Asymmetric Catalysis of the Diels−Alder Reaction Using Dicationic Zirconocene Complexes

Enantio- and diastereoselective Diels−Alder reactions between dienophiles 1 and cyclopentadiene are catalyzed by a chiral, dicationic complex formed on reaction of the chiral ansa -zirconocene 2 (X = Me) with 2 equiv of the strong acid [(Et 2 O) 2 H][B(Ar F ) 4 ] ( 3; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ), in either dichloromethane or 2-nitropropane solvent. The enantioselectivities are higher at lower catalyst loadings, particularly in dichloromethane solution, and spectroscopic studies suggest that this is related to competitive formation of ether-coordinated complexes at relatively low oxazolidinone:catalyst ratios.