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6395This is a very large rate constant. Its size shows that the activating effect which a negatively charged oxygen substituent exerts upon electrophilic attack of H30+ on carbon-carbon double bonds is very powerful indeed. This activation is many orders of magnitude greater than that of a nonionized hydroxyl group, as shown by the ratio kH+//kH+ = (3.3 * 0.9) X 10'.Although this ratio IS very large, it is nevertheless an order of magnitude smaller than those we have observed for other enolate ion-enol p a i r~.~*~O This suggests that in this case k d is reduced because the enolate reaction here might be a diffusion-controlled process, an idea reinforced by the very large value of this rate constant. This rate constant is, in fact, so great as to imply that proton transfer occurs via hydrogen-bonded solvent bridges between the substrate and H30+. Such a Grotthuss chain mechanism is the accepted mode of proton transfer between oxygen and nitrogen acids and bases in aqueous solution?' but it has heretofore been generally believed that such solvent bridges do not play a role in proton transfers involving carbon.32It is possible, on the other hand, that the uncatalyzed term observed here is due to reaction by a non-diffusive process, for (30) Kresge, A. J.; Pruszynski, P., unpublished work.

5-Indolylboronic acid (1) reversibly forms boronic esters with reducing sugars in water. The resulting boronates are stronger Lewis acids than the starting boronic acid in the deprotonation of water; pKa = 10 for 1 and < 7 for its sugar-boronates. Thus, at pH 9.0, there exists an experimentally-evaluable sugar-binding equilibrium between neutral trivalent acid 1 and anionic tetravalent ester thereof, which can be conveniently monitored by fluorescence or 11B NMR spectroscopy. A characteristic aspect of the present sugar-binding process is the notable selectivity of host 1 for oligosaccharides. The binding constants for maltodextrins (α-1,4-linked glucose oligomers) increase with increasing chain lengths or repetition numbers of the glucose unit. Such a chain length selectivity and those among stereoisomers and linkage isomers of disaccharides are discussed in terms of intracomplex oligosaccharide–indole interaction.

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