Dibenzo-30-crown-10: Synthetic optimization and studies of the binding conformation
Wessels, Hanlie R
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Dibenzo-30-crown-10 (DB30C10) is one of the first-generation macrocyclic hosts discovered by Pedersen. Crown ethers originally attracted attention due to their ability to encapsulate metal cations and render them soluble in organic solvents. These studies helped to launch host-guest chemistry as a discipline within supramolecular chemistry. Crown ethers form complex molecules containing organic cations and neutral organic molecules. Additionally, they form components in supramolecular architectures such as catenanes, rotaxanes, and supramolecular polymers. They have been used as selective hosts in diverse applications such as wastewater treatment, switchable catalysis, therapeutic agents, sensors, molecular machines, and stimuli responsive materials "smart polymers". Despite the vigorous research activity in the field, DB30C10 has received surprisingly little attention. DB30C10 was reported in 1967 and has been commercially available since 1992; however, it has been mostly overlooked as a host in favour of smaller crown ethers such as DB24C8, B15C5, 18C6 and 15C5. Herein we present an improved synthetic route that improves the yield of the cyclization step in the synthesis of DB30C10 from 25% to 88% enabling us to prepare multiple grams of the material without the use of pseudo high-dilution techniques. The same methodology was applied to three other crown ethers with similar improvement in yield. Four new rotaxanes based on the DB30C10-paraquat binding motif were used to investigate the binding conformation of DB30C10 and paraquat. The new rotaxanes were characterized by 1H, 13C and 2D-NOESY NMR, mp, and HRMS. A single crystal X-ray structure of one of the rotaxanes was obtained. To our knowledge, this is the first crystal structure of a rotaxane based on this particular binding motif. This result illustrated that DB30C10 was a suitable host for the construction of supramolecular systems and polymers. Our eventual goal is to use DB30C10 in the construction of supramolecular polymers with novel topologies. Therefore, the relative threading efficiency of DB30C10 in solution had to be determined. A series of segmented polyurethane poly(pseudorotaxanes) with paraquats in the backbone were synthesized with different crown ether or cyptand hosts. The threading efficiency was determined by 1H NMR.
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