Synthesis, characterization and catalytic studies of bis(chloro)dioxomolybdenum(VI)-chiral diimine complexes

Abstract

Chiral 1,4-diazabutadienes (DAB) of the type R * N CPh CPh N R * were prepared in quantitative yields by condensation of benzil with two equivalents of R-(+)-α-methylbenzylamine or S-(−)-α-methylbenzylamine, using ZnCl 2 as catalyst. The chiral diimine (1 R,2 R)- N, N′-dibenzylidenecyclohexane-1,2-diamine was also prepared by condensation of (1 R,2 R)-cyclohexane-1,2-diamine with two equivalents of benzaldehyde using a Dean–Stark adapter for the removal of water. Six-coordinate dioxomolybdenum(VI) complexes of the type [MoO 2Cl 2L] containing the bidentate chiral ligands were prepared and characterized by FT-IR, FT Raman and NMR spectroscopy. The complexes were evaluated as catalysts for the asymmetric epoxidation of cis- and trans-β-methylstyrene by tert-butylhydroperoxide at either room temperature or 55 °C. The reactions proceeded with high retention of olefin configuration and high selectivity to the epoxide, but only for cis-β-methylstyrene were significant enantiomeric excesses (e.e.) obtained. With this substrate and the complex containing (1 R,2 R)- N, N′-dibenzylidenecyclohexane-1,2-diamine, (1 S,2 R)- cis-β-methylstyrene oxide was obtained in 77% e.e. at room temperature (24% conversion). Increasing the reaction temperature increased the epoxide yields but good enantiomeric excesses (≥65%) could only be achieved at low conversions (≤12%). The two complexes containing the chiral DAB ligands generally exhibited higher catalytic activity than the third complex but lower optical yields.