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-(+)-alpha-methylbenzylamine or S-(-)-alpha-methylbenzylamine, using ZnCl(2) as catalyst. The chiral diimine (1R,2R)-N,N'-dibenzylidenecyclohexane-1,2-diamine was also prepared by condensation of (1R,2R)-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(2)Cl(2)L] 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-beta-methylstyrene by tert-butylhydroperoxide at either room temperature or 55 degrees C. The reactions proceeded with high retention of olefin configuration and high selectivity to the epoxide, but only for cis-beta-methyl styrene were significant enantiomeric excesses (e.e.) obtained. With this substrate and the complex containing (1R,2R)-N,N'-dibenzylidenecyclohexane-1,2-diamine, (1S,2R)-cis-beta-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. (c) 2005 Elsevier B.V All rights reserved.
keywords
ASYMMETRIC OLEFIN EPOXIDATION; ENANTIOSELECTIVE EPOXIDATION; DIOXOMOLYBDENUM(VI) COMPLEXES; ALKENE EPOXIDATION; CRYSTAL-STRUCTURES; OXAZOLINE LIGANDS; OXO COMPLEXES; MOLYBDENUM(VI); COORDINATION; PORPHYRINS
subject category
Chemistry
authors
Gago, S; Rodriguez-Borges, JE; Teixeira, C; Santos, AM; Zhao, J; Pillinger, M; Nunes, CD; Petrovski, Z; Santos, TM; Kuhn, FE; Romao, CC; Goncalves, IS
our authors
Cátia Teixeira
Post-doc Fellowship
Isabel Maria de Sousa Gonçalves
Associate Professor
Martyn Pillinger
Principal Researcher