ISSN:0947-6539    

DOI:10.1002/chem.19970030403

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractDouble‐stranded DNA can be viewed as a multifunctional, modular receptor that can be read sequence‐selectively in a digital way (base pair per base pair) by a complementary, similarly modular ligand. This principle has been exploited in several approaches to design sequence‐specific DNA‐binding ligands, such as triplex‐forming oligonucleotides, peptide nucleic acids and minor groove binding p

ISSN:0947-6539    

DOI:10.1002/chem.19970030404

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIn this paper we describe photophysical and mechanistic studies of spiro[fluorene‐9,1′‐pyrrolo[2,1‐a]quinolines](DHI)1–4and of their styryl homologues5–11(styryl DHI). DHIs1–11have low fluorescence quantum yields (approximately ϕf= 10‐3). The position of the fluorescence emission band is temperature‐dependent. The ring‐opened betaines1′–11′ do not fluoresce at all. DHIs1–11exhibit phosphorescence, which is hypsochromically shifted by NO2substitution of the fluorene ring. The triplet can be populated only by sensitization (with benzophenone as sensitizer) and, therefore, the reaction that occurs under direct excitation of the molecule involves the singlet excited state only. On irradiation under time‐resolved conditions, two transients were seen to be formed in the picosecond and microsecond domains. Slightly tilted educt or product‐like geometries are attributed to these transients. An energy diagram is proposed for the photoreaction of the model compound spiro[pyrroloquinoline]1 → 1′, which takes into account the singlet pathway and the different transients of ca. 100 ps lifetimes formed f

ISSN:0947-6539    

DOI:10.1002/chem.19970030405

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractTwo‐step and practical asymmetric syntheses of enantiomerically pure 4‐trifluoromethyl‐2,2‐dioxo‐1,3,2‐dioxathiolane and 4‐trichloromethyl‐2,2‐dioxo‐1,3,2‐dioxathiolane (>98%ee) have been achieved. Catalytic asymmetric dihydroxylation (AD) of 3,3,3‐trifluoropropene and 3,3,3‐trichloropropene, respectively, is followed by direct cyclic sulfate formation by reaction with sulfuryl chloride. Opening of the cyclic sulfates with various nucleophiles provides easy access to

ISSN:0947-6539    

DOI:10.1002/chem.19970030406

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe [1,5]‐sigmatropic rearrangements of hydrogen and other groups, such as methyl,tert‐butyl, allyl, benzyl, and azafulvenium, in pyrroles and, to some extent, in furans and thiophenes, has been studied by using semiempirical and ab initio methods. These systems are used as models to explain the ring D inversion in the biosynthesis of uroporphyrinogen III and the stereoselective shift of a methyl group in the biosynthesis of vitamin B12. The difference in energy between the competing pathways of hydrogen and methyl shifts is significantly lower than for cyclopentadiene. The rearrangements are usually concerted, except for in strongly resonance‐stabilized systems, such as azafulvenium cations. Furthermore, ab initio calculations of the 1,3‐allylic strain for a range of substituted pyrroles has been performed, and the results compared with semiempiric

ISSN:0947-6539    

DOI:10.1002/chem.19970030407

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractIn this theoretical study, we investigate the molecular and electronic structures of symmetrical and unsymmetrical squaraine dyes. Such dyes can be represented by the formula D‐A‐D, where D is a donor group and A an acceptor moiety. We analyze the evolution in geometric structure that results from changing both donor substituents simultaneously or from varying only one donor group to produce an asymmetrical system. The changes in geometric and electronic structures are compared and found to be consistent. The trends in linear and nonlinear optical properties, and in particular in second‐order polarizabilities, are investigated in several ways. The two‐state model appears to be inadequate in describing the second‐order polarizability, β. Consequently, we are unable to deduce simple structure/property relationships that might help in the design of quadrupolar compounds for nonlinear optics. Finally, a series of unsymmetrical squaraines with OH substitution and enol‐ketone isomerism are investigated; the calculated nonlinear properties follow a similar trend to the experimental results: the OH substitution and isomerization contribute to increasing the ground‐stat

