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Unexpected formation of a cyclopentadienylruthenium alkoxycarbonyl complex with a coordinated C=C bond
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
2009 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 38, 13622-13624 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
American Chemical Society , 2009. Vol. 131, no 38, 13622-13624 p.
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-30232DOI: 10.1021/ja905741wISI: 000270186600027OAI: oai:DiVA.org:su-30232DiVA: diva2:242241
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Mechanistic Studies on Ruthenium-Catalyzed Hydrogen Transfer Reactions
Open this publication in new window or tab >>Mechanistic Studies on Ruthenium-Catalyzed Hydrogen Transfer Reactions
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mechanistic studies on three different ruthenium-based catalysts have been performed. The catalysts have in common that they have been employed in hydrogen transfer reactions involving alcohols and ketones, amines and imines or both.

Bäckvall’s catalyst, η5-(Ph5C5)Ru(CO)2Cl, finds its application as racemization catalyst in dynamic kinetic resolution, where racemic alcohols are converted to enantiopure acetates in high yields. The mechanism of the racemization has been investigated and both alkoxide and alkoxyacyl intermediates have been characterized by NMR spectroscopy and in situ FT-IR measurements. The presence of acyl intermediates supports a mechanism via CO assistance. Substantial support for coordination of the substrate during the racemization cycle is provided, including exchange studies with both external and internal potential ketone traps. We also detected an unexpected alkoxycarbonyl complex from 5-hydroxy-1-hexene, which has the double bond coordinated to ruthenium.

Shvo’s catalyst, [Ru2(CO)4(μ-H)(C4Ph4COHOCC4Ph4)] is a powerful catalyst for transfer hydrogenation as well as for dynamic kinetic resolution. The mechanism of this catalyst is still under debate, even though a great number of studies have been published during the past decade. In the present work, the mechanism of the reaction with imines has been investigated. Exchange studies with both an external and an internal amine as potential traps have been performed and the results can be explained by a stepwise inner-sphere mechanism. However, if there is e.g. a solvent cage effect, the results can also be explained by an outer-sphere mechanism. We have found that there is no cage effect in the reduction of a ketone containing a potential internal amine trap. If the mechanism is outer-sphere, an explanation as to why the solvent cage effect is much stronger in the case of imines than ketones is needed.

Noyori’s catalyst, [p-(Me2CH)C6H4Me]RuH(NH2CHPhCHPhNSO2C6H4-p-CH3), has successfully been used to produce chiral alcohols and amines via transfer hydrogenation. The present study shows that the mechanism for the reduction of imines is different from that of ketones and aldehydes. Acidic activation of the imine was found necessary and an ionic mechanism was proposed.

Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University, 2009. 77 p.
Keyword
mechanistic studies, catalysis, ruthenium, hydrogen transfer, racemization, transfer hydrogenation, alcohols, ketones, amines, imines, inner-sphere, outer-sphere, solvent cage effect
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-27596 (URN)978-91-7155-862-6 (ISBN)
Public defence
2009-06-12, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12A, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-05-21 Created: 2009-05-08 Last updated: 2010-01-14Bibliographically approved
2. Ruthenium-Catalyzed Hydrogen Transfer Reactions: Mechanistic Studies and Chemoenzymatic Dynamic Kinetic Resolutions
Open this publication in new window or tab >>Ruthenium-Catalyzed Hydrogen Transfer Reactions: Mechanistic Studies and Chemoenzymatic Dynamic Kinetic Resolutions
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main focus of this thesis lies on transition metal-catalyzed hydrogen transfer reactions. In the first part of the thesis, the mechanism for racemization of sec-alcohols with a ruthenium complex, Ru(CO)2Cl(η5-C5Ph5) was studied.

The reaction between 5-hexen-2-ol and Ru(CO)2(Ot-Bu)(η5-C5Ph5) was studied with the aim to elucidate the origin of the slow racemization observed for this sec-alcohol. Two diastereomers of an alkoxycarbonyl complex, which has the double bond coordinated to ruthenium, were characterized by NMR and in situ FT-IR spectroscopy. The observed inhibition of the rate of racemization for substrates with double bonds provided further confirmation of the importance of a free coordination site on ruthenium for β-hydride elimination. Furthermore, we observed that CO exchange, monitored by 13C NMR using 13CO, occurs with both the precatalyst, Ru(CO)2Cl(η5-C5Ph5), and the active catalytic intermediate, Ru(CO)2(Ot-Bu)(η5-C5Ph5). It was also found that added CO has an inhibitory effect on the rate of racemization of (S)-1-phenylethanol. Both these observations provide strong support for reversible CO dissociation as a key step in the racemization mechanism.

In the second part of this thesis, Ru(CO)2Cl(η5-C5Ph5) was combined with an enzymatic resolution catalyzed by a lipase, leading to several efficient dynamic kinetic resolutions (DKR). DKR of exocyclic allylic alcohols afforded the corresponding acetates in high yields and with excellent enantiomeric excess (ee). The products were utilized as synthetic precursors for α-substituted ketones and lactones. DKR of a wide range of homoallylic alcohols afforded the products in good to high yields and with high ee. The homoallylic acetates were transformed into 5,6-dihydropyran-2-ones in a short reaction sequence. Furthermore, DKR of a wide range of aromatic β-chloroalcohols afforded the products in high yields and with excellent ee. The β-chloro acetates were further transformed into chiral epoxides.

Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University, 2013. 84 p.
Keyword
Hydrogen transfer, ruthenium catalysis, kinetic resolution, dynamic kinetic resolution, racemization, asymmetric synthesis
National Category
Natural Sciences
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-89263 (URN)978-91-7447-706-1 (ISBN)
Public defence
2013-05-24, Magnélisalen, Kemiska Övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 5: Mansucript.

Available from: 2013-04-18 Created: 2013-04-17 Last updated: 2013-11-06Bibliographically approved

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