resumo
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.
palavras-chave
ASYMMETRIC OLEFIN EPOXIDATION; ENANTIOSELECTIVE EPOXIDATION; DIOXOMOLYBDENUM(VI) COMPLEXES; ALKENE EPOXIDATION; CRYSTAL-STRUCTURES; OXAZOLINE LIGANDS; OXO COMPLEXES; MOLYBDENUM(VI); COORDINATION; PORPHYRINS
categoria
Chemistry
autores
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
nossos autores
Cátia Teixeira
Bolseiro de pós-Doutoramento
Isabel Maria de Sousa Gonçalves
Professor associado
Martyn Pillinger
Investigador Principal