Here we report the first detailed study of a Diels-Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ 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(NO 3) 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.

The enantioselective cleavages of p-nitrophenyl esters of phenylalanine (PhePNP), phenylglycine (PhgPNP), and leucine (LeuPNP) catalyzed by Cu(II) complexes of homochiral ligands have been investigated in conditions of very fast change of pH (“pH-jump”) at the beginning of the reaction. Using lipophilic ligands comicellized with hexadecyltrimethylammonium bromide and decreasing the pH from 9 to 5, a remarkable amplification of the enantioselectivity has been observed when compared to the same reaction performed at constant pH (from 24 to 58 for PhgPNP, as the ratios of the rate constants measured for the faster and slower reacting enantiomers). This has been correlated with the changes in the ligand/Cu(II) complex concentration induced by the change of pH. In fact, it has been established that under the conditions employed the faster enantiomer of the substrate reacts with a higher concentration of catalyst than the slower one:  in the first case the reaction occurs before decomplexation takes place and in the second case after decomplexation is virtually complete.

1996 cycloaddition reactions cycloaddition reactions O 0070 -051Lewis Acid Catalysis of a Diels-Alder Reaction in Water.-The effect of Co2+, Ni2+, Cu2+, and Zn2+ ions as Lewis acid catalysts on the rate and endo-exo selectivity of the Diels-Alder reaction between the bidendate dienophiles (I) and cyclopentadiene in water is studied. The reaction is accelerated greatly by Lewis acid catalysis combined with an aqueous solvent effect. The kinetics of the catalyzed reaction are analyzed in terms of equilibrium constants for complexation of the Lewis acid with (I) and rate constants for the reaction of the resulting complexes with cyclopentadiene. The rate enhancing effect of water on the catalyzed reaction is less pronounced than the corrresponding effect on the uncatalyzed reaction and the increase of the endo-exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. -(OTTO, S.; BERTONCIN, F.; ENGBERTS, J.