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The importance of alkali cations in the {RuCl2(p-cymene)}(2) -pseudodipeptide-catalyzed enantioselective transfer hydrogenation of ketones
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
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2006 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 12, no 12, 3218-3225 p.Article in journal (Refereed) Published
Abstract [en]

We studied the role of alkali cations in the [{RuCl2(p-cymene)}2]- pseudo-dipeptide-catalyzed enantioselective transfer hydrogenation of ketones with isopropanol. Lithium salts were shown to increase the enantioselectivity of the reaction when iPrONa or iPrOK was used as the base. Similar transfer-hydrogenation systems that employ chiral amino alcohol or monotosylated diamine ligands are not affected by the addition of lithium salts. These observations have led us to propose that an alternative reaction mechanism operates in pseudo-dipeptidebased systems, in which the alkali cation is an important player in the ligand-assisted hydrogen-transfer step. DFT calculations of the proposed transition-state (TS) models involving different cations (Li+, Na+, and K+) confirm a considerable loosening of the TS with larger cations. This loosening may be responsible for the fewer interactions between the substrate and the catalytic complex, leading to lower enantiodifferentiation. This mechanistic hypothesis has found additional experimental support; the low ee obtained with [BnNMe3]OH (a large cation) as base can be dramatically improved by introducing lithium cations into the system. Also, the complexation of Na+, K+, and Li+ cations by the addition of [15]crown-5 and [18]crown-6 ethers and cryptand 2.1.1 (which selectively bind to these cations and, thus, increase their bulkiness), respectively, to the reaction mixture led to a significant drop in the enantioselectivity of the reaction. The lithium effect has proved useful for enhancing the reduction of different aromatic and heteroaromatic ketones.

Place, publisher, year, edition, pages
2006. Vol. 12, no 12, 3218-3225 p.
Keyword [en]
amino acids, amino alcohols, asymmetric catalysis, hydrogen transfer, ruthenium, asymmetric transfer-hydrogenation, alcohol)-catalyzed transfer hydrogenation, catalyzed transfer hydrogenation, ligand bifunctional catalysis, molecular-orbital methods, alpha-amino-acids, carbonyl-compounds, aerobic oxidation, transfer reduction, aromatic ketones
URN: urn:nbn:se:kth:diva-15630DOI: 10.1002/chem.200501384ISI: 000236989400005ScopusID: 2-s2.0-33646071853OAI: diva2:333672
QC 20100525Available from: 2010-08-05 Created: 2010-08-05Bibliographically approved

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