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Conformationally restricted dynamic supramolecular catalysts for substrate-selective epoxidations.

A second generation of a substrate-selective dynamic supramolecular catalytic system consisting of a catalyst part and a receptor part, connected by a hydrogen-bonding motif, has been realized based on rational design. The results from analyses of the equilibrium mixture of the species generated by the components of the first generation system led us to selectively lock the cisoid conformation of the catalyst part to increase the amount of the substrate-selective catalytic cavity in the equilibrium mixture. This was realized by strapping the catalyst part by organic synthesis. This strapping led to an increase in substrate selectivity in the pair-wise competitive epoxidations of pyridyl- vs. phenyl-appended styrenes and pyridyl- vs. phenyl-appended stilbenes of both Z- and E- configuration compared to the first generation system, reaching 3.4 : 1 as the highest substrate selectivity for Z-mono-pyridyl-stilbene (27a) vs. the corresponding all-carbon analogue (28a) and for E-dipyridyl-stilbene (26b) vs. the corresponding all-carbon analogue (28b), respectively.