Highly enantioselective organocatalytic asymmetric Mukaiyama-aldol reaction of difluoroenoxysilanes with β, γ-unsaturated α-ketoesters

We report the first example of catalytic asymmetric reaction of difluoroenoxysilanes 1 and β, γ-unsaturated α-ketoesters 2. In the presence of tertiary amine or tertiary amine-H-bonding donor bifunctional catalysts, it was found that the reaction selectively took place at the ketone moiety of ketoesters 2, and no conjugate reaction happened. Hydronquinine derived urea catalyst 11 was identified as a powerful catalyst for this Mukaiyama-aldol reaction to furnish α-difluoroalkyl substituted tertiary alcohols in good to high yield (44%-81%), with moderate to excellent enantioselectivity (72%-96%). The optimum reaction condition was determined to run the reaction at -40°C using THF as the solvent, in the presence of 10 mol% of chiral catalyst 11. The reaction possibly proceeded via the dual activation of both reaction partners: the tertiary amine moiety of catalyst 11 working as a Lewis base to activate difluoroenoxysilanes 1, while the urea part of the catalyst as a Brønsted acid to activate carbonyl group of ketoesters 2. Different aryl substituted difluoroenoxysilanes 1 and γ-aryl substituted β, γ-unsaturated α-ketoesters 2 all worked well under the established reaction condition, with 15 examples provided. The thus obtained chiral α-difluoroalkyl substituted tertiary alcohols were not only valuable in the medicinal research, but also versatile building blocks for the synthesis of a variety of difluoroalkyl-substituted chiral compounds. For example, the selective reduction of the ketone or ester group of product 3 g has been realized by using NaBH₄ or Et₃SiH as the reducing agents, giving the triol 14 or diol 15 in moderate yield without the loss of ee.