Lewis Acid Catalysis of a Diels−Alder Reaction in Water

Here we report the first detailed study of a Diels−Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co2+, Ni2+, Cu2+ and Zn2+ ions as Lewis acid catalysts on the rate and endo−exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-1-(2-pyridyl)-2-propen-1-ones (1a−e) and cyclopentadiene (2) in water has been studied. Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M Cu(NO3)2 in water accelerates the reaction by a factor of 79 300. The kinetics of the catalyzed reaction were analyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a−e and rate constants for the reaction of the resulting complexes with 2. The rate enhancement imposed upon the uncatalyzed DA reaction of substrates 1 with 2 by water is much more pronounced than that for the catalyzed reaction. The increase of the endo−exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. The modest solvent and substituent effects observed for the catalyzed reaction indicate that the change in charge separation during the activation process is not larger than the corresponding change for the uncatalyzed reaction.