Diels-Adler Reactions in Nonaqueous Polar Solvents. Kinetic Effects of Chaotropic and Antichaotropic Agents and of β-Cyclodextrin

Article abstract of DOI:10.1021/ja00225a003

The Diels-Adler addition reactions of nitrosobenzene with 1,3-cyclohexadiene and of methyl vinyl ketone with 1,3-cyclopentadiene are faster in formamide or in ethylene glycol than in other organic solvents, but not as fast as in water solution.The reactions in these organic solvents are also accelerated by β-cyclodextrin.The kinetic results indicate that there is solvophobic binding reactants to each other, or into the cyclodextrin cavity, in these polar solvents.In spite of this, there is no striking increase in endo/exo selectivity, as there is in water.We detect two classes of chaotropic agents.Urea and guanidinium ion, which are normally chaotropic in water and thus decrease the hydrophobic effect and the rate of our Diels-Adler reactions, show no such effect in the organic solvents.These solutes also decrease the solubility of benzene in ethylene glycol or in formamide, in contrast to their effects in water.The contrasts indicate that "waterlike" organic solvents still do not share some of the most striking properties of water itself.The results also indicate that the chaotropic effects of urea and guanidinium ion in water reflect primarily the interaction of the chaotropes with the water, not with the hydrocarbon solutes.By contrast, tetramethylammonium bromide, and even more so tetrabutylammonium bromide, belong to a second class of salting agents.They decrease the rate of the Diels-Adler addition of cyclopentadiene to methyl vinyl ketone, and tetramethylammonium bromide increases the solubility of benzene in ethylene glycol and formamide (as it does in water).These tetraalkylammonium cations probably act as pseudodetergents and interact primarily with the solutes, not the solvents.