Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 65
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Org Chem ; 2020 Sep 16.
Artigo em Inglês | MEDLINE | ID: mdl-32856915

RESUMO

A catalytic approach for the preparation of indolines by dearomatizing cyclization is presented. FeCl3 acts as a catalyst to afford tetracyclic 5a,6-dihydro-12H-indolo[2,1-b][1,3]benzoxazin-12-ones in good yields. The cyclization also proceeds with tosylamides forming C-N bonds in 53% yield.

2.
Chempluschem ; 85(4): 715-724, 2020 04.
Artigo em Inglês | MEDLINE | ID: mdl-32286742

RESUMO

Supramolecular chemistry is based on weak intermolecular forces which nevertheless are of importance for chemical processes. In this report the relevance of alkyl-alkyl interactions in supramolecular assemblies is discussed. We show how hierarchically formed helicates can be used to evaluate weak interactions of alkyl groups based on solvent-supported London dispersion. In addition, the role of nano-segregation of alkyl groups in the periphery of supramolecular assemblies is described, as well as how this can be used to improve the properties of liquid-crystalline materials by controlling the alkyl-chain-mediated aggregation process.

3.
Chemistry ; 2020 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-32222003

RESUMO

Hierarchical helicates based on ketone-substituted titanium(IV)triscatecholates show different monomer-dimer behavior depending on different solvents. The dimerization constants of a whole series of differently alkyl-substituted complexes is analyzed to show that the solvent has a very strong influence on the dimerization. Hereby, effects like solvophobicity/philicity, sterics, electronics of the substituents and weak side chain-side chain interactions seem to act in concert.

4.
Chemistry ; 26(17): 3829-3833, 2020 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-31899932

RESUMO

The thioester moiety is introduced as a lithium binding unit for the hierarchical formation of titanium(IV) catecholate-based lithium-bridged helicates. In solution, the coordination compounds show a monomer-dimer equilibrium which -in comparison to the oxo esters- is significantly shifted towards the monomers. In addition, the influence of the thioester side chain on the dimerization behavior is investigated and an expansible/compressible molecular switch is synthesized. In the latter case expansion and compression are performed reversibly in methanol, whereas in DMSO spontaneous expansion occurs.

5.
Inorg Chem ; 59(3): 1758-1762, 2020 Feb 03.
Artigo em Inglês | MEDLINE | ID: mdl-31967799

RESUMO

A cyclohexyl substituent strongly prefers the chair conformation with large substituents in equatorial positions, while other cycloalkyls are structurally more flexible. In hierarchically formed dimeric titanium(IV) tris(catecholates) equatorial versus axial connection of the cyclohexane to the ester results in either a more compact (axial) or more expanded (equatorial) structure. In DMSO solution the axial position results in a compact structure which minimizes solvophobic effects, leading to higher stability. However, computational investigations indicate that additionally intramolecular London dispersion interactions significantly contribute to the stability of the dimer. Thus, weak side-chain-side-chain interactions are responsible for the high stability of cyclohexyl ester derivatives with axial compared to equatorial ester connection.

6.
Chemistry ; 26(6): 1396-1405, 2020 Jan 27.
Artigo em Inglês | MEDLINE | ID: mdl-31737953

RESUMO

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1 H NMR dimerization constants for the monomer-dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π-π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n-alkyls, n-Ω-alkenyls or n-Ω-alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

7.
Dalton Trans ; 48(28): 10574-10580, 2019 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-31215927

RESUMO

Hierarchically assembled helicates consisting of lithium-bridged triscatecholate titanium(iv) complexes represent a powerful self-assembled supramolecular system with applications as e.g. molecular balances for the evaluation of weak interactions, stereoselectivity switches in asymmetric synthesis or molecular switches. Their applications or properties are based on the monomer-dimer equilibrium which can be easily observed in solution. The dimer is the only species in the crystal. After dissolution, the dimer slowly dissociates into the monomer until the equilibrium is reached. This can be observed by NMR spectroscopy to investigate the kinetics of the dissociation process. A strong steric effect can be observed by studying differently substituted ligands. Activation energies of the dissociation process can be correlated with the size of the ester substituents and the system may be described as a molecular buckle.

