Using NMR Spectroscopy and Chemical Synthesis to Establish Diastereoselectivity of NaBH4 and Meerwein–Ponndorf–Verley (MPV) Reduction of (±)-Benzoin Isopropyl Ether: A Collaborative Discovery-Based Laboratory Experiment

• Shahrokh Saba ^[1] ; James A. Ciaccio ^[1] ; Lauren McMann ^[1] ; Tamilla Tariverdieva ^[1]
  1. [1] Fordham University

     Fordham University

     Estados Unidos

     
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 97, Nº 5, 2020, págs. 1418-1424
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • A discovery-based undergraduate organic chemistry lab experiment has been developed in which students establish the diastereoselectivity of hydride reduction of the α-chiral ketone (±)-benzoin isopropyl ether to erythro and threo diastereomeric alcohols by two different reducing agents. Students achieve this goal by working collaboratively and using chemical, chromatographic (TLC), and NMR spectroscopic methods to identify the major diastereomers produced from each reduction. To this end, three groups of students perform three different reductions using three different reducing agents. Students in two groups carry out reduction of (±)-benzoin isopropyl ether: one uses NaBH4/ethanol, a widely used metal hydride reagent for reduction of aldehydes and ketones, and the other uses Al(OiPr)3/2-propanol, a hydride transfer reagent used in the Meerwein–Ponndorf–Verley (MPV) reaction. Students in the third group use diisobutylaluminum hydride (DIBAL-H) to reductively ring-open meso-2,2-dimethyl-4,5-diphenyl-1,3-dioxolane (an acetal they synthesize in a previous experiment) to obtain an authentic sample of the erythro diastereomer unaccompanied by its threo counterpart. Students perform TLC analysis of NaBH4 and MPV reaction products vs the authentic erythro sample to qualitatively establish the diastereoselectivity exhibited by NaBH4 and MPV reduction of the ketone substrate. Afterward, students from all three groups share 1H NMR spectra of their products and perform quantitative analysis by establishing the ratios of each diastereomer present using NMR integration values. Through experimentation, students learn how diastereomers can differ chromatographically and spectroscopically and yet not be readily distinguishable by such techniques. This experiment also serves as a pedagogically valuable complement to a previously described and highly popular discovery-based experiment that probes the diastereoselectivity in the NaBH4 reduction of (±)-benzoin.