Lewis-Acid Catalysis of a Diels-Alder Reaction in Water

The reaction between the diene 1 and cyclopentadiene (2) is efficiently catalysed by small amounts of Lewis acids (Co2+, Ni2+, Cu2+, Zn2+). The reaction in an aqueous solution containing 0.010 M Cu(NO3)2 250.000 times faster than that in acetonitrile. This huge acceleration is the result of a combination of the accelerative solvent effect of water and catalysis by Cu2+ as the Lewis acid.