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| - Effects of Host−Guest Recognition on Kinetics of Diels−AlderReaction of Quinocrown Ethers with Cyclopentadiene
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| - Diels−Alder reactions of 15−21-membered quinocrown ethers 1a−c and 18-membered quinobenzocrown ether 1d with cyclopentadiene were catalyzed by the addition of alkali, alkaline earthmetal and ammonium perchlorates, and scandium trifluoromethane-sulfonate. The alkali metaland ammonium ions brought about a fairly selective rate-acceleration for each crown ether due tothe size-fitted ion-in-the-hole complexation. However, such a hole-size-selectivity was not observedfor the reactions catalyzed by divalent alkaline-earth (Mg2+ to Ba2+) and trivalent Sc3+ ions. Thewrapping complexation played a significant role in rate-acceleration in such a way that the smallestMg2+ caused 160 times rate-enhancement for the most flexible 1c and the Sc3+ performed maximal3700 times rate-increment for the 18-membered quinobenzocrown 1d. These effects of cationrecognition were rationalized by the reduction of LUMO energy that is favored by the orbitalinteraction with the HOMO of cyclopentadiene. The magnitude of rate-enhancement was discussedin terms of the cation binding affinity and coordination geometry of quinocrown ethers as well asthe valence of cations.
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