Solvent effects on Diels-Alder reactions. The use of aqueous mixtures of fluorinated alcohols and the study of reactions of acrylonitrile

Abstract

Rate, endo/exo, regio- and diastereo-facial selectivities of several Diels–Alder reactions were measured in a series of fluorinated alcohol–water mixtures, whose solvophobicity has been determined by means of the solvophobic power (Sp) parameter. Solvophobicity is the main factor influencing the reaction rate, although in some reactions hydrogen bond donating (HBD) ability may also play a role. Both solvophobicity and HBD ability are important to account for changes in endo/exo selectivity. HBD ability is the main factor responsible for the changes in regio- and diastereo-facial selectivities, induced by the reaction medium. On the other hand, the kinetic rate constants and endo/exo selectivity of the reaction of acrylonitrile with cyclopentadiene, as well as the regioselectivity of the reaction of acrylonitrile with isoprene, have been determined in 23 reaction media. The analysis of the results using empirical solvent parameters show that the reaction rate depends on solvophobic, HBD and dipolarity interactions, whereas endo/exo selectivity is influenced by solvophobic and dipolarity interactions, and the regioselectivity only by HBD effects.