Resumen de Using NMR Spectroscopy To Probe the Chemo- and Diastereoselectivity in the NaBH4 Reduction of Benzoin Acetate and Benzoin Benzoate
Shahrokh Saba, Kristen Cagino, Caitlin Bennett
A pedagogically useful discovery-based undergraduate organic chemistry lab experiment probing the chemo- and diastereoselectivity in the NaBH4 reduction of two chiral ketoesters (benzoin acetate and benzoin benzoate) has been developed. This experiment complements a previously described and highly popular discovery-based experiment that probes the stereoselectivity in the NaBH4 reduction of the chiral ketone (±)-benzoin. Using reactions described in standard textbooks, students convert (±)-benzoin to the ketoester derivatives (±)-benzoin acetate and (±)-benzoin benzoate. In contrast to benzoin that has a single reducible ketone functional group, the ketoester derivatives have two reducible functional groups. In addition, the structural modifications made to the hydroxyl group of the benzoin molecule lead to an increase in the effective steric bulk of the OH group of benzoin, affording a larger acetoxy group and an even larger benzoxy group. Using NMR spectroscopy, students probe not only the chemoselectivity but also the diastereoselectivity associated with the NaBH4 reduction of their difunctional substrates. In contrast to the NaBH4 reduction of benzoin, which is highly diastereoselective (de ∼ 100%) and affords (R,S)-hydrobenzoin with near exclusion of its (R,R)- and (S,S)- counterparts, students discover a decreasing trend in diastereoselectivity going from benzoin to benzoin acetate and benzoin benzoate. These findings are then assessed with the help of qualitative predictions made by the Felkin-Anh model for nucleophilic addition to the carbonyl group.
