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.