Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80−99% ee). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechanistic experiments. It was corroborated by nuclear magnetic resonance titration that the catalyst binds the two enantiomers by two-point hydrogen bonding. In one of the diastereomeric complexes, the hydrogen atom at the stereogenic carbon atom is ideally positioned for hydrogen atom transfer (HAT) to the photoexcited benzophenone. Detection of the protonated ketyl radical by transient absorption revealed hydrogen abstraction to occur from only one but not from the other hydantoin enantiomer. Quantum chemical calculations allowed us to visualize the HAT within this complex and, more importantly, showed that the back HAT does not occur to the carbon atom of the hydantoin radical but to its oxygen atom. The achiral enol formed in this process could be directly monitored by its characteristic transient absorption signal at λ ≅ 330 nm. Subsequent tautomerization leads to both hydantoin enantiomers, but only one of them returns to the catalytic cycle, thus leading to an enrichment of the other enantiomer. The data are fully consistent with deuterium labeling experiments and deliver a detailed picture of a synthetically useful photochemical deracemization reaction.
The Guerbet reaction from two alcohols to a long-chain alcohol and water requires a redox catalyst and a strong base in homogeneous liquid systems. Especially, the reaction from ethanol to n-butanol is a challenging example of the reaction that suffers from low yields and selectivities in comparison with reactions of higher alcohols. The most important side reactions are the polymerization of acetaldehyde to C 6 + -components and the saponification of ethyl acetate under consumption of the base. This work pursues the systematic kinetic investigation of the Guerbet reaction network by experiments with isolated subsystems of the network. In-situ-infrared spectroscopy is applied to determine time-resolved concentration profiles. Adapted kinetic models of the single steps are integrated into a microkinetic model of the whole network. The simulation of the reaction network reveals dependencies between temperature, hydrogen pressure, initial concentrations and the yield and selectivity of n-butanol. Finally, it is shown that Ru-MACHO does not lead to high yields in the reaction, because the dehydrogenation to ethyl acetate exhibits a too low activation barrier.
Invited for this month's cover is the group of Marcel A. Liauw at the RWTH Aachen University (Germany). The cover picture shows a playable version of the homogeneously catalyzed Guerbet‐coupling from ethanol to n‐butanol. The 2D ball‐in‐a‐maze‐game is shown here because the reaction steps are either performed in presence of a redox catalyst (top‐down) or in presence of a base (left‐right). Both axes can be moved independently from each other and substances are connected with humps in the height of the activation barrier. Playing the game repeatedly for a defined time and writing down the outcomes (see blackboard) yields the composition of the reaction mixture at a given time. Alas, the highly active redox catalyst Ru‐MACHO shows unsatisfying yields and selectivities of n‐butanol. But why? Read the full text of their Full Paper at 10.1002/cmtd.202000056.
The concept of distinct bonds within molecules has proven to be successful in rationalizing chemical reactivity. However, bonds are not a well-defined physical concept, but rather vague entities, described by different and often contradicting models. With probability density analysis, which can---in principle---be applied to any wave function, bonds are recovered as spin-coupled positions within most likely electron arrangements in coordinate space. While the wave functions of many systems are dominated by a single electron arrangement which is built from two-center two-electron bonds, some systems require several different arrangements to be well described. In this work, a range of these multi-center bonded molecules are classified and investigated with probability density analysis. The results are compared with valence bond theory calculations and data from collision-induced dissociation experiments.
The Front Cover shows a playable version of the homogeneously catalyzed Guerbet‐coupling from ethanol to n‐butanol. The 2D ball‐in‐a‐maze‐game is shown here because the reaction steps are either performed in presence of a redox catalyst (top‐down) or in presence of a base (left‐right). Both axes can be moved independently from each other and substances are connected with humps in the height of the activation barrier. Playing the game repeatedly for a defined time and writing down the outcomes (see blackboard) yields the composition of the reaction mixture at a given time. Alas, the highly active redox catalyst Ru‐MACHO shows unsatisfying yields and selectivities of n‐butanol. But why? More information can be found in the Full Paper by Andreas Ohligschläger et al.
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