Quantitative Antihydrophobic Effects as Probes for Transition State Structures. 2. Diels—Alder Reactions

摘要

In a previous paper we described the use of antihydrophobic alcohol cosolvents in water to establish quantitative relationships between solubilities and rates in the benzoin condensation and in some displacement reactions. The idea, in brief, is that a small amount of an alcohol cosolvent will not interfere with the ability of water to solvate ionic transition states (activated complexes), as we demonstrated, but that the cosolvent will be recruited into the transition state to help solvate the hydrophobic portions. The induced solubility perturbations reflect the solvation of the reactants by the cosolvent. We showed that the δΔG° induced by such antihydrophobic cosolvation is proportional to the amount of hydrophobic surface that is exposed to water, and we present further evidence for this here. The rate effects caused by the cosolvents reflect the difference in solvation of reactants and transition states.