1041191-85-8Relevant academic research and scientific papers
Stabilization of ketone and aldehyde enols by formation of hydrogen bonds to phosphazene enolates and their aldol products
Kolonko, Kristopher J.,Reich, Hans J.
supporting information; experimental part, p. 9668 - 9669 (2009/02/04)
Solution properties of enolates generated using the phosphazene (Schwesinger) base P4-tBu were investigated by NMR spectroscopy. With a full equivalent of base the benzyl ketones 1a and 1b, the acetophenone 2, the arylacetaldehyde 1c, and the methyl arylacetate 1d formed the expected "naked" (P4H+) enolates 3 and 7. However, at a half-equivalent of base the ketones 1a and 1b as well as the aldehyde 1c formed solutions of stable hydrogen-bonded dimeric (enol-enolate) structures (4). The acetophenone 2, on the other hand, forms only traces of the H-bonded dimer 8 during deprotonation of 2. The thermodynamic product was the isomeric self-aldol condensation product 12. The mechanism of this condensation was elucidated by low temperature rapid-injection (RI) NMR spectroscopy. Solutions of 8 stable enough for NMR characterization could be transiently generated by semiprotonation of the enolate 7 with HCl·OEt2 at -130 °C using RINMR. The ester enolate 1d gave no trace of 4d even on a time scale as short as a few seconds at -130 °C either during the semideprotonation of 1d, or during semiprotonation of the enolate 3d. Long-lived solutions of the enols derived from 1a, 1b, 1c, and 2 (but not 1d) could be produced by full protonation of the phosphazene enolates with HCl·OEt2 at low temperature. Copyright
