Synthesis, characterization and catalytic studies of bis(chloro)dioxomolybdenum(VI)-chiral diimine complexes
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
nationality International
journal JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL
author keywords molybdenum; oxide complexes; chiral diimines; homogeneous catalysis; asymmetric epoxidation
keywords ASYMMETRIC OLEFIN EPOXIDATION; ENANTIOSELECTIVE EPOXIDATION; DIOXOMOLYBDENUM(VI) COMPLEXES; ALKENE EPOXIDATION; CRYSTAL-STRUCTURES; OXAZOLINE LIGANDS; OXO COMPLEXES; MOLYBDENUM(VI); COORDINATION; PORPHYRINS
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.
publisher ELSEVIER SCIENCE BV
issn 1381-1169
year published 2005
volume 236
issue 1-2
beginning page 1
ending page 6
digital object identifier (doi) 10.1016/j.molcata.2005.03.032
web of science category Chemistry, Physical
subject category Chemistry
unique article identifier WOS:000230389700001

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