Organocatalytic Asymmetric Conjugate Addition of Heteroaryl Moieties to α,β-Unsaturated Enones Using Boronates and 3,3ʹ-Disubstituted Binaphthols
Cheung, Didi Chiu Yee
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Asymmetric conjugate addition is an important methodology for carbon-carbon bond formation in organic synthesis. While there is extensive literature on the asymmetric conjugate addition of a wide selection of donors to a variety of acceptors, there are relatively few reports on the asymmetric conjugate addition of heteroaryl groups. Almost all of these reports utilize catalytic amounts of both a rhodium compound and a chiral ligand, and usually employ organoboron reagents. Furthermore, these additions have been problematic due to protodeboronation. Other organocatalytic methods have offered low yields and moderate enantioselectivies. The binaphthol / boronate catalyst system developed by the Chong group is effective in promoting asymmetric conjugate addition of 2-thienyl, 3-thienyl, 2-furyl, and 2-benzo[b]thienyl moieties to acyclic enones. The combination of 3,3ʹ-disubstituted binaphthols with diethyl heteroarylboronates generated 1,4-adducts with good yields and high enantioselectivities (up to 100% yield and 99.9:0.1 er). This catalyst system complements known methods of conjugate addition of heteroaryl groups involving transition metal catalysts and cyclic enone acceptors and is the most effective organocatalytic system reported to date.