ISSN:0018-019X    

DOI:10.1002/hlca.19920750702

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19920750703

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe titanates derived from α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanols (TADDOLs, prepared from tartrate) act as catalysts for enantioselective additions of dialkylzinc compounds to aldehydes. For the standard reaction chosen for this investigation of the mechanism, the addition of diethylzinc to benzaldehyde, there is very little change of selectivity with different aryl substituents on the TADDOLate ligands (Tables 2–4, examples). With 0.02 to 0.2 equiv. of the chiral titanates, selectivities above 90% are observed only in the presence of excess tetraisopropyl titanate! According to NMR measurements (Fig. 2), the chiral bicyclic titanate and the achiral titanate do not react to give new species under these conditions. From experiments with different stoichiometries of the components, and with different achiral or chiral OR groups on the Ti‐atom of the seven‐membered ring titanate, it is concluded (i) that a single chiral titanate is involved in the product‐forming step, (ii) that the bulky TADDOLate ligand renders the Ti‐center catalytically more active than that of (i‐PrO)4Ti, due to fast dynamics of ligand exchange on the sterically hindered Ti‐center (Table 5, Fig. 3), and (iii) that the role of excess (i‐PrO)4Ti is to remove – by ligand exchange – the product alkoxides (R*O) from the catalytically active Ti‐center (Scheme 4, Table 6). Three new crystal structures of TADDOL derivatives (two clathrates with secondary amines, and a dimethyl ether) have been determined by X‐ray diffraction (Figs. 5–7), and are compared with those previously reported. The distances between the C(aryl)2O oxygen atoms in theC2‐ andC1‐symmetrical structures vary from 2.58 to 2.94 Å, depending upon the conformation of their dioxolane rings and the presence or absence of an intramolecular H‐bond (Fig. 8). A single‐crystal X‐ray structure of a spiro‐titanate, with two TADDOLate ligands on the Ti‐atom, is described (Fig. 9); it contains six different seven‐membered titanate‐ring conformations in the asymmetric unit (Fig. 10), which suggests a highly flexible solution structure. The structures of Ti TADDOLate complexes are compared with those ofC2‐symmetrical Ru, Rh, and Pd disphosphine chelates (Table 7). A common topological model is presented for all nucleophilic additions to aldehydes involving Ti TADDOLates (Siattack with (R,R)‐derivatives, relative topicityunlike; Fig. 11). Possible structures of complexes containing bidentate substrates for Ti TADDOLate‐mediated ene reactions and cycloadditions are proposed (Fig. 12). A simple six‐membered ring chair‐type arrangement of the atoms involved can be used to describe the result of TADDOLate‐mediated nucleophilic additions to aldehydes and ketones, with Ti, Zr, Mg, or Al bearing the chiral ligand (Scheme 6). A proposal is also made for the geometry of the intermediate responsible for enantioselective hydrogenation ofN‐(acetylamino)cinnama

ISSN:0018-019X    

DOI:10.1002/hlca.19920750704

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractDimethyl azulene‐1,2‐dicarboxylates are reduced with a 4–5 molar excess of DIBAH in Et2O/hexane at 0° to yield the corresponding 2‐(hydroxymethyl)‐1‐methylazulenes which can be further reduced to 1,2‐dimethylazulenes on treatment with Et3SiH in TFA at 60

ISSN:0018-019X    

DOI:10.1002/hlca.19920750705

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe title compound FeF+has been generated in aFourier‐transform ion‐cyclotron‐resonance mass spectrometerviaCF bond activation by reacting hexafluorobenzene with bare FeO+. Bracketing experiments provide a bond‐dissociation energy (BDE) for FeF+of 86

ISSN:0018-019X    

DOI:10.1002/hlca.19920750706

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractIt is shown that azulenes react with dimethyl acetylenedicarboxylate (ADM) in solvents such as toluene, dioxan, or MeCN in the presence of 2 mol‐% [RuH2(PPh3)4] already at temperatures as low as 100° and lead to the formation of the corresponding heptalene‐1,2‐dicarboxylates in excellent yields (Tables 1and2). The Ru‐catalyzed reaction of ADM with 1‐(tert‐butyl)‐4,6,8‐trimethylazulene (31) takes place even at room temperature, yielding the primary tricyclic addition product32and its thermalretro‐Diels‐Alderproduct dimethyl 4,6,8‐trimethylazulene‐1,2‐dicarboxylate (21;Scheme 4). At 100° in MeCN,32yields 90% of21and only 10% of the corresponding heptalene. These observations demonstrate that [RuH2(PPh3)4] catalyzes the first step of the thermal formation of heptalenes from azulenes and ADM which occurs in apolar solvents such as tetralin or decalin at tempe

