ABSTRACT
As a rule, helical structures at the molecular level are formed by non-planar units. This makes the design of helices, starting from planar building blocks via self-assembly, even more fascinating. Until now, however, this has only been achieved in rare cases, where hydrogen and halogen bonds were involved. Here, we show that the carbonyl-tellurium interaction motif is suitable to assemble even small planar units into helical structures in solid phase. We found two different types of helices: both single and double helices, depending on the substitution pattern. In the double helix, the strands are connected by additional Teâ â â Te chalcogen bonds. In the case of the single helix, a spontaneous enantiomeric resolution occurs in the crystal. This underlines the potential of the carbonyl-tellurium chalcogen bond to generate complex three-dimensional patterns.
ABSTRACT
For a molecular system, size and shape are of elementary importance for its function and properties. Therefore, the determination of distances within a molecule is essential. However, the commonly used methods are only suitable for distances smaller than 4â Å or larger than 15â Å. Here, we show that by incorporating a molecular spring, we can measure distances in macrocycles in the range of 10â Å using 13 C NMR spectroscopy. The accuracy of the method also allows to determine the temperature dependence of the distances. In one case, we find a contraction of the length by almost 10 % upon heating. This shrinking due to heating can be considered as inverse thermoelasticity at the molecular level and is a previously completely overlooked phenomenon that can be used in the future as a tool to change the length and, thus, the function of a system.
Subject(s)
Temperature , Magnetic Resonance Spectroscopy/methodsABSTRACT
Azobenzenes are without a doubt the most widely used light-induced switching units, and there is a plethora of application examples ranging from supramolecular chemistry to material science and biological chemistry. Here, we present a smart azobenzene, in which the photoswitching capability of the azobenzene moiety can be reversibly switched on and off using a second unit (redox switch). This second switching unit is based on the variation of the strength of a chalcogen bond between the azo group and a Te-Ph unit in ortho position to the azo group. This allows the selective switching of only one azobenzene unit in the presence of other azobenzene switches. The entire double-switch is a very simple, small system that can also be easily synthesized. As a result, this double-switch can be used as a smarter replacement for the established azobenzene system in the future. For example, in contrast to the latter this double-switch could be employed to store state information analogous to a flip-flop in digital electronic systems.
ABSTRACT
In the last years, chalcogen bonding, the noncovalent interaction involving chalcogen centers, has emerged as interesting alternative to the ubiquitous hydrogen bonding in many research areas. Here, we could show by means of high-level quantum chemical calculations that the carbonylâ â â tellurazole chalcogen bond is at least as strong as conventional hydrogen bonds. Using the carbonylâ â â tellurazole binding motif, we were able to design complex supramolecular networks in solid phase starting from tellurazole-substituted cyclic peptides. X-ray analyses reveal that the rigid structure of the cyclic peptides is caused by hydrogen bonds, whereas the supramolecular network is held together by chalcogen bonding. The type of the supramolecular network depends on peptide used; both linear wires and a honeycomb-like supramolecular organic framework (SOF) were observed. The unique structure of the SOF shows two channels filled with different types of solvent mixtures that are either locked or freely movable.
ABSTRACT
The combination of photo-switchable units with macrocycles is a very interesting field in supramolecular chemistry. Here, we present the synthesis of a foldable container consisting of two different types of Lissoclinum macrocyclic peptides which are connected via two azobenzene units. The container is controllable by light: irradiation with UV light causes a switching process to the compact cis,cis-isomer, whereas by the use of visible light the stretched trans,trans-isomer is formed. By means of quantum chemical calculations and CD spectroscopy we could show that the transâcis isomerization is spatially directed; that means that one of the two different macrocycles performs a definite clockwise rotation to the other, caused by irradiation with UV light. For the cisâtrans isomerization counterclockwise rotations are found. Furthermore, quantum chemical calculations reveal that the energy of the cis,cis-isomer is only slightly higher than the energy of the cis,trans-isomer. This effect can be explained by the high dispersion energy in the compact cis,cis-isomer.
ABSTRACT
An efficient approach for the synthesis of pyrazolopyridines containing the aminochromane motif through a base-catalyzed cyclization reaction is reported. The synthesis was carried out through a three-component reaction of (arylhydrazono)methyl-4H-chromen-4-one, malononitrile, primary amines in the presence of Et3N at room temperature. However, carrying out the reaction under the same conditions without base led to a fused chromanyl-cyanopyridine. High selectivity, high atom economy, and good to high yields in addition to mild reaction conditions are the advantages of this approach.
ABSTRACT
An efficient approach for the synthesis of functionalized ß-lactams and pyrrolidine-2,5-diones was achieved through a sequential Ugi-4CR/cyclization reaction. Diversity-oriented synthesis, good to high yields, easy workup, and short reaction times are advantages of this procedure.
Subject(s)
Metals/chemistry , Pyrrolidinones/chemical synthesis , beta-Lactams/chemical synthesis , Combinatorial Chemistry Techniques/methods , Cyclization , StereoisomerismABSTRACT
An efficient sequential four-component reaction of chromone carbaldehydes, Meldrum's acid, 4-hydroxyl coumarin or 6-methyl-4-hydroxyl-pyrone and primary alcohols is reported which leads to 5a-i in aqueous media. Replacing the primary alcohol with isopropyl alcohol and tert-butyl alcohol results in different products 10 and 11. The environmentally friendly features, good to high yields and easy work-up are advantages of this approach.
Subject(s)
Chromones/chemistry , Chromones/chemical synthesis , Water/chemistry , Butanols/chemistry , Molecular ConformationABSTRACT
Novel analogs of 2-pyridone-3-carboxylic acids 4a-l have been prepared by the three-component reaction of 3-formyl chromone, Meldrum's acid, and primary amines in the presence of a catalytic amount of diammonium hydrogen phosphate in water. Good-to-high yields, easy work-up, and an environmentally friendly profile are the advantages of this method for the synthesis of 2-pyridone-3-carboxylic acid derivatives.