Stabilization of ketone and aldehyde enols by formation of hydrogen bonds to phosphazene enolates and their aldol products.
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 degrees 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 degrees 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.
Kolonko
Kristopher J
KJ
Department of Chemistry, University of Wisconsin, , Madison, Wisconsin 53706, USA.
Reich
Hans J
HJ
D000438:Alcohols / Q000138:chemical synthesis / Q000737:chemistry; D000447:Aldehydes / Q000737:chemistry*; D000475:Alkenes / Q000138:chemical synthesis / Q000737:chemistry; D006860:Hydrogen Bonding; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D009943:Organophosphorus Compounds / Q000737:chemistry*; D012996:Solutions; D013816:Thermodynamics
D000438:Alcohols; D000447:Aldehydes; D000475:Alkenes; D007659:Ketones; D009943:Organophosphorus Compounds; D012996:Solutions