Resumen de Highly Reversed Diastereoselective Meerwein–Ponndorf–Verley (MPV) Reductions of Benzoin and Benzoin Pivalate Leading to meso- and dl-Hydrobenzoins: A Collaborative Stereochemical Investigation Using NMR Spectroscopy and Chemical Synthesis
Shahrokh Saba, James A. Ciaccio, Pooja Grewal
In this second-semester organic chemistry lab experiment, students work collaboratively using a combination of chemical synthesis and NMR spectroscopy to investigate the diastereoselectivity associated with the Meerwein–Ponndorf–Verley (MPV) reduction of two ketone substrates having carbonyl groups flanked by an α-chiral carbon with substituents of contrasting sizes. One group of students performed the MPV reduction on (±)-benzoin, which has an unprotected hydroxyl group, while the other group performed the same reaction on (±)-benzoin pivalate, which has a hydroxyl with an easily removed, bulky protecting group. Students in the second group perform ester hydrolysis after the MPV reduction to obtain the corresponding diols. Students in both groups indirectly establish the relative configuration of the resulting diols by converting them to the corresponding acetonides and then examining the latter’s 1H and 13C NMR spectra, which afford distinct resonances for their stereochemically homotopic or diastereotopic geminal methyl groups. Students discover that these MPV reactions have highly reversed diastereoselectivities, affording meso- and dl-hydrobenzoins as their major products. Through experimentation, students learn that while diastereomers can differ spectroscopically, they may be indistinguishable by such techniques. Students share 1H and 13C NMR spectra of their products and establish the ratio of the diastereomers present by using NMR peak integration values. The highly reversed diastereoselectivity displayed by these MPV reactions is consistent with the Felkin–Anh model for predicting the preferred diastereoselection in nucleophilic addition to α-chiral ketones.
