For organic chemists, the construction of CC bonds is the most essential aspect of the assembly of molecules. Transition‐metal‐catalyzed coupling reactions have evolved as one of the key tools for ...this task. Lately, gold has also emerged as a catalyst for this kind of transformation. Gold, with its special properties as a mild carbophilic π Lewis acid, its ability to insert into CH bonds, and, as discovered recently, its ability to undergo redox transformations, offers the opportunity to apply all this potent proficiency for the construction of compounds in an efficient and economical way. This Minireview critically presents the CC coupling reactions enabled by gold catalysts to encourage further research activities in this promising area of oxidation/reduction gold catalysts.
More than just a pretty face: Gold excels as a catalyst for organic synthesis owing to its unique reactivity pattern: its π Lewis acidity as well as its ability to insert into CH bonds and to undergo oxidation/reduction processes. This last property in particular offers an efficient means to construct CC bonds. The current state of the art of gold‐catalyzed CC coupling is discussed along with mechanistic aspects of these reactions.
A series of substituted azothiophenes was prepared and investigated toward their isomerization behavior. Compared to azobenzene (AB), the presented compounds showed red‐shifted absorption and almost ...quantitative photoisomerization to their (Z) states. Furthermore, it was found that electron‐withdrawing substitution on the phenyl moiety increases, while electron‐donating substitution decreases the thermal half‐lives of the (Z)‐isomers due to higher or lower stabilization by a lone pair–π interaction. Additionally, computational analysis of the isomerization revealed that a pure singlet state transition state is unlikely in azothiophenes. A pathway via intersystem crossing to a triplet energy surface of lower energy than the singlet surface provided a better fit with experimental data of the (Z)→(E) isomerization. The insights gained in this study provide the necessary guidelines to design effective thiophenylazo‐photoswitches for applications in photopharmacology, material sciences, or solar energy harvesting applications.
Switch it up! A series of substituted thiophenylazobenzenes was synthesized. The thermal (Z)→(E) isomerization rates can be tuned by attractive lone pair⋅⋅⋅π interactions, which stabilize the (Z) isomer: Lowering the electron density at the phenyl or increasing the electron density at the thiophenyl moieties increases the half‐lives up to two orders of magnitude. Computations support involvement of a triplet TS in the thermal (E)→(Z) isomerization process.
Herein we present a systematic study of the influence of different alkyl chains in malonyl ester fullerene adducts with 10cycloparaphenylene (10CPP) and a tert‐butyl (tBu) ester‐substituted 10CPP ...analogue. The association constants between the nanoring hosts and the fullerene guests were determined by fluorescence quenching experiments. The trends in association were rationalized by an interplay of repulsion arising from an extended volume and London dispersion as an attractive counterpart.
The supramolecular assembly between substituted 10cycloparaphenylene (s10CPP) and fullerenes was studied to investigate the subtle interplay of steric repulsion, conformation and attractive London dispersion interactions. For this, 10CPP and a tert‐butyl ester‐substituted analogue were employed as host molecules whose fluorescence is quenched upon addition of C60 as well as alkyl malonyl ester‐substituted fullerenes.
Tighten your belt: The first aromatic nanobelt has been prepared by Itami and co‐workers. This synthesis of a truncated Vögtle belt constitutes a milestone in long‐standing efforts of numerous ...chemists. The realization of such a molecule provides not only new opportunities in materials science, but also paves the way to the rational synthesis of carbon nanotubes.
London Dispersion in Alkane Solvents Strauss, Marcel A.; Wegner, Hermann A.
Angewandte Chemie (International ed.),
January 11, 2021, Letnik:
60, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The importance of London dispersion interactions in solution is an ongoing debate. Although the significance of dispersion for structure and stability is widely accepted, the degree of its ...attenuation in solution is still not properly understood. Quantitative evaluations are derived mostly from computations. Experimental data provide guidelines to include London dispersion in solution phase design. Herein, dispersive interactions were examined with an azobenzene probe. Alkyl substituents in meta positions of the azobenzene core were systematically varied and the effect on the half‐lives for the thermally induced Z to E isomerization in several alkane solvents was determined. The results show that intramolecular dispersion is only marginally influenced. In solvents with low surface tension, reduced destabilizing solvent‐solvent interactions increase the half‐life up to 20 %. Specific individual interactions between alkyl chains on the azobenzene and those of the solvent lead to additional fluctuations of the half‐lives. These presumably result from structural changes of the conformer ensemble.
The degree of attenuation of London dispersion in solution is evaluated. Dispersive interactions were examined with azobenzene probes. The effect on half‐lives for the thermally induced Z to E isomerization in several alkane solvents was determined. The results show only a marginally influence on intramolecular dispersion. In solvents with low surface tension, reduced destabilizing solvent‐solvent interactions increase the half‐life up to 20 %.
The development of metal‐free catalysts as an alternative to the use of transition metals has gained tremendous interest in the past. In catalysis, Lewis acidity is one of the major principles used ...for the activation of organic compounds. Improving the reactivity and selectivity of Lewis acids by utilizing bidentate interactions was already proposed 50 years ago. Nevertheless, product inhibition due to strong binding has made applications of bidentate Lewis acids challenging for many years. Recently, bis‐boron compounds have been found to be very effective and several applications in Diels–Alder reactions, carbon dioxide reduction, and ammonia‐borane dehydrogenation were reported. All three transformations are enabled by the catalyst at different stages during the course of the reaction. These new and useful examples illustrate the great potential of the concept.
Exploring all options: Metal‐free catalysts as an alternative to the use of transition metals has gained tremendous interest in the past. Recently, bis‐boron compounds were found to be very effective and several applications in Diels–Alder reactions, carbon dioxide reduction, and ammonia‐borane dehydrogenation were reported. These new and useful examples illustrate the great potential of the concept.
Herein, we report a modular synthesis providing access to substituted cycloparaphenylenes (CPPs) of different sizes. A key synthon introducing two geminal ester units was efficiently prepared by ...2+2+2 cycloaddition. This building block can be conveniently converted to macrocyclic precursors controlling the ring size of the final CPP. Efficient reductive aromatization through single‐electron transfer provided the substituted nanohoops in a straightforward manner. The tBu ester substitution pattern enables a tube‐like arrangement in the solid‐state governed by van der Waals interactions that exhibits one of the tightest packings of CPPs in tube direction, thus opening new avenues in the crystal design of CPPs.
A new synthesis provides access to different sized substituted cycloparaphenylenes (CPPs). The use of a key synthon in a sequence of macrocyclization by cross‐coupling with different reaction partners followed by aromatization allows a modular build‐up. The controlled introduction of substituents enables a tube‐like arrangement of these molecular carbon allotropes in the solid state.
Large substituents are commonly seen as entirely repulsive through steric hindrance. Such groups have additional attractive effects arising from weak London dispersion forces between the neutral ...atoms. Steric interactions are recognized to have a strong influence on isomerization processes, such as in azobenzene‐based molecular switches. Textbooks indicate that steric hindrance destabilizes the Z isomers. Herein, we demonstrate that increasing the bulkiness of electronically equal substituents in the meta‐position decreases the thermal reaction rates from the Z to the E isomers. DFT computations revealed that attractive dispersion forces essentially lower the energy of the Z isomers.
London’s calling: It is generally accepted that large alkyl substituents destabilize the Z isomer of double bonds. In the case of meta‐substituted azobenzenes, it could be shown that the stability of the Z isomer increases as the size of the substituent grows. Computations show that the enhancement in stability is based on intramolecular London dispersion forces, which compete with steric hindrance in the Z isomers.
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