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Article: A mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins

TitleA mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins
Authors
Issue Date1999
PublisherAmerican Chemical Society. The Journal's web site is located at http://pubs.acs.org/joc
Citation
Journal Of Organic Chemistry, 1999, v. 64 n. 20, p. 7365-7374 How to Cite?
AbstractThe highly substituted dioxoruthenium(VI) porphyrins [Ru(VI)(DPP)O2] (1a; H2DPP = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin), [Ru(VI)(TDCPP)O2] (1b; H2TDCPP = meso-tetrakis(2,6- dichlorophenyl)porphyrin), and [Ru(VI)(TMOPP)O2] (1c; H2TMOPP = meso- tetrakis(2,4,6-trimethoxyphenyl)porphyrin) are competent oxidants for alkene epoxidation. The oxidations were carried out in a CH2Cl2/Hpz solution, and a paramagnetic bis(pyrazolato)ruthenium(IV) porphyrin, [Ru(IV)(Por)(pz)2] (2; H2Por = H2DPP, H2TDCPP, H2TMOPP), was isolated and characterized. For the oxidation of cis-alkenes, stereoselectivity is dependent upon both the alkenes and the ruthenium oxidants, and it decreases in the order: cis- stilbene > cis-β-methylstyrene > cis-β-deuteriostyrene. The observation of inverse secondary KIE for the oxidation of β-d2-styrene [k(H)/k(D) = 0.87 (1a); 0.86 (1b)] but not for the α-deuteriostyrene oxidations suggests that the C-O bond formation is more advanced at the C(β) atom than at the C(α) atom of styrene, consistent with a nonconcerted mechanism. By consideration of spin delocalization and polar effects, the second-order rate constants for the oxidation of para-substituted styrenes by complexes 1a-c can linearly correlate with the carboradical substituent constants σ(mb) and σ(JJ)· (Jiang, X.-K. Acc. Chem. Res. 1997, 30, 283). This implies that the styrene oxidation by the dioxoruthenium(VI) porphyrins should involve rate-limiting generation of a benzylic radical intermediate, and the magnitude of |ρ·(JJ)/ρ(mb)| > 1 suggests that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The spontaneous epoxidation of trans-β-methylstyrene by the sterically encumbered [Ru(VI)(TDCPP)O2] and [Ru(VI)(TMOPP)O2] complexes and the comparable ΔS((+)) values for their reactions with trans-β-methylstyrene and styrene are incompatible with the 'side-on approach' model; a 'head-on approach' model is implicated.
Persistent Identifierhttp://hdl.handle.net/10722/69500
ISSN
2015 Impact Factor: 4.785
2015 SCImago Journal Rankings: 2.095
ISI Accession Number ID
References

 

DC FieldValueLanguage
dc.contributor.authorLiu, CJen_HK
dc.contributor.authorYu, WYen_HK
dc.contributor.authorChe, CMen_HK
dc.contributor.authorYeung, CHen_HK
dc.date.accessioned2010-09-06T06:14:14Z-
dc.date.available2010-09-06T06:14:14Z-
dc.date.issued1999en_HK
dc.identifier.citationJournal Of Organic Chemistry, 1999, v. 64 n. 20, p. 7365-7374en_HK
dc.identifier.issn0022-3263en_HK
dc.identifier.urihttp://hdl.handle.net/10722/69500-
dc.description.abstractThe highly substituted dioxoruthenium(VI) porphyrins [Ru(VI)(DPP)O2] (1a; H2DPP = 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin), [Ru(VI)(TDCPP)O2] (1b; H2TDCPP = meso-tetrakis(2,6- dichlorophenyl)porphyrin), and [Ru(VI)(TMOPP)O2] (1c; H2TMOPP = meso- tetrakis(2,4,6-trimethoxyphenyl)porphyrin) are competent oxidants for alkene epoxidation. The oxidations were carried out in a CH2Cl2/Hpz solution, and a paramagnetic bis(pyrazolato)ruthenium(IV) porphyrin, [Ru(IV)(Por)(pz)2] (2; H2Por = H2DPP, H2TDCPP, H2TMOPP), was isolated and characterized. For the oxidation of cis-alkenes, stereoselectivity is dependent upon both the alkenes and the ruthenium oxidants, and it decreases in the order: cis- stilbene > cis-β-methylstyrene > cis-β-deuteriostyrene. The observation of inverse secondary KIE for the oxidation of β-d2-styrene [k(H)/k(D) = 0.87 (1a); 0.86 (1b)] but not for the α-deuteriostyrene oxidations suggests that the C-O bond formation is more advanced at the C(β) atom than at the C(α) atom of styrene, consistent with a nonconcerted mechanism. By consideration of spin delocalization and polar effects, the second-order rate constants for the oxidation of para-substituted styrenes by complexes 1a-c can linearly correlate with the carboradical substituent constants σ(mb) and σ(JJ)· (Jiang, X.-K. Acc. Chem. Res. 1997, 30, 283). This implies that the styrene oxidation by the dioxoruthenium(VI) porphyrins should involve rate-limiting generation of a benzylic radical intermediate, and the magnitude of |ρ·(JJ)/ρ(mb)| > 1 suggests that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The spontaneous epoxidation of trans-β-methylstyrene by the sterically encumbered [Ru(VI)(TDCPP)O2] and [Ru(VI)(TMOPP)O2] complexes and the comparable ΔS((+)) values for their reactions with trans-β-methylstyrene and styrene are incompatible with the 'side-on approach' model; a 'head-on approach' model is implicated.en_HK
dc.languageengen_HK
dc.publisherAmerican Chemical Society. The Journal's web site is located at http://pubs.acs.org/jocen_HK
dc.relation.ispartofJournal of Organic Chemistryen_HK
dc.titleA mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrinsen_HK
dc.typeArticleen_HK
dc.identifier.openurlhttp://library.hku.hk:4550/resserv?sid=HKU:IR&issn=0022-328X&volume=64&spage=7365&epage=7374&date=1999&atitle=A+mechanistic+investigation+of+alkene+epoxidation+by+sterically+encumbered+trans-Dioxoruthenium(VI)+porphyrinsen_HK
dc.identifier.emailChe, CM:cmche@hku.hken_HK
dc.identifier.authorityChe, CM=rp00670en_HK
dc.description.naturelink_to_subscribed_fulltext-
dc.identifier.doi10.1021/jo990517xen_HK
dc.identifier.scopuseid_2-s2.0-0033215419en_HK
dc.identifier.hkuros53007en_HK
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-0033215419&selection=ref&src=s&origin=recordpageen_HK
dc.identifier.volume64en_HK
dc.identifier.issue20en_HK
dc.identifier.spage7365en_HK
dc.identifier.epage7374en_HK
dc.identifier.isiWOS:000083036800012-
dc.publisher.placeUnited Statesen_HK
dc.identifier.scopusauthoridLiu, CJ=37033272100en_HK
dc.identifier.scopusauthoridYu, WY=7403913673en_HK
dc.identifier.scopusauthoridChe, CM=7102442791en_HK
dc.identifier.scopusauthoridYeung, CH=7201354141en_HK

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