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This report describes the design and development of chiral Co(III)−salen catalysts for enantioselective Diels−Alder reactions. A crystal structure of a Co−salen catalyst with two equivalents of benzaldehyde provided insight on the factors that may be important for enantioselectivity. On the basis of this structural information, new catalysts were prepared in which the “bay region” tert-butyl groups were replaced by trimethylsilyl groups. The new silyl-substituted catalysts were exceptionally effective (catalyst loadings down to 0.05 mol %) and convenient to useroom temperature, under an air atmosphere, using a minimum amount of solvent.