Michael reaction of β-ketoesters with vinyl ketones by iron(III)-exchanged fluorotetrasilicic mica: catalytic and spectroscopic studies

Michael reaction of β-ketoesters with vinylketones at room temperature under solvent-free condition is investigated with various Fe3+ catalysts, including FeCl3 ⋅ 6H2O supported on various supports (Fe-mica, Fe-mont, Fe-SiO2, Fe-Al2O3, Fe-NaY) and homogeneous catalysts, FeCl3 ⋅ 6H2O and Fe(NO3)3 ⋅ 9H2O. Fe3+-exchanged fluorotetrasilicic mica (Fe-mica) shows highest activity. Fe-mica exhibits almost quantitative yields of Michael adducts, high turnover numbers (TON = 1000), and a low level of Fe leaching. After simple work-up procedures, Fe-mica can be recycled without a loss in activity. The relationship between catalytic activity and the catalyst structure determined by XRD, UV-vis, and Fe K-edge XANES/EXAFS is discussed in terms of the effect of clay support on the structure and reactivity of Fe3+ species. The Fe3+ cation, highly dispersed in the interlayer of clay (mica or mont) or on SiO2, is more active than the cluster-like Fe3+ oxide or hydroxide species in Fe-NaY and Fe-Al2O3. UV-vis and XAFS results for the catalysts treated with reactants suggest that, during the reaction, the FeCl2(O)4 octahedral species in FeCl3 ⋅ 6H2O or those on Fe-SiO2 are converted to the β-diketonato complexes with two β-diketonato ligands, whereas in Fe-mica β-diketonato complexes with one β-diketonato ligand are formed. The formation of β-diketonato complexes results in a slight lowering of the Fe oxidation number from 3+, probably as a result of the electron donation from the β-diketonato ligand to Fe3+ as a Lewis acid site. The lower numbers of β-diketonato ligand coordinated with Fe3+ in Fe-mica should result in a larger coordination strength for β-diketonato ligand than that in Fe-SiO2, which was confirmed by acetylacetone-TPD. Thus, the central carbon atom of the β-diketonato ligand in Fe-mica is more reactive toward nucleophilic attack by the coordinated enone, leading to higher activity for the Michael reaction.