<i>C</i>₂‐Symmetric Cationic Copper(<scp>II</scp>) Complexes as Chiral Lewis Acids: Counterion Effects in the Enantioselective Diels–Alder Reaction

Abstract

Alone a tailor-made chiral cationic Lewis acid is not sufficient to achieve high enantiomic excesses in Diels–Alder reactions; the counterion can dramatically affect the efficiency of the chiral catalyst. Thus, for example, the enantioselectivities reached in these reactions in the presence of the copper complex 1 with X = SbF6 as counterion are in some cases substantially greater than those with X = OSO2CF3.