Highly Chemoselective Direct Crossed Aliphatic-Aromatic Acyloin Condensations with Triazolium-Derived Carbene Catalysts
Sarah E. O'Toole, Christopher A. Rose, Sivaji Gundala, Kirsten Zeitler, and Stephen J. Connon, J. Org. Chem. 2010, in press.
Web edition: http://dx.doi.org/10.1021/jo101791w
Abstract: It has been shown for the first time that triazolium precatalysts promote (in the presence of base) highly chemoselective crossed acyloin condensation reactions between aliphatic and ortho-substituted aromatic aldehydes. An o-bromine atom can serve as a temporary directing group to ensure high chemoselectivity (regardless of the nature of the other substituents on the aromatic ring) which then can be conveniently removed. The process is of broad scope and is operationally simple as it does not require the preactivation of any of the coupling partners to ensure selectivtiy. Preliminary data indicate that highly enantioselective variants of the reaction are feasible using chiral precatalysts.
Metal-Free, Cooperative Asymmetric Organophotoredox Catalysis with Visible Light
M. Neumann, S. Füldner, B. König, K. Zeitler, Angew. Chem. Int. Ed. 2010, in press.
Web edition: http://dx.doi.org/10.1002/anie.201002992
Abstract: "The dawn of old stars" Classic xanthene dyes like eosin Y (gr. εος=goddess of dawn) and green-light irradiation can replace precious metal complexes for the organocatalytic asymmetric-alkylation of aldehydes rendering the process purely organic.
Efficient Catalytic, Oxidative Lactonization for the Synthesis of Benzodioxepinones using Thiazolium Derived Carbene Catalysts
C.A. Rose, K. Zeitler, Org. Lett. 2010, 12, 4552-4555.
Web edition: http://dx.doi.org/10.1021/ol101854r
Abstract: An efficient, oxidative carbene-catalyzed lactonization reaction has been developed. Using thiazolium precatalysts, a variety of benzodioxepinone products are accessible in good to excellent yields under mild and operationally simple conditions. The reaction does not require high dilution conditions and proceeds via mild and selective oxidation with azobenzene, which can easily be recovered and reused applying inexpensive FeCl3 as a formal terminal oxidant.
Efficient, Enantioselective Iminium Catalysis with an Immobilized, Recyclable Diarylprolinol Silyl Ether Catalyst
I. Mager, K. Zeitler, Org. Lett. 2010, 12, 1480-1483.
Web edition: http://dx.doi.org/10.1021/ol100166z
Abstract: A highly efficient approach for the synthesis, application, and recycling of immobilized diarylprolinol silyl ethers was developed. The MeOPEG-supported Jørgensen-Hayashi catalyst provides unchanged reactivity and selectivity as compared to the homogeneous catalyst, as demonstrated for the Michael addition of nitromethane to α,β-unsaturated aldehydes via iminium activation. In addition, the immobilization allows for a simple, column-free isolation of pure, sensitive aldehyde products and therefore may be useful for application in more complicated syntheses.
Photoredox Catalysis with Visible Light
K. Zeitler, Angew. Chem. Int. Ed. 2009, 48, 9785-9789
Web edition: http://dx.doi.org/10.1002/anie.200904056
Abstract: Recent examples of visible light promoted photoredox catalysis in the presence of [Ru(bipy)3]2+ as efficient photocatalyst have set new standards for effecting challenging reactions under mild and environmental benign conditions.
Highly Enantioselective Benzoin Condensation Reactions Involving a Bifunctional Protic Pentafluorophenyl-Substituted Triazolium Precatalyst
L. Baragwanath, C. A. Rose, K. Zeitler, S. J. Connon, J. Org. Chem. 2009, 74, 9214-9217.
Web edition: http://dx.doi.org/10.1021/jo902018j
Abstract: Improved catalyst design by incorporating a hydrogen bond donating substituent to improve enantiocontrol together with an acidifying pentafluorophenyl substituent to enhance catalyst efficiency results in a triazolium ion precatalyst that promotes the asymmetric archetypal benzoin condensation with excellent efficiency and unprecedented enantioselectivity.
An Efficient Carbene-Catalyzed Access to 3,4-Dihydrocoumarins
K. Zeitler, C. A. Rose, J. Org. Chem. 2009, 74, 1759-1762
Web edition: http://dx.doi.org/10.1021/jo802285r
Abstract: Dihydrocoumarins play an important role as flavor and fragrance compounds and can be prepared efficiently from o-hydroxycinnamaldehydes in a mild, atom-economic N-heterocyclic carbene-catalyzed redox lactonization. Corresponding coumarins are accessible via a one-pot domino oxidation lactonization procedure in the presence of oxidants.
N-Heterocyclic Carbenes - Organocatalysts Displaying Diverse Modes of Action
K. Zeitler, E. Schering Found. Symp. Proc. 2007, 2, 183-206
S. Jaroch, H. Weinmann, K. Zeitler, ChemMedChem 2007, 2, 1261-1264
Web edition: http://dx.doi.org/10.1002/cmdc.200700109
Abstract: Despite considerable efforts to explore and extend the scope of asymmetric organocatalytic reactions throughout the last years, their use in medicinal and process chemistry is still rather low. This minireview highlights some of the recent developments in the rapidly evolving field of organocatalysis.
An Efficient and Versatile Approach for the Immobilization of Carbene Precursors via Copper-catalyzed [3+2]-Cycloaddition and their Catalytic Application
K. Zeitler, I. Mager, Adv. Synth. Cat. 2007, 349, 1851-1857
Web edition: http://dx.doi.org/10.1002/adsc.200700174
Abstract: Different classes of alkynyl-substituted heterazolium derivatives could be covalently immobilized on an azide-functionalized support via copper-catalyzed 1,3-dipolar cycloaddition, which efficiently yields a rigid and robust 1,2,3-triazole linkage. The catalytic performance of the corresponding nucleophilic carbenes (NHCs) was examined in intramolecular Stetter reactions (chroman-4-one products) and organocatalytic redox esterifications (α,β-unsaturated esters). The MeOPEG-immobilized organocatalysts are highly active, and show comparable diastereoselectivities to non-supported derivatives. Additionally, they allow simplified work-up procedures and also have proven to be recyclable.
Highlighted in Synfacts 2007, 1221: "MeOPEG-Supported Carbene Catalysts"
Stereoselective Synthesis of (E)-α,β-Unsaturated Esters via Carbene-Catalyzed Redox Esterification
K. Zeitler,Org. Lett. 2006, 8, 637-640
Web edition: http://dx.doi.org/10.1021/ol052826h
Abstract: Stereoselective, carbene-mediated redox esterification of alkynyl aldehydes provides mild and atom economical access to (E)-configurated, α,β-unsaturated carboxylic esters. The organocatalytic method relies on the generation of activated carboxylates via extended/conjugated umpolung in the presence of catalytic amounts of carbene precursor and base.
Extending Mechanistic Routes in Heterazolium Catalysis-Promising Concepts for Versatile Synthetic Methods
K. Zeitler,Angew. Chem. Int. Ed. 2005, 44, 7506-7510
Web edition: http://dx.doi.org/10.1002/anie.200502617
Abstract: A broad spectrum of valuable cyclic and acyclic products are available in diastereo- and/or enantioselective form by sustainable, organocatalytic processes like the heterazolium-catalyzed transformations shown in the scheme. The carbene-mediated generation of activated carboxylate intermediates provides new possibilities for the straightforward synthesis of multifunctionalized compounds.