8.
Angew Chem Int Ed Engl ; 58(37): 12879-12882, 2019 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-31165534

RESUMO

Two biscatecholester ligands with oligoether spacers were used to prepare dinuclear titanium(IV) triscatecholate based helicates. In the case of Li4 [(1/2)3 Ti2 ], "classical" helicates with three internally bound Li+ ions and syn-oriented ligands in the complex units (fac/fac isomer) were obtained. In the case of the sodium salt Na4 [(2)3 Ti2 ], a different homochiral dinuclear triple-stranded helicate with two internally bound Na+ ions was formed. The complex units are anti-configured, and two of the ligand spacers are connecting internal with external positions of the helicate (mer/mer isomer). Removal of the sodium ions and addition of lithium ions leads to the switching from one topology to the other with an expanded helicate [(2)3 Ti2 ]4- as an intermediate. Switching back to the "non-classical" helicate cannot be observed because severe structural rearrangements would be required.

9.
Chemistry ; 25(17): 4265-4273, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30394588

RESUMO

Catechol ligands with aldehyde, ketone or ester groups attached in 3-position form, in the presence of titanium(IV) triscatecholate, titanium(IV) complexes. If lithium cations are the counterions, they can bind in a successive step to the salicylate units of the complex and form a dimeric triple-lithium-bridged dinuclear helicate. In solution, the dimer is in equilibrium with the monomer and the thermodynamics of the dimerization can be easily evaluated. Thus, the hierarchically assembled titanium(IV) helicates represent a lithium-dependent molecular switch. The investigation of different derivatives of the complex allows for an estimation of the influence of side chain functionalities on the energetics of the dimerization. Thus, the hierarchically assembled helicates can be used as a kind of molecular balance to determine weak interaction energies (solvophobic effects and even dispersive effects). In addition, tethering of two ligands leads to "classical" helicates. Removal or addition of lithium cations allows for a reversible switching between a compressed and expanded state, which in the case of chiral ligands can be even performed stereospecifically.

10.
Angew Chem Int Ed Engl ; 57(36): 11817-11820, 2018 Sep 03.
Artigo em Inglês | MEDLINE | ID: mdl-30027682

RESUMO

The control of structural transformations triggered by external signals is important for the development of novel functional devices. In the present study, it is demonstrated that helicates can be designed to structurally respond to the presence of different counterions and to adopt either a compressed or an expanded structure. Reversible switching is not only possible between those two states, furthermore, the twist of the aggregate also can be controlled. Thus, three out of four possible states of a helicate (expanded/left-handed, expanded/right-handed, compressed/left-handed) based on an enantiomerically pure ester bridged dicatecholate ligand are specifically addressed by introduction, exchange, or removal of countercations. This approach is used to reversibly switch between the different states or to successively address them.

11.
J Am Chem Soc ; 139(46): 16959-16966, 2017 11 22.
Artigo em Inglês | MEDLINE | ID: mdl-29068687

RESUMO

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

12.
Angew Chem Int Ed Engl ; 55(33): 9625-9, 2016 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-27346062

RESUMO

Coordinatively unsaturated double-stranded helicates [(H2 L)2 Eu2 (NO3 )2 (H2 O)4 ](NO3 )4 , [(H2 L)2 Tb2 (H2 O)6 ](NO3 )6 , and [(H2 L)2 Tb2 (H2 O)6 ]Cl6 (H2 L=butanedioicacid-1,4-bis[2-(2-pyridinylmethylene)hydrazide]) are easily obtained by self-assembly from the ligand and the corresponding lanthanide(III) salts. The complexes are characterized by X-ray crystallography showing the helical arrangement of the ligands. Co-ligands at the metal ions can be easily substituted by appropriate anions. A specific luminescence response of AMP in presence of ADP, ATP, and other anions is observed. Specificity is assigned to the perfect size match of AMP to bridge the two metal centers and to replace quenching co-ligands in the coordination sphere.

13.
Chemistry ; 22(20): 6956-63, 2016 05 10.
Artigo em Inglês | MEDLINE | ID: mdl-27061308

RESUMO

The ability of multiple CF3 -substituted arenes to act as acceptors for anions is investigated. The results of quantum-chemical calculations show that a high degree of trifluoromethyl substitution at the aromatic ring results in a positive quadrupole moment. However, depending on the polarizability of the anion and on the substitution at the arene, three different modes of interaction, namely Meisenheimer complex, side-on hydrogen bonding, or anion-π interaction, can occur. Experimentally, the side-on as well as a η(2) -type π-complex are observed in the crystal, whereas in solution only side-on binding is found.

