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 se lectivity of the DA reaction between the bidentate dienophiles 3-pheny l-1-(2-pyridyl)-2-propen-1-ones (1a-e) and cyclo-pentadiene (2) in wat er has been studied. Relative to the uncatalyzed reaction in acetonitr ile, 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 an alyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a-e and rate constants for the reaction of the resulting c omplexes 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 selectiv ity induced by water in the uncatalyzed process is completely absent f or the Lewis acid catalyzed reaction. The modest solvent and substitue nt effects observed for the catalyzed reaction indicate that the chang e in charge separation during the activation process is not larger tha n the corresponding change For the uncatalyzed reaction.