Stereodivergent Quaternization of 2‐Alkyl‐2‐p‐tolylsulfinylacetonitriles: NMR Spectroscopic Evidence of Planar and Pyramidal Benzylic Carbanions |
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Authors: | José?Luis García?Ruano Prof Ana?M Martín‐Castro Prof Francisco Tato Dr Esther Torrente Ana?M Poveda Dr |
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Institution: | 1. Departamento Quimica Orgánica, Cantoblanco, 28049 Madrid (Spain), Fax: (+34)?914973966;2. Servicio Interdepartamental de Investigación, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid (Spain) |
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Abstract: | Enantiomerically enriched α,α‐disubstituted phenylacetonitriles have been readily prepared by stereoselective quaternization of 2‐alkyl‐2‐2‐(p‐tolylsulfinyl)phenyl]acetonitriles with different alkylating electrophiles in the presence of bases. The use of potassium hexamethyldisilazane (KHMDS)/18]crown‐6 ether and NHMDS with alkyl halides afforded S,SS and R,SS diastereoisomers, respectively, in high enantiomeric purities, thus providing stereodivergent processes for synthesizing both isomers. The dependence of the stereochemical course of the reactions on the experimental conditions (mainly on the counterion) has been rationalized by assuming a planar or pyramidal structure for the benzylic carbanions. This hypothesis has been supported by NMR spectroscopic studies, which permit one to assign a chelated pyramidal structure to the sodium benzylic carbanions and an almost planar naked carbanionic structure to the potassium benzylic carbanions generated in the presence of 18]crown‐6 ether. |
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Keywords: | asymmetric synthesis carbanions chirality diastereoselectivity NMR spectroscopy |
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