ISSN:0947-6539    

DOI:10.1002/chem.19970030408

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe titanium tetrachloride promoted reaction of allylsilanes1with 1‐acetylcyclohexene is shown to afford the silylbicyclo[4.3.0]nonanes9([3 + 2] cycloaddition products) along with the 1‐acetyl‐2‐allylcyclohexane4(Hosomi‐Sakurai product). Here we report that systematic variation of the substituents at the silicon atom of1allows suppression of the classical Hosomi‐Sakurai reaction in favor of the [3 + 2] cycloaddition. Cycloaddition of the allylsilanes1d,1i, and1 kwith 1‐acetylcycloalkenes10, containing a 5‐, 6‐, 7‐, 8‐, or 12‐membered ring, gives rise to the corresponding silylbicyclo[n.3.0]alkanes11 – 13.The cycloaddition of allyltriisopropylsilane (1 k) and 1‐acetyl‐2‐methylcycloalkenes15provides silylbicyclo[n.3.0]alkanes16with two contiguous quaternary carbon centers. The stereochemistry of the silylbicyclo[n.3.0]alkanes11 a ‐ cand14is unambiguously determined by X‐r

ISSN:0947-6539    

DOI:10.1002/chem.19970030409

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe report molecular dynamics simulations on ionophores of different topologies and on their complexes with alkali and alkaline‐earth cations, with or without counterion, at the water/chloroform interface. As ionophores we consider two phosphoryl‐containing podands (the “chainlike” monopodandMPand the “octopuslike” tripodandTP) and the bicyclic cryptand222.We find that all the solutes behave as surfactants: they remain adsorbed at the interface, without migrating to bulk phases. Their precise location and solvation depend on the nature and conformation of the ionophore, of the cation and of the counterion. Schematically, two types of solutes can be distinguished, depending on their hydrophilic/hydrophobic character. The first type (cryptand222and its complexes, or the [MP⋅K+] complex), which have a hydrophobic exterior, stay on the chloroform side of the interface and are partially hydrated by “water fingers”. The second type (freeMPandTP, [MP⋅K+]Pic‐and [MP⋅Sr2+](Pic‐)2complexes), which are more hydrophilic, are partitioned to a greater extent between the two liquid phases. The status of the ion pairs at the interface depends on the interplay between cation … anion and anion … solvent interactions. When cation‐anion interactions are strong enough (as in [MP⋅Sr2+](Pic‐)2), the ion pairs remain intimate. Otherwise they dissociate, leading to solvent‐separated ion pairs adsorbed at the interface (in the [222⋅K+] Pic‐complex) or to the migration of the anion to the

ISSN:0947-6539    

DOI:10.1002/chem.19970030410

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe synthesis, complete characterization and molecular dynamics simulations of dendrimers4–8involving [60]fullerene as core tecton and first‐ to thirdgeneration benzyl‐ether‐based dendrons as branches is described. In dendrimer7a core branching multiplicity of 12, the highest known to date, is realized for the first time with aTh‐symmetrical C60core having an octahedral addition pattern. This unique structural type of core building block is only possible on the base of C60and has no precedent in organic chemistry. NMR investigations as well as molecular modelling studies show that, owing to high core branching multiplicity, dendrimer7and to a minor extent the mixed adduct8already represent globular and densely packed macromolecules, although only first‐generation dendrons are involved in their c

ISSN:0947-6539    

DOI:10.1002/chem.19970030411

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe reaction of CsCl with In(CH2Ph)3yields the triorganochloroindate Cs[(PhCH2)3InCl] (1). Treatment of In(CH2Ph)3with dry O2gives the alkoxide [(PhCH2)2InOCH2Ph]2(2), whereas the reaction of1with O2leads to Cs2[O{PhCH2In(OCH2Ph)2}4] (3). According to X‐ray structural investigations,1contains Cs2Cl2four‐membered rings as basic structural motif. The dimers are connected by Cs‐π‐electron interactions to form a layer structure. Compound3can be described as a metalla(crown ether) fragment coordinating two Cs+ions. Additional shielding of the Cs centers by weak Cs‐π‐electron contacts

ISSN:0947-6539    

DOI:10.1002/chem.19970030412

出版商:WILEY‐VCH Verlag    

年代:1997

数据来源: WILEY