Synthesis and NMR-Spectral Analysis of Achiral O,O- and N,N-Acetals: Anisochronous pro-R and pro-S Ligands in NMR Spectra
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Shahrokh Saba [1] ; Ariel Corozo-Morales [1]
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[1] Fordham University
Fordham University
Estados Unidos
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- Localización: Journal of chemical education, ISSN 0021-9584, Vol. 96, Nº 2, 2019, págs. 354-359
- Idioma: inglés
- Texto completo no disponible (Saber más ...)
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Resumen
A pedagogically useful undergraduate organic-chemistry-laboratory experiment has been developed that involves the synthesis of two structurally simple, achiral O,O- and N,N-acetal substrates with no stereocenters. The experiment introduces students to a “new” and more effective variant of classical acetal synthesis that uses an ortho ester for the preparation of an acyclic O,O-acetal. Students also use a solvent-free protocol for the preparation of an N,N-acetal (aminal). Using NMR spectroscopy, students explore the diastereotopicity and nonequivalence of the ligands within the acetals, especially those bonded to a carbon on a freely rotating σ bond. Students discover that the presence of stereocenters is not a structural precondition for diastereotopicity and nonequivalence of ligands and that just like chiral or achiral molecules with stereocenters, achiral substrates with no stereocenters can contain diastereotopic moieties that display anisochronous NMR signals. Students use pro-R and pro-S descriptors to specify the nonequivalent ligands attached to the prochiral centers and accordingly assign unique stereochemical descriptors (RR, RS, SS, and SR) to each of the four methyl groups in their substrates.
