The development of enantio-divergent catalysts capable of producing in a controlled fashion each of the two enantiomeric products from one substrate, is highly challenging. It requires the implementation of a switching function into the catalyst, which can reversibly change the chiral reaction environment during catalysis. Here we report a photo-switchable phosphate ligand derived from 2,2'-biphenol, which axially coordinates as the counter ion to an achiral manganese(III) salen catalyst, providing the latter with the ability to dynamically switch stereoselectivity in the epoxidation of alkenes. Both enantiomers of the chiral ligand exist as a pair of pseudo-enantiomers, which can be e ciently interconverted by irradiation with light of different wavelengths. The opposite axial chiralities of these pseudo-enantiomers are e ciently transferred to the manganese(III) salen catalyst. With this switchable ion-paired catalyst, the enantioselectivity of the epoxidation of a variety of alkenes can be controlled, resulting in opposite enantiomeric excesses of the epoxide products. This dynamic transfer of chirality from a photoswitchable counter ion to a metal complex will greatly broaden the scope of supramolecular catalysts. In future studies this photo-switchable catalyst will be used to write digital information in the form of chiral epoxides on single polymer chains.
Porphyrin cage-compounds are used as biomimetic models and substrate-selective catalysts in supramolecular chemistry. In this work we present the resolution of planar-chiral porphyrin cages and the determination of their absolute configuration by vibrational circular dichroism in combination with density functional theory calculations. The chiral porphyrin-cages form complexes with achiral and chiral viologen-guests and upon binding one of the axial enantiomorphs of the guest is bound selectively, as is indicated by induced-electronic-dichroism-spectra in combination with calculations. This host-guest binding also leads to unusual enhanced vibrational circular dichroism, which is the result of a combination of phenomena, such as rigidification of the host and guest structures, charge transfer, and coupling of specific vibration modes of the host and guest. The results offer insights in how the porphyrin cage-compounds may be used to construct a future molecular Turing machine that can write chiral information onto polymer chains.
Molecular motors and switches change conformation under the influence of an external stimulus, e.g. light. They can be incorporated into functional systems, allowing the construction of adaptive materials and switchable catalysts. Here, we present two molecular motor-functionalized porphyrin macrocycles for future photo-switchable catalysis. They display helical, planar and point chirality, and are diastereomers, which differ in the relative orientation of the motor and macrocyclic components. Fluorescence, UV-vis, and 1H NMR experiments reveal that the motor-functionalized macrocycles can bind and thread different variants of viologen guests, including a one-side blocked polymeric one of 30 repeat units. The latter feature indicates that the motor systems can find the open end of a polymer chain, thread on it, and move along the chain to eventually bind at the viologen trap, opening possibilities for catalytic writing on single polymer chains via chemical routes.
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