The kinetic effects of water and of cyclodextrins on Diels–Alder reactions. Host–guest chemistry. Part 18

Abstract

The rates of 19 different dienophiles with mostly cyclopentadiene have been measured in a range of solvents. Water, by comparison with methanol, is found to accelerate the cycloadditions by factors ranging from 15–7 680. While there is no simple correlation of the rate enhancement with the hydrophobicity of the substrates, a correlation can be made in the case of acrylates, which show a decreasing rate effect due to water, and with β-cyclodextrin, which is found to lead to inhibition in these and several other cases. Computer analysis of enzyme-like saturation kinetics—observed with some fumarates in the presence of β-cyclodextrin—indicates apparent binding constants for the ternary complex of ca. 10² dm³ mol^–1 and catalytic k_cat/k_o ratios of up to 100. Heptakis-6-piperidino-β-cyclodextrin in the form of its ammonium salt leads to rate retardation in most of the cases investigated; sodium dodecylsulphate also decreases the constants. α-Cyclodextrin generates small effects, rate retardation, or in one case complete inhibition.