ISSN:0018-019X    

DOI:10.1002/hlca.19920750707

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe reaction of dodecacarbonyltriruthenium with 1,3,5‐trithiacyclohexane (1,3,5‐trithiane) in refluxing THF yields [Ru3(CO)6(μ2‐CO)3{μ3‐(η3‐C3S3H6)}] (1) in which the three S‐atoms of the cyclic ligand coordinate to the three Ru‐atoms of the metal core. X‐Ray diffraction reveals a nearly perfectC3vsymmetry for the molecule. The variable‐temperature13C‐NMR spectra show the carbonyl ligands to be

ISSN:0018-019X    

DOI:10.1002/hlca.19920750708

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA new, convenient, and safe route to 1,3,5‐triamino‐1,3,5‐trideoxy‐cis‐inositol (taci) was investigated by hydrogenation of azo‐coupled derivatives of phloroglucinol. In the presence of acetic anhydride, the reduction of trisphenylazophloroglucinol (H2/Pd(5%) on C) resulted in the formation of tri‐, hexa‐, and nona‐acetylated derivatives of triaminophloroglucinol. All three compounds are air‐stable, colorless solids. However, the succeeding hydrogenation to the cyclohexane derivative failed. Trisodiumtris(p‐sulfonatophenylazo)phloroglucinol could be hydrogenated in a one‐pot reaction to the desired taci· 1.5H2SO4using a Pt/Rh oxide as catalyst. taci provides two distinct chair conformations with either three amino or three hydroxy groups for metal binding. The unique metal‐binding properties are discussed in terms of minimal conformational changes required for coordination. Conformational analysis, based on X‐ray structural data of [BiCl6][H3(taci)] ·2 H2O (Pnma,a= 24.314 (5) Å,b= 10.215 (2) Å,c= 7.422 (8) Å,R= 5.8%) and [Co(taci)2(NO3)3]·2H2O (C2/c, a = 22.912 (8) Å,b= 8.942 (2) Å,c= 14.731 (3) Å, β = 128.66 (2)°,R= 4.9%) and the previously investigated [Cr(taci)2]3+revealed an almost ideal cha

ISSN:0018-019X    

DOI:10.1002/hlca.19920750709

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractThe complexes [Fe(tdci)2]Cl3and [Al(tdci)2]Cl3(tdci = 1,3,5‐trideoxy‐1,3,5‐tris(dimethylamino)‐cis‐inositol) were prepared and characterized by mass spectrometry, NMR spectroscopy, and magnetic‐susceptibility measurements. The formation constants were determined in aqueous solution (25°, 0.1MKCl) by potentiometric titration. pKvalues of H3(tdci)3+: 5.89, 7.62, 9.68; FeIIIcomplexes: log βML= 18.8, log β ML 2= 32.6; AlIIIcomplexes: log βML= 14.3, logβ ML 2= 26.4. The protonated complex [FeH(tdci)2]4+has also been identified. In contrast to the high stability of the FeIIIand AlIIIcomplexes, only weak interactions of tdci with CuIIhave been observed in aqueous

ISSN:0018-019X    

DOI:10.1002/hlca.19920750710

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY

摘要:

AbstractA new base pair (called κ‐π) ofWatson‐Cricktype, with an H‐bond pattern different from that in A‐T and G‐C base pairs, has been recently synthesized byBennerand coworkers and shown to be stable and incorporable into duplex DNA and RNA by polymerases. This new base pair, which contains three H‐bonds, is compared with G‐C, in the framework of modern dynamical theory of quantum nonlocality and quantum correlations (also calledEinstein‐Podolsky‐Rosencorrelations). Connection with the traditional treatment of proton transfer in DNA base pairs, which uses the adiabatic approximation (thus considering the protons as classical particles), is explicitly made. As a result, the dynamics of the H‐bond pattern of G‐C is shown to exhibit a specific quantum‐mechanical phase stability (or: rigidity, stiffness), which is clearly missing in the case of κ‐π. This finding is discussed and illustrated, also in connection with recent quantum chemical calculations of proton transfers in DNA base pairs. Additionally, certain speculations concerning a probable ‘evolutionary advantage’ of G‐C with re

ISSN:0018-019X    

DOI:10.1002/hlca.19920750711

出版商:WILEY‐VCH Verlag GmbH    

年代:1992

数据来源: WILEY