14.
Chemistry ; 22(10): 3255-3258, 2016 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-26833393

RESUMO

Dimeric hierarchically-assembled titanium(IV) helicates are in solvent-dependent equilibrium with the corresponding monomers. Statistically formed mixtures of such complexes bearing chiral stereocontrolling ligands and achiral diene-substituted ligands show high diastereoselectivity and reasonable enantioselectivity in the Diels-Alder reaction with maleimides if the reaction proceeds with the dimer but not with the monomer. Thus, solvent dependent switching between the monomer and dimer enables on/off switching of the enantioselectivity.

15.
Chem Rev ; 115(16): 8867-95, 2015 Aug 26.
Artigo em Inglês | MEDLINE | ID: mdl-26278927
16.
Chem Sci ; 6(1): 354-359, 2015 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-28966761

RESUMO

The present study gives a comprehensive insight into anion-π interactions in the solid state, focusing on purely organic and charge-neutral fluorophenyl groups bearing a positive charge located at a side chain. The detailed statistical analysis of a series of structural data sets shows the geometrical variability of anion-π bonding in the solid state. It reveals the directing substituents at the arene as key elements for the positional preferences of anions above π-systems. The structural variety of the interaction between anions and electron-deficient arenes is considered by use of the hapticity concept. Together with new evaluation criteria, two helpful tools to understand and describe anion-π interactions in the solid are used.

17.
J Phys Chem B ; 119(1): 301-6, 2015 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-25516141

RESUMO

Anion receptors based on an 8-thiourea substituted quinoline with pentafluorinated (1a) or nonfluorinated (1b) biarylamide groups in the 2-position show similar binding of halide anions with somewhat higher association constants for the more acidic fluorinated derivative. Surprisingly, binding affinities for the halides in the case of the nonfluorinated 1b are similar in nonpolar chloroform or polar DMSO as solvent. Thorough thermodynamic investigations based on NMR van't Hoff analysis show that anion binding in chloroform is mainly enthalpically driven. In DMSO, entropy is the driving force for the binding of the ions with replacement of attached solvent.

18.
Nat Chem ; 6(9): 761-2, 2014 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-25143208
19.
Chemistry ; 20(22): 6650-8, 2014 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-24782405

RESUMO

Compounds 1 a-f-H2 form "monomeric" triscatecholate titanium(IV) complexes [Ti(1 a-f)3](2-), which in the presence of Li cations are in equilibrium with the triple lithium-bridged "dimers" [Li3(Ti(1 a-f)3)2](-). The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is "locked". The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti(1 c-e)3](2-) or [Li3(Ti(1 c-e)3)2](-). A surprising exception was found for the dimer [Li3(Ti(1 f)3)2](-). Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li3(Ti(1 f)3)2](-) the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal-complex units of [Li3(Ti(1 c-e)3)2](-) or [Li3(Ti(1 f)3)2](-).

20.
Chemistry ; 20(20): 6047-53, 2014 May 12.
Artigo em Inglês | MEDLINE | ID: mdl-24692292

RESUMO

Phenanthroline-based chiral ligands L(1) and L(2) as well as the corresponding Eu(III) and Tb(III) complexes were synthesized and characterized. The coordination compounds show red and green emission, which was explored for the sensing of a series of anions such as F(-), Cl(-), Br(-), I(-), NO3(-), NO2(-), HPO4(2-), HSO4(-), CH3COO(-), and HCO3(-). Among the anions, HPO4(2-) exhibited a strong response in the emission property of both europium(III) and terbium(III) complexes. The complexes showed interactions with the nucleoside phosphates adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP). Owing to this recognition, these complexes have been applied as staining agents in the microalgal cell Chlorella vulgaris. The stained microalgal cells were monitored through fluorescence microscopy and scanning electron microscopy. Initially, the complexes bind to the outer cell wall and then enter the cell wall through holes in which they probably bind to phospholipids. This leads to a quenching of the luminescence properties.


Assuntos
Chlorella vulgaris/química , Complexos de Coordenação/química , Európio/química , Substâncias Luminescentes/química , Fosfatos/análise , Térbio/química , Difosfato de Adenosina/análise , Monofosfato de Adenosina/análise , Trifosfato de Adenosina/análise , Chlorella vulgaris/citologia , Complexos de Coordenação/síntese química , Ligantes , Microscopia de Fluorescência
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA