We report an original catalytic molecular tetrahedron. By threading through the cavity of the tetrahedron, polymeric substrates are unfolded or broken apart. Our catalyst distinguishes between polymer chains of different lengths, functionalizing the shorter polymers selectively over the longer ones-as a proof of concept for selective catalysis to modify polymers. Our findings advance the fundamental understanding of the thermodynamic and kinetic phenomena controlling the interactions between molecular cages and synthetic polymers, offering valuable ability to create complex materials in the future.
Multi-gigawatt-scale hydrogen production by water electrolysis is central in the green transition when it comes to storage of energy and forming the basis for sustainable fuels and materials. Alkaline water electrolysis plays a key role in this context, as the scale of implementation is not limited by the availability of scarce and expensive raw materials. Even though it is a mature technology, the new technological context of the renewable energy system demands more from the systems in terms of higher energy efficiency, enhanced rate capability, as well as dynamic, part-load, and differential pressure operation capability. New electrode separators that can support high currents at small ohmic losses, while effectively suppressing gas crossover, are essential to achieving this. This Focus Review compares the three main development paths that are currently being pursued in the field with the aim to identify the advantages and drawbacks of the different approaches in order to illuminate rational ways forward.
Enantioselective electrophilic aromatic nitration methodology is needed to advance chirality-assisted synthesis (CAS). Reported here is an enantioselective aromatic nitration strategy operating with chiral diester auxiliaries,and it provides an enantioselective synthesis of aC 3v -symmetric tribenzotriquinacene (TBTQ). These axially-chiral structures are much sought-after building blocks for CAS, but they were not accessible prior to this work in enantioenriched form without resolution of enantiomers.T his nitration strategy controls the stereochemistry of threefold nitration reactions from above the aromatic rings with chiral diester arms.Dicarbonyl-to-arenium chelation rigidifies the reaction systems,s ot hat remote stereocenters position the ester-directing groups selectively over specific atoms of the TBTQ framework. Closely guided by computational design, am ore selective through-space directing arm was first predicted with density functional theory (DFT), and then confirmed in the laboratory,t o outperform the initial structural design. This enantio-and regioselective TBTQ synthesis opens anew pathwaytoaccess building blocks for CAS.Classical electrophilic aromatic nitration reactions are among the best known and most frequently employed aromatic transformations. [1] They readily proceed [2] with unactivated aromatic substrates and provide exquisite control over the number of NO 2 groups introduced, unlike for instance most Friedel-Crafts alkylations and aromatic brominations. [1b-d, 3] Nevertheless,r esolution-free,e nantioselective electrophilic aromatic nitration processes (even with chiral auxiliaries) have yet to be reported. While enantioselective Friedel-Crafts-type reactions [4] and enantioselective aromatic brominations [5] have been studied extensively,o nly the enantioselective nitration of crotylsilanes, [6] but not of aromatic substrates,h as been described.To advance the growing field of chirality-assisted synthesis [7] (CAS), we now describe an enantioselective nitration strategy which makes use of chiral diester auxiliaries with remote stereocenters to induce axial chirality upon trifold nitration. We employ our chiral auxiliaries to achieve the first enantioselective synthesis of a C 3v -symmetric tribenzotriqui-nacene (TBTQ) [8] derivative,w hich is not based on either ak inetic, [9] crystallization-based, [10] or chromatographic [11] resolution. Directing arms with remote stereocenters are positioned at equal distances from the favored and disfavored sites of reactivity to provide for an enantioselective nitration methodology after removal (Figure 1b)ofthe through-space directing groups.T he success of our strategy stems from 1) chelation (Figure 1b)o ft wo ester carbonyl groups to the cationic Wheland intermediates positioned underneath these diester directing arms as well as 2) A 1, 3 strain (Figure 3b), which can be controlled by the size of the alkyl substituents attached to the remote chirality centers of the through-space directing groups.T his combination of A 1,3 strain with ...
Crystal-packing forces can have a significant impact on the relative stabilities of different molecules and their conformations. The magnitude of such effects is, however, not yet well understood. Herein we show, that crystal packing can completely overrule the relative stabilities of different stereoisomers in solution. Heating of atropoisomers (i.e. "frozen-out" conformational isomers) in solution leads to complex mixtures. In contrast, solid-state heating selectively amplifies minor (<25 mole %) components of these solution-phase mixtures. We show that this heating strategy is successful for compounds with up to four rotationally hindered σ bonds, for which a single stereoisomer out of seven can be amplified selectively. Our results demonstrate that common supramolecular interactions-for example, [methyl⋅⋅⋅π] coordination and [C-H⋅⋅⋅O] hydrogen bonding-can readily invert the relative thermodynamic stabilities of different molecular conformations. These findings open up potential new avenues to control the folding of macromolecules.
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