Lewis acid properties of some metal salts for lactic acid formation in water: 31P NMR spectroscopy with trimethylphosphine oxide as a molecular probe

• Lewis acid catalysts are effective for the isomerization of pyruvic aldehyde.
• 31P NMR spectra of TMPO with metal salts reflect each Lewis acid property in water.
• Catalytic performance of metal salts accords with the 31P chemical shifts of TMPO.
• Sc(OTf)3, ScCl3 and In(OTf)3 enhance lactic acid formation than other metal salts.

31P NMR spectroscopy was applied to the evaluation of water-tolerant Lewis acid catalysis of various metal triflates and chlorides in water using trimethylphosphine oxide (TMPO) as a probe molecule. Direct interaction of the TMPO molecule with the Lewis acid in water changes the original 31P NMR chemical shift and line width. ScCl3, Sc(OTf)3 and In(OTf)3 exhibit larger changes in the 31P chemical shift by the formation of a Lewis acid–TMPO complex than other triflates and chlorides. This is due to strong interaction between the Lewis acid and TMPO, which leads to the slow exchange between TMPO and H2O in ScCl3, Sc(OTf)3 and In(OTf)3. Lewis acids with large change in the 31P chemical shift exhibit high catalytic activities for lactic acid formation from 1,3-dihydroxyacetone (1.3-DHA) and pyruvic aldehyde in water: Strong interaction with TMPO on the Lewis acid is a dominant factor for lactic acid formation.

Figure optionsDownload high-quality image (212 K)Download as PowerPoint slide