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Article: A mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins
Title | A mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins |
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Authors | |
Issue Date | 1999 |
Publisher | American 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? |
Abstract | The 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 Identifier | http://hdl.handle.net/10722/69500 |
ISSN | 2023 Impact Factor: 3.3 2023 SCImago Journal Rankings: 0.724 |
ISI Accession Number ID | |
References |
DC Field | Value | Language |
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dc.contributor.author | Liu, CJ | en_HK |
dc.contributor.author | Yu, WY | en_HK |
dc.contributor.author | Che, CM | en_HK |
dc.contributor.author | Yeung, CH | en_HK |
dc.date.accessioned | 2010-09-06T06:14:14Z | - |
dc.date.available | 2010-09-06T06:14:14Z | - |
dc.date.issued | 1999 | en_HK |
dc.identifier.citation | Journal Of Organic Chemistry, 1999, v. 64 n. 20, p. 7365-7374 | en_HK |
dc.identifier.issn | 0022-3263 | en_HK |
dc.identifier.uri | http://hdl.handle.net/10722/69500 | - |
dc.description.abstract | The 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.language | eng | en_HK |
dc.publisher | American Chemical Society. The Journal's web site is located at http://pubs.acs.org/joc | en_HK |
dc.relation.ispartof | Journal of Organic Chemistry | en_HK |
dc.title | A mechanistic investigation of alkene epoxidation by sterically encumbered trans-dioxoruthenium(VI) porphyrins | en_HK |
dc.type | Article | en_HK |
dc.identifier.openurl | http://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)+porphyrins | en_HK |
dc.identifier.email | Che, CM:cmche@hku.hk | en_HK |
dc.identifier.authority | Che, CM=rp00670 | en_HK |
dc.description.nature | link_to_subscribed_fulltext | - |
dc.identifier.doi | 10.1021/jo990517x | en_HK |
dc.identifier.scopus | eid_2-s2.0-0033215419 | en_HK |
dc.identifier.hkuros | 53007 | en_HK |
dc.relation.references | http://www.scopus.com/mlt/select.url?eid=2-s2.0-0033215419&selection=ref&src=s&origin=recordpage | en_HK |
dc.identifier.volume | 64 | en_HK |
dc.identifier.issue | 20 | en_HK |
dc.identifier.spage | 7365 | en_HK |
dc.identifier.epage | 7374 | en_HK |
dc.identifier.isi | WOS:000083036800012 | - |
dc.publisher.place | United States | en_HK |
dc.identifier.scopusauthorid | Liu, CJ=37033272100 | en_HK |
dc.identifier.scopusauthorid | Yu, WY=7403913673 | en_HK |
dc.identifier.scopusauthorid | Che, CM=7102442791 | en_HK |
dc.identifier.scopusauthorid | Yeung, CH=7201354141 | en_HK |
dc.identifier.issnl | 0022-3263 | - |