Tetrabutylammonium iodide-catalyzed oxidative α-azidation of ß-ketocarbonyl compounds using sodium azide.

We herein report the oxidative α-azidation of carbonyl compounds by using NaN3 in the presence of dibenzoyl peroxide catalyzed by tetrabutylammonium iodide (TBAI). By utilizing these readily available bulk chemicals a variety of cyclic ß-ketocarbonyl derivatives can be efficiently α-azidated under operationally simple conditions. Control experiments support a mechanistic scenario involving in situ formation of an ammonium hypoiodite species which first facilitates the α-iodination of the pronucleophile, followed by a phase-transfer-catalyzed nucleophilic substitution by the azide. Furthermore, we also show that an analogous α-nitration by using NaNO2 under otherwise identical conditions is possible as well.

(Thio)urea Containing Chiral Ammonium Salt Catalysts.

(Thio)-urea-containing bifunctional quaternary ammonium salts emerged as powerful non-covalently interacting organocatalysts over the course of the last decade. The most commonly employed catalysts in this field are either based on Cinchona alkaloids, α-amino acids, or trans-cyclohexane-1,2-diamine. Our group has been heavily engaged in the design and use of such catalysts, i. e. trans-cyclohexane-1,2-diamine-based ones for around 10 years now, and it is therefore the intention of this short personal account to provide an overview of the, at least in our opinion, most significant and pioneering achievements in this field by looking on catalyst design and asymmetric method development, with a special focus on our own contributions.

Oxidative decarboxylative ammonium hypoiodite-catalysed dihydrobenzofuran synthesis.

The catalytic use of quaternary ammonium iodides under oxidative conditions allows for the direct conversion of readily available ß-ketolactones into dihydrobenzofurans via a decarboxylative oxidative cycloetherification sequence facilitated by an in situ formed ammonium hypoiodite species.

Asymmetric α-Chlorination of ß-Keto Esters Using Hypervalent Iodine-Based Cl-Transfer Reagents in Combination with Cinchona Alkaloid Catalysts.

We herein report an unprecedented strategy for the asymmetric α-chlorination of ß-keto esters with hypervalent iodine-based Cl-transfer reagents using simple Cinchona alkaloid catalysts. Our investigations support an α-chlorination mechanism where the Cinchona species serves as a nucleophilic catalyst by reacting with the chlorinating agent to generate a chiral electrophilic Cl-transfer reagent in situ. Using at least 20 mol-% of the alkaloid catalyst allows for good yields and enantioselectivities for a variety of different ß-keto esters under operationally simple conditions.

Enantiospecific deoxyfluorination of cyclic α-OH-ß-ketoesters.

We herein report the deoxyfluorination of cyclic α-hydroxy-ß-ketoesters using diethylaminosulfur trifluoride (DAST). The reaction proceeds with excellent levels of stereospecificity, giving the configurationally inverted α-fluoro-ß-ketoesters in high yields under operationally simple conditions.

Synergistic Ammonium (Hypo)Iodite/Imine Catalysis for the Asymmetric α-Hydroxylation of ß-Ketoesters.

The synergistic use of chiral bifunctional ammonium iodide catalysts in combination with simple catalytically relevant aldimines allows for an unprecedented asymmetric α-hydroxylation reaction of ß-ketoesters using H2O2. The reaction proceeds via in situ formation of a hypervalent iodine species, which then reacts with the used aldimine to generate an activated electrophilic oxygen transfer reagent.