Creation of monomeric La complexes on apatite surfaces and their application as heterogeneous catalysts for Michael reactions
作者:Kohsuke Mori、Michitaka Oshiba、Takayoshi Hara、Tomoo Mizugaki、Kohki Ebitani、Kiyotomi Kaneda
DOI:10.1039/b512030f
日期:——
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.
采用阳离子交换法,通过将化学计量的羟基磷灰石(HAP:Ca\(}_10}\)(PO\(}_4}\))\(}_6}\)(OH)\(}_2}\))与La(OTf)\(}_3}\)的水溶液处理,实现了La\(}^3+}\)对Ca\(}^2+}\)的等摩尔取代,得到了单体羟基磷灰石结合的镧配合物(LaHAP)。通过XRD、XPS、IR和La K边XAFS分析的物理化学表征证明,在其表面生成了单体La\(}^3+}\)磷酸盐配合物。这种单体La\(}^3+}\)物种作为高效的非均相催化剂,在水相或无溶剂条件下对1,3-二羰基化合物与烯酮的迈克尔加成反应具有催化作用。后处理过程简单,且催化剂可回收,无催化活性损失。此外,还研究了通过各种含手性有机配体的磷灰石催化剂的进一步应用,以实现不对称版本。在甲基1-氧茚-2-羧酸酯与甲基乙烯基酮的反应中,产物的对映选择性取决于手性配体、溶剂和稀土金属三氟甲磺酸盐前体(RE(OTf)\(}_3}\))。在优化的反应条件下,经过(R,R)-酒石酸(TA-LaFAP)修饰的单体氟磷灰石结合的镧配合物催化剂,能使迈克尔加成产物达到定量收率,最高对映体过量值达60%。