantiomeric pair. Thermodynamic parameters for this process in a mixture of MeOH‐toluene have been determined with NMR and show this process to be influenced by the steric nature of the alkyl substituent (R) on nitrogen. A conformational analysis based on1H‐NMR coupling constants within theN,O‐chelate ring of complexes1–4provides details on the solution structure of

ISSN:0009-2940    

DOI:10.1002/cber.19971300106

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractMethyl isopropenyl ether (1) has been metallated at low temperature with a 1:1 molar mixture ofn‐BuLi/t‐BuOK in THF‐hexane, and subsequently functionalized with a variety of electrophilic reagents. At temperatures higher than – 30°C, the metallated methyl isopropenyl ether (2) decomposes with formation of allene. When the suspension of2is allowed to warm up to room temperature in the presence of an additional equivalent ofn‐BuLi, the intermediary allene is converted into 1‐propynyllithium (2c), which reacts with propylthiocyanate to give 1‐propylthio‐1‐propyne (4). Some of the functionalization products were subjected to acidic hydrolysis affording the expec

ISSN:0009-2940    

DOI:10.1002/cber.19971300107

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe solid‐state structures of (p‐bromoselenobenzoyl)morpholine (2a) and [p‐(dimethylamino)selenobenzoyl]morpholine (2b) were determined by X‐ray diffraction. Both molecules show a flat selenoamide group. The larger contribution of resonance stabilization by the aromatic ring carrying thep‐dimethylamino substituent is reflected by the smaller interplanar angle Θ between the aromatic ring and the selenoamide group [53.3(1)° vs. 81.1(1)°] and by the shorter length of the CSe bond [1.824(5) Å vs. 1.840(3) Å]. The Gibbs free energy of activation of C‐N bond rotation (ΔG≠rot) of fivep‐substituted (selenobenzoyl)morpholines was determined by dynamic13C NMR. The activation barriers were found to range from 61.6 kJ/mol (X = NNMe2) to 75.1 kJ/mol (X = H). The ΔG≠rotvalues of the corresponding (thiobenzoyl)morpholines were found to be from 3.2 kJ/mol (X = NMe2) to 5.0 kJ/mol (X = H) lower. In both cases, ΔG≠rotshowed an excellent linear

ISSN:0009-2940    

DOI:10.1002/cber.19971300108

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractFor the hydrogenation of the CN bonds in the pyrazine ring of the vitamin folic acid (1) optically active rhodium(I)/diphosphane complexes immobilized on supports such as silica gel or Al2O3were used. The reduction was carried out at 50 bar hydrogen pressure in an aqueous solution buffered to pH 7. Thus, 5,6,7,8‐tetrahydrofolic acid (2) was obtained which contains a new asymmetric center at C‐6 of the pterine system. Therefore, in combination with the (S) configuration of the naturalL‐glutamic acid part of the molecule two diastereomers with (6S,S) and (6R,S) configuration arise. The relatively unstable tetrahydrofolic acid (2) was converted into its 5‐formyl derivative folinic acid (4) by treatment with methyl formate/formic acid in a 5:1 mixture of DMSO/pyridine. The Ca salt of folinic acid (4) is the widely used drug leucovorin. The diastereomers were separated by silica gel HPLC. To the column bovine serum albumine (BSA) is covalently bound. With optically active rhodium(I)/diphosphane catalysts, immobilized on silica gel supports, a diastereoselectivity of up to 90% could be achieved in the hydrogenation of folic

ISSN:0009-2940    

DOI:10.1002/cber.19971300109

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractTwo new compounds Cu4(AsO4)2O (1) and Ba2Cu7As6O24(2) were obtained from a mixture of CuO and As2O5· xH2O heated to 1090°C for 72 h in a BaCO3flux. Compound1was obtained as dark green crystals, belonging to the orthorhombic crystal system. Compound2was obtained as light gray to colorless crystals belonging to the triclinic crystal system. The structure of1contains two copper ions with distorted trigonal bipyramidal geometries and a third with a distorted square pyramidal geometry. One ‚oxide’︁ ion is surrounded by four copper ions in a distorted tetrahedral arrangement. Compounds2consists of two chains with alternating units of trigonal bipyramidal CuO5and square planar CuO4units, with the oxygen atoms derived from the AsO4groups. Magnetic susceptibility measurements of1and2both show effective magnetic moments, μeff1.90–1.95 B.M./Cu at 25°C, which is consistent with Cu2+ions having one unpaired electron. Significant antiferromagnetic coupling was revealed by variable temperature measurements. At very low temperatures (15–25 K) samples of both1and2undergo a transition to a weak ferr

ISSN:0009-2940    

DOI:10.1002/cber.19971300110

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe performed a systematic investigation of [2 + 1] cycloadditions of HNC to dipolarophiles of the type CH2X and PHX with XCH2, NH, O, SiH2, PH, and S, as well as HNNH and H2SiSiH2. Ab initio MO calculations at the QCISD(T)/6‐311G(d,p)/MP2/6‐31G(d,p)+ZPE level were applied to construct the minimum‐energy reaction pathways. Calculated results concerning the regiochemistry of the approaches and the stereochemistry of the product formation were analyzed in relation with established concepts such as frontier orbital, net charge distribution and stereoelectronic effect. For some systems, up to four distinct transition structures for cycloaddition were found. In general, there is a certain similarity between the behavior of both C and P series of dipolarophiles. The characteristics of the transition structures can be, in most cases, recovered by using qualitative concepts. The [2 + 1] cycloadditions are highly regioselective and stereospecific. Silicon‐containing dipolarophiles are particularly attractive as they exhibit very small energy barriers to addi

ISSN:0009-2940    

DOI:10.1002/cber.19971300111

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY