Synthesis and catalytic properties in olefin epoxidation of chiral oxazoline dioxomolybdenum(VI) complexes

Abstract

Dioxomolybdenum(VI) complexes with the general formula [MoO 2X 2(N,N)] (X = Cl, OSiPh 3) containing a chiral bidentate oxazoline ligand (N,N = 2,2′-bis[(4 S)-4-benzyl-2-oxazoline]) have been prepared and characterised by 1H NMR, IR spectroscopy and thermogravimetric analysis. The bis(chloro) complex was heterogenised in the ordered mesoporous silica MCM-41 by direct grafting in dichloromethane. Elemental analysis and 29Si MAS NMR spectroscopy of the derivatised material indicated the presence of monopodally anchored species of the type MoO 2[(–O) 3SiO]Cl(N,N). The complex [MoO 2Cl 2(N,N)] and the derivatised material exhibited initial activities of 147 and 255 mol mo l Mo − 1 h − 1 , respectively, in the catalytic epoxidation of cyclooctene using tert-butylhydroperoxide ( tBuOOH) as the oxidant, both yielding 1,2-epoxycyclooctane quantitatively within 24 h at 55 °C. The MCM-41 grafted catalyst could be recycled with no loss in performance with respect to the epoxide yields obtained for reaction times above 2 h. With trans-β-methylstyrene as the substrate, the bis(chloro) complex and the derivatised material gave epoxides as the only products with yields in the range of 56–64% after 24 h, but no catalytic asymmetric induction was observed. The triphenylsiloxy complex was more active than the bis(chloro) complex for the epoxidation of trans-β-methylstyrene, but the enantiomeric excess was negligible and the corresponding diols were also formed. For the reaction catalysed by the supported material, changing the oxidant from tBuOOH to cumene hydroperoxide greatly improved the catalytic activity but the enantiomeric excess continued very low and the corresponding diol was the main product.