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CO Assistance in ligand exchange of a ruthenium racemization catalyst: identification of an acyl intermediate
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.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
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2009 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 27, 9500-9501 p.Article in journal (Refereed) Published
Abstract [en]

An acyl intermediate in the activation of eta(5)-(Ph(5)Cp)Ru(CO)(2)Cl by t-BuOK was identified by means of in situ FT-IR measurements and NMR spectroscopy. This strongly supports the conclusion that the ligand exchange takes place via CO assistance, i.e., that the activation occurs via nucleophilic attack by tert-butoxide on one of the CO ligands. The tert-butoxycarbonyl intermediate shows stretching vibrations at 1933 and 1596 cm(-1), corresponding to the CO and COOt-Bu groups, respectively. In the (13)C NMR spectrum, the CO group appears at 209.5 ppm and the COOt-Bu group at 208.7 ppm. The NMR assignments were confirmed by density functional theory calculations. The subsequent alcohol-alkoxide exchange is also thought to take place via CO assistance. However, no intermediate in that step could be detected.

Place, publisher, year, edition, pages
American Chemical Society , 2009. Vol. 131, no 27, 9500-9501 p.
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-29966DOI: 10.1021/ja9038455ISI: 000268239400016OAI: oai:DiVA.org:su-29966DiVA: diva2:236580
Available from: 2009-09-24 Created: 2009-09-24 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. Theoretical modeling of metal- and enzyme catalyzed transformations
Open this publication in new window or tab >>Theoretical modeling of metal- and enzyme catalyzed transformations
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on describing and predicting catalytic reactions. The major part of the work is based on density functional theory (DFT). In some cases where the size of the investigated system precluded the use of more accurate methods molecular dynamics was employed. In several cases the proposed mechanism was later tested in the laboratory. A few examples where the predictions were confirmed are:

  • The formation of an acyl intermediate in the activation of a ruthenium catalyst used for racemizing alcohols. This intermediate was observed by both NMR and in situ FT-IR.
  • The improvement of the substrate specificity and catalytic activity of Candida antarctica lipase A by modifying amino acids close to the active site.
  • The improved specificity of Candida antarctica lipase B toward δ-substituted secondary alcohols by an enzyme variant where the alanine in position 281 was exchanged for a serine.

In other cases experimental results were complemented with a theoretical investigation, for example:

  • The observed second order rate constant for a ruthenium based catalyst used for water oxidation was explained and a novel intramolecular mechanism based on a high valent ruthenium dimer was suggested.
  • The effects of electron withdrawing/donating axial ligands on the performance of ruthenium catalyzed water oxidation were addressed.
  • Mechanisms of H2 activation by Lewis acid/Lewis base adducts were rationalized. One example of the predictive power of computational chemistry is the mechanism of hydrogen uptake by phosphanylboranes; the potential energy barrier for the transition state could be predicted within a few kcal/mol based on the orbital energies of the starting material.
Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University, 2010. 96 p.
Keyword
density functinal theory, computational chemistry, directed evolution, enzyme, mechanistic studies, catalysis, ruthenium, hydrogen transfer, racemization, artificial photosynthesis, frustrated lewis pairs, hydrogen storage
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-38344 (URN)978-91-7447-063-5 (ISBN)
Public defence
2010-05-12, Magnelisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. Paper 8: In press.Available from: 2010-04-20 Created: 2010-04-08 Last updated: 2010-05-28Bibliographically approved

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