Effects of host-guest recognition on kinetics of Diels-alder reaction of quinocrown ethers with cyclopentadiene

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 earth metal and ammonium perchlorates, and scandium trifluoromethane-sulfonate. The alkali metal and ammonium ions brought about a fairly selective rate-acceleration for each crown ether due to the size-fitted ion-in-the-hole complexation. However, such a hole-size-selectivity was not observed for the reactions catalyzed by divalent alkaline-earth (Mg2+ to Ba2+) and trivalent Sc3+ ions. The wrapping complexation played a significant role in rate-acceleration in such a way that the smallest Mg2+ caused 160 times rate-enhancement for the most flexible 1c and the Sc3+ performed maximal 3700 times rate-increment for the 18-membered quinobenzocrown 1d. These effects of cation recognition were rationalized by the reduction of LUMO energy that is favored by the orbital interaction with the HOMO of cyclopentadiene. The magnitude of rate-enhancement was discussed in terms of the cation binding affinity and coordination geometry of quinocrown ethers as well as the valence of cations.