Showing papers by "Bernd Plietker published in 2005"

RuCl3/CeCl3/NaIO4: a new bimetallic oxidation system for the mild and efficient dihydroxylation of unreactive olefins.

Abstract: [reaction: see text] The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO(4) has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO(4) has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO(4)-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO(4)-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl(3) allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.

Selectivity versus reactivity : Recent advances in RuO4-catalyzed oxidations

Abstract: Since its introduction into organic chemistry more than fifty years ago, RuO 4 has been used mainly as a strong oxidizing agent. Owing to its high reactivity, catalytic reactions employing RuO 4 were often considered to be sluggish or unselective. However, within the past ten years, several groups have reported the development of new and selective oxidative transformations possible only under RuO 4 catalysis. The present article summarizes the state of research in this field and tries to give a systematic overview of the reactivity and the reaction mode of RuO 4 . In the final section, relative reactivities between olefins or olefins and functional groups are discussed. The information provided in the present paper might serve as a good starting point for chemists interested in applying RuO 4 -catalyzed reactions in organic synthesis or methodology.

New oxidative pathways for the synthesis of α-hydroxy ketones—the α-hydroxylation and ketohydroxylation

Abstract: α-Hydroxy ketones serve as versatile intermediates in organic chemistry. Although this functional group combination allows a variety of synthetic manipulation, catalytic methods for the selective generation of this structural motif, via oxidation of carbonyl compounds (α-hydroxylation) or olefins (ketohydroxylation), have recently been developed. The present review gives a brief overview of the recent developments in the field of catalytic α-hydroxylations and ketohydroxylations, with a focus on the latter reaction.

Enantioselective palladium-catalyzed addition of 1,3-dicarbonyl compounds to an allene derivative.

Abstract: Enhancing atom economy of the metal-catalyzed asymmetric allylic alkylation (AAA) shifts from the usual nucleophilic displacement of a leaving group to an addition of a pronucleophile to a double bond. Using 1-alkoxyallenes as proelectrophiles, the palladium-catalyzed AAA proceeds with 1,3-dicarbonyl compounds as pronucleophiles with excellent regioselectivity and enantiomeric excess under optimized conditions. The pH of the medium proved crucial for reactivity/ selectivity. By using the more acidic Meldrum's acids, the reactions required a co-catalytic amount of Bronsted acid. such as trifluoroacetic acid. Single regioisomeric products of 82-99% ee were obtained. On the other hand, the less acidic 1,3-diketones failed to react under such conditions. The fact that a less acidic acid like benzoic acid sufficed, suggested the need for general base catalysis as well. Thus, a mixture of triethylamine and benzoic acid proved optimal (ee's 93-99). Employment of the (/?,/?)-phenyl Trost ligand gave a product with S configuration. A model to rationalize the results has been developed.

RuCl3/CeCl3/NaIO4: A New Bimetallic Oxidation System for the Mild and Efficient Dihydroxylation of Unreactive Olefins.

Abstract: [reaction: see text] The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO(4) has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO(4) has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO(4)-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO(4)-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl(3) allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.

The RuO4-Catalyzed Ketohydroxylation. Part 1. Development, Scope, and Limitation.

Abstract: A new straightforward oxidation of C,C-double bonds to unsymmetrical alpha-hydroxy ketones using catalytic amounts of RuCl(3) and stoichiometric amounts of Oxone under buffered conditions has been developed, a reaction for which we coined the expression "ketohydroxylation". The transformation allows the direct formation of alpha-hydroxy ketones (acyloins) from olefins without intermediate formation of syn-diols. The present paper will provide detailed information starting with the underlying concept and the subsequent development of the reaction. The effect of base, solvent stoichiometry, and temperature will be discussed resulting in an improved mechanistic model that might help to explain the influence of different reaction parameters on reactivity and selectivity in RuO(4)-catalyzed oxidations of C,C-double bonds. Furthermore, an improved workup procedure allows the recovery of the ruthenium catalyst by precipitation while simplifying the overall product purification. The second part of the paper focuses on exploration of scope and limitation. A variety of substituted olefins are oxidized to alpha-hydroxy ketones in good to excellent regioselectivities and yield. Cyclic substrates proved to be problematic to oxidize; however, a careful analysis of temperature effects resulted in the development of a successful protocol for the ketohydroxylation of cyclic substrates by simply decreasing the reaction temperature.

Eisen-katalysierte allylische Alkylierung

Abstract: Die Erfindung betrifft ein Verfahren zur Durchfuhrung einer Eisen-katalysierten allylischen Alkylierung, umfassend - die Herstellung eines Reaktionsgemisches erhaltlich aus (i) einem allylischen Substrat mit dem Strukturelement C=C-C-X, wobei X eine Abgangsgruppe umfasst, welche ein Carbonat darstellt, (ii) einem aktiven Fe(-II)-Katalysatorkomplex, (iii) mindestens einem Ligand, (iv) mindestens einem Losungsmittel und (v) einem Nukleophil oder Pronukleophil.