Creation of monomeric La complexes on apatite surfaces and their application as heterogeneous catalysts for Michael reactions

Abstract

Using a cation-exchange method, an equimolar substitution of La³⁺ for Ca²⁺ occurred by the treatment of stoichiometric hydroxyapatite (HAP: Ca₁₀(PO₄)₆(OH)₂) with an aqueous solution of La(OTf)₃, affording a monomeric hydroxyapatite-bound La complex (LaHAP). Physicochemical characterization by means of XRD, XPS, IR, and La K-edge XAFS analyses proved that a monomeric La³⁺ phosphate complex was generated on its surface. Such monomeric La³⁺ species function as an efficient heterogeneous catalyst for the Michael reaction of 1,3-dicarbonyls with enones under aqueous or solvent-free conditions. The work-up procedure is straightforward and the spent catalyst could be recycled without any loss of the catalytic activity. Further application to an asymmetric version was also investigated using various apatite catalysts modified with chiral organic ligands. Enantioselectivity was found to depend on the chiral ligand, solvent, and rare earth metal triflate precursor (RE(OTf)₃) for the reaction of methyl 1-oxoindan-2-carboxylate with methyl vinyl ketone. Under optimized reaction conditions, a monomeric fluoroapatite-bound La complex catalyst modified with (R,R)-tartaric acid (TA-LaFAP) provided the Michael adduct quantitatively in up to 60% ee.