Dynamics of crown and lariat ether cation complexation assessed by carbon-13 NMR relaxation times

Self-activated supramolecular reactions: effects of host-guest recognition on the kinetics of the Diels-Alder reaction of open-chain oligoether quinones with cyclopentadiene

Diels-Alder reactions of acyclic oligoether-substituted quinones 1b, 1c, 2b, and 2c with cyclopentadiene were accelerated by the addition of alkali and alkaline earth metal perchlorates, and scandium trifluoromethane sulfonate (k(c)/k(f) = 1.2-23 for univalent cations, 11-1160 for divalent cations, and 1700-192 000 for Sc(3+), where k(c) and k(f) are the rate constants for the metal complexed and uncomplexed quinones, respectively). The shorter-armed 1a, 2a, and 3, however, exhibited no such acceleration effects. The rate accelerations can be rationalized by the FMO consequence in which the bound guest cation withdraws electron density from the quinone dienophile and lowers the LUMO energy suitable for the orbital interaction with the HOMO of cyclopentadiene. Despite the poor cation selectivity, these acyclic oligoether quinones showed larger rate accelerations than the relevant quinocrown ethers 4 (k(c)/k(f) = 1.3-3.0 for univalent cations, 5.0-160 for divalent cations, and 100-2020 for Sc(3+)). The effective electron withdrawal, which leads to the enhanced rate acceleration, can be caused by the direct interaction between the metal cation accommodated in the pseudo-cyclic oligoether linkage and the quinone carbonyl oxygen, as indicated by (1)H NMR spectroscopy. In addition, the larger rate enhancement is rather achieved in the complex with low binding constant K, because the strong encapsulation of metal cation by the oligoether chain diminishes the crucial interaction to the quinone carbonyl oxygen. As a whole, the smaller and higher valent cations tend to bring about notable rate acceleration due to the more enhanced ion-dipole interaction with the quinone carbonyl oxygen. Spectroscopic titration (absorption and (1)H NMR) and kinetic experiments indicated that only the longest di-armed 2c constructs 1:1, and then 1:2, host/guest complexes with Ca(2+), Sr(2+), and Ba(2+). These 1:2 complexes exhibited the most effective acceleration for the respective metal cations.

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.