Linked Life Data

12180328

Source:http://linkedlifedata.com/resource/pubmed/id/12180328

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PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
11
pubmed:dateCreated
2002-8-15
pubmed:abstractText
The mechanism of stoichiometric enantioselective alkene epoxidations by the D4- and D2-symmetric homochiral trans-dioxoruthenium(VI) porphyrins, [RuVI(D4-Por*)O2] (1) and [RuVI(D2-Por*)O2] (2a), in the presence of pyrazole (Hpz) was studied by UV/Vis spectrophotometry and analysis of the organic products. The enantioselectivity of styrene oxidations is more susceptible to steric effects than to substituent electronic effects. Up to 72% ee was achieved for epoxidation of 3-substituted and cis-disubstituted styrenes by employing 1 as the oxidant, whereas entantioselectivities of only 20-40% were obtained in the reactions with 2-substituted and trans-disubstituted styrenes. Complex 2a oxidized 2-substituted styrenes to their epoxides in up to 88% ee. Its reactions with trans-alkenes are more enantioselective (67% ee) than with the cis-alkenes (40% ee). Based on a two-dimensional NOESY NMR study, 2a was found to adopt a more open conformation in benzene than in dichloromethane, which explains the observed solvent-dependent enantioselectivity of its reactions with alkenes. The oxidation of aromatic alkenes by the chiral dioxoruthenium(VI) porphyrins proceeds through the rate-limiting formation of a benzylic radical intermediate; the observed enantioselectivity (eeobs) depends on both the facial selectivity of the first C-O bond formation step and the diastereoselectivity of the subsequent epoxide ring closure. To account for the observed facial selection, "side-on" and "top-on" approach transition state models are examined (see: B. D. Brandes, E. N. Jacobsen, Tetrahedron Lett. 1995, 36, 5123).
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
Jun
pubmed:issn
0947-6539
pubmed:author
pubmed:issnType
Print
pubmed:day
3
pubmed:volume
8
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
2495-507
pubmed:dateRevised
2009-8-4
pubmed:meshHeading
pubmed:year
2002
pubmed:articleTitle
Stereo- and enantioselective alkene epoxidations: a comparative study of D4- and D2-symmetric homochiral trans-dioxoruthenium(VI) porphyrins.
pubmed:affiliation
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong.
pubmed:publicationType
Journal Article, Research Support, Non-U.S. Gov't