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Racemization of alcohols catalyzed by [RuCl(CO)25-pentaphenylcyclopentadienyl)] – Mechanistic insights from theoretical modeling
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: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 15, no 21, 5220-5229 p.Article in journal (Refereed) Published
Abstract [en]

Two possible pathways of inner-sphere racemization of sec-alcohols by using the [RuCl(CO)(2)(eta(5)-pentaphenylcyclopentadienyl)] catalyst (1) have been thoroughly investigated by means of density function calculations. To be able to racemize alcohols, catalyst 1 needs to have a free coordination site on the metal. This can be achieved either by a eta(5)-->eta(3) ring slippage or by dissociation of a carbon monoxide (CO) ligand. The eta(5)-->eta(3) ring-slip pathway was found to have a high potential energy barrier, 42 kcal mol(-1), which can be explained by steric congestion in the transition state. On the other hand, CO dissociation to give a 16-electron complex has a barrier of only 22.6 kcal mol(-1). We have computationally discovered a mechanism involving CO participation that does not require eta(5)-->eta(3) ring slippage. The key features of this mechanism are 1) CO-assisted exchange of chloride for alkoxide, 2) alcohol-alkoxide exchange, and 3) generation of an active 16-electron complex through CO dissociation with subsequent beta-hydride elimination as the racemization step. We have found a low-energy pathway for reaction of 1 with potassium tert-butoxide and a pathway for fast alkoxide exchange with interaction between the incoming/leaving alcohol and one of the two CO ligands. We predict that dissociation of a Ru-bound CO ligand does not occur in these exchange reactions. Dissociation of one of the two Ru-bound CO ligands has been found necessary only at a later stage of the reaction. Though this barrier is still quite high, our results indicate that it is not necessary to cross the CO dissociation barrier for the racemization of each new alcohol. Thus, the dissociation of a CO ligand is interpreted as a rate-limiting reaction step in order to create a catalytically active 16-electron complex.

Place, publisher, year, edition, pages
Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA , 2009. Vol. 15, no 21, 5220-5229 p.
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-29969DOI: 10.1002/chem.200900291ISI: 000266419600010OAI: oai:DiVA.org:su-29969DiVA: diva2:236584
Available from: 2009-09-24 Created: 2009-09-24 Last updated: 2017-12-13Bibliographically approved
In thesis
1. 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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