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Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of Imine to Ruthenium Prior to Hydrogen Transfer
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.
Kungliga Tekniska Högskolan.
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2006 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 128, no 44, 14293-14305 p.Article in journal (Refereed) Published
Abstract [en]

Reaction of [2,3,4,5-Ph4(η5-C4COH)Ru(CO)2H] (2) with different imines afforded ruthenium amine complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed to give [Ru2(CO)4(μ-H)(C4Ph4COHOCC4Ph4)] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope effect (kRuHOH/kRuDOD = 1.05) observed in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the rate-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine 8 by [2,3,4,5-Ph4(η4-C4CO)Ru(CO)2] (A), the kinetic isotope effects observed support a stepwise hydrogen transfer where the isotope effect for C−H cleavage (kCHNH/kCDNH = 3.24) is equal to the combined (C−H, N−H) isotope effect (kCHNH/kCDND = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine (14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded 90−100% of complex [2,3,4,5-Ph4(η4-C4CO)]Ru(CO)2NH(CH3)(CHPhCH3) (15), which is the complex between ruthenium and the amine newly generated from the imine. At −80 °C the reaction of hydride 2 with 4-BnNH-C6H9=NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph44-C4CO)](CO)2RuNH(Ph)(C6H10-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above −8 °C complex 19 rearranged to the thermodynamically more stable [Ph4(η4-C4CO)](CO)2RuNH(Bn)(C6H10-4-NHPh) (20). These results are consistent with an inner sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are difficult to explain with the outer sphere pathway previously proposed.

Place, publisher, year, edition, pages
Washington, DC: American Chemical Society , 2006. Vol. 128, no 44, 14293-14305 p.
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-27591DOI: 10.1021/ja061494oOAI: oai:DiVA.org:su-27591DiVA: diva2:216394
Available from: 2009-05-08 Created: 2009-05-08 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

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