Carbon-Substituted Co(III) Salens as Effective Catalysts for Enantioselective Diels-Alder Reactions

Abstract

Seven newly synthesized Co(III) salen hexafluoroantimonate complexes having aryl or alkyl-aryl substituents in place of the bay region t-Bu or silyl substituents were synthesized and examined as catalysts for the enantioselective Diels-Alder reaction of 1-benzylaminocarbamate-1,3-butadiene and methacrolein. The results were consistent with a working model for rationalizing enan­tioselectivity in these cycloaddition reactions: an increase in the steric bulk of the internal substituents gives, in most instances, a predictable increase in the ee of adducts. The best results were obtained with a homobenzyl-substituted salen, which yielded cyclo­adducts in >97% ee, and with an increased reaction rate compared to the previously studied TMS-Co(III) salen. Insights from this study suggest that helical asymmetry, induced by increasing the steric size of the internal substituents, alone does not account for increased ee of the products. The present study also shows that fully carbon-based substituents can be as effective enantiodiscriminating elements as trialkylsilanes, and, unlike silanes, are not succeptible to protodesilylation.

Key words

asymmetric catalysis - Lewis acids - enantioselective Diels-Alder - substituent effects - aminodienes

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9

Values for the enantiomer ratio are included to distinguish subtle differences in selectivity for systems that would otherwise appear identical due to rounding of the ee values.

10

These results show that even a small amount of mono-aryl substituted salen complex (˜2%), similar to that which would be produced from partial protodesilation of the silyl salens, produces cycloadducts with significantly lower ee values. Presumably, the less encumbered complex catalyzes a faster, less enantioselective reaction.