Enantio- and Diastereoselectivity in 1,3-Dipolar Cycloaddition Reactions of Nitrones with 3-Crotonoyl-2-oxazolidinone Catalyzed by Ni(II)- Binaphthyldiimine Complexes

Chiral Ni(II)-1,1ʹ-binaphthyl-2,2ʹ-diimine complexes were found to be effective Lewis-acid catalysts for an asymmetric 1,3-dipolar cycloaddition reaction of Nbenzylideneaniline N-oxide with 3-crotonoyl-2-oxazolidinone. In the presence of molecular sieves (4 A), when the chiral N,Nʹ-bis(2,6-dichlorobenzylidene)-1,1ʹbinaphthyl-2,2ʹ-diamine (BINIM-DC) and Ni(ClO4)2·6H2O were used to prepare the catalyst, up to 81% ee of the corresponding endo-cycloadduct was obtained with endo-selectivity (87 : 13, and up to 96 : 4). The use of N,Nʹ-bis(3-chloro-substituted 2-hydroxybenzylidene) derivatives as ligands under similar conditions showed high exo-selectivity (up to 95 : 5) with moderate enantioselectivity. Although almost no diastereoselectivity was observed, endo- and exo-cycloadducts were obtained with promising levels of enantioselectivity (up to > 98%) in the presence of catalysts, which were prepared from chiral BINIM-DC, NiBr2, and AgBF4, AgSbF6, or AgPF6.