The importance of copper placement in chiral catalysts supported on heteropolyanions: Lacunary vs external exchanged

• Published in: Molecular catalysis 2020-06, Vol.489, p.110935, Article 110935
• Main Authors: Torviso, M. Rosario, Mansilla, Daniela S., Fraile, José M., Mayoral, José A.
• Format: Article
• Language: English
• Subjects: Bis(oxazolines); Cyclopropanation; Enantioselective catalysis; Heteropolyacids; Immobilized catalysts
• ISSN: 2468-8231; 2468-8231
• DOI: 10.1016/j.mcat.2020.110935

[Display omitted] •Copper-containing lacunary phosphotungstate was modified with chiral bis(oxazoline).•Its poor reductability made it inefficient as catalyst in cyclopropanation.•Partial neutralization of phosphotungstic acid produces a loss in symmetry.•Non-symmetrical phosphotungstate shows an IR spectrum similar to lacunary species.•The most efficient catalysts are prepared by external cationic exchange. Lacunary [PCuW11O39]5− species modified with chiral bis(oxazoline) leads to very poor results as catalyst in the enantioselective cyclopropanation, in contrast with the Cu-bis(oxazoline) complex exchanged on the Keggin [PW12O40]3− species. The incomplete neutralization and/or exchange of the Keggin species produces a loss in symmetry that leads to spectra in solid phase (IR and NMR) similar to those obtained for the lacunary species. The symmetry is averaged in solution, but additional characterization methods are necessary to determine the true nature of the solid heteropolyanionic species. These results demonstrate that the efficiency of copper-bis(oxazoline) complexes is related to its placement in an external exchange position, whereas the copper included in the heteropolyanion structure is not active for cyclopropanation reactions.