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

Michael reaction of beta-ketoesters with vinylketones at room temperature under solvent-free condition is investigated with various Fe(3+) catalysts, including FeCl(3) (.) 6H(2)O supported on various supports (Fe-mica, Fe-mont, Fe-SiO(2), Fe-Al(2)O(3), Fe-NaY) and homogeneous catalysts, FeCl(3) (.) 6H(2)O and Fe(NO(3))(3) (.) 9H(2)O. 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 workup procedures, Fe-rnica 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 Fe3+ 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(2)O(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(2)O or those on Fe-SiO(2) are converted to the beta-diketonato complexes with two beta-diketonato ligands, whereas in Fe-mica beta-diketonato complexes with one beta-diketonato ligand are formed. The formation of beta-diketonato complexes results in a slight lowering of the Fe oxidation number from 3+, probably as a result of the electron donation from the beta-diketonato ligand to Fe(3+) as a Lewis acid site. The lower numbers of beta-diketonato ligand coordinated with Fe(3+) in Fe-mica should result in a larger coordination strength for beta-diketonato ligand than that in Fe-SiO(2), which was confirmed by acetylacetone-TPD. Thus, the central carbon atom of the beta-diketonato ligand in Fe-mica is more reactive toward nucleophilic attack by the coordinated enone, leading to higher activity for the Michael reaction. (C) 2004 Elsevier Inc. All rights reserved.