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

Abstract

Michael reaction of β-ketoesters with vinylketones at room temperature under solvent-free condition is investigated with various Fe 3+ catalysts, including FeCl 3 ⋅ 6H 2O supported on various supports (Fe–mica, Fe–mont, Fe–SiO 2, Fe–Al 2O 3, Fe–NaY) and homogeneous catalysts, FeCl 3 ⋅ 6H 2O and Fe(NO 3) 3 ⋅ 9H 2O. Fe 3+-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 Fe 3+ species. The Fe 3+ cation, highly dispersed in the interlayer of clay (mica or mont) or on SiO 2, is more active than the cluster-like Fe 3+ oxide or hydroxide species in Fe–NaY and Fe–Al 2O 3. UV–vis and XAFS results for the catalysts treated with reactants suggest that, during the reaction, the FeCl 2(O) 4 octahedral species in FeCl 3 ⋅ 6H 2O or those on Fe–SiO 2 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 Fe 3+ as a Lewis acid site. The lower numbers of β-diketonato ligand coordinated with Fe 3+ in Fe–mica should result in a larger coordination strength for β-diketonato ligand than that in Fe–SiO 2, 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.