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NMR Detection of the Enamine Intermediate in Proline-Catalyzed Aldol Reactions

M. Schmid, Kirsten Zeitler, Ruth M. Gschwind

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Abstract

Significance An enamine intermediate in proline-catalyzed aldol reactions was indentified in situ by NMR studies on self-aldolization of aldehydes in DMSO. Only E -configured s- trans -enamines were detected in accordance with the generally accepted mechanism of enamine catalysis. The enamines were found to form directly from oxazoli­din­ones (e.g., 3 ), and not via central iminium or iminium-like intermediates (e.g., 2 ) as evidenced by NMR exchange spectroscopy (EXSY). These results indicate a possible role of oxazolidinones in the catalytic cycle, beyond their involvement in ‘parasitic equilibria’.

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Significance An enamine intermediate in proline-catalyzed aldol reactions was indentified in situ by NMR studies on self-aldolization of aldehydes in DMSO. Only E -configured s- trans -enamines were detected in accordance with the generally accepted mechanism of enamine catalysis. The enamines were found to form directly from oxazoli­din­ones (e.g., 3 ), and not via central iminium or iminium-like intermediates (e.g., 2 ) as evidenced by NMR exchange spectroscopy (EXSY). These results indicate a possible role of oxazolidinones in the catalytic cycle, beyond their involvement in ‘parasitic equilibria’.

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Available abstract

Significance An enamine intermediate in proline-catalyzed aldol reactions was indentified in situ by NMR studies on self-aldolization of aldehydes in DMSO. Only E -configured s- trans -enamines were detected in accordance with the generally accepted mechanism of enamine catalysis. The enamines were found to form directly from oxazoli­din­ones (e.g., 3 ), and not via central iminium or iminium-like intermediates (e.g., 2 ) as evidenced by NMR exchange spectroscopy (EXSY). These results indicate a possible role of oxazolidinones in the catalytic cycle, beyond their involvement in ‘parasitic equilibria’.

Key concepts: Enamine, Iminium, Aldol reaction, Chemistry, Catalysis, Organocatalysis, Catalytic cycle, Stereochemistry

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