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
β-ketoesters using H
2
O
2
. 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.
Dibenzoylperoxide emerged as a versatile oxidant for quaternary ammonium iodide or bromide‐catalyzed reactions of phenol derivatives with NaN3. While the use of iodides allowed for efficient benzylic azidations under these oxidative conditions, the use of bromides allowed for dearomative azidations instead. Both approaches have been successfully applied to different phenol derivatives and a first proof‐of‐concept for an enantioselective variant using chiral quat. ammonium bromides has been obtained as well.
The introduction of hypervalent iodine-based electrophilic functional group transfer reagents and the development of catalysis concepts making use of in situ generated hypervalent iodine species has significantly contributed to the advancement of organic synthesis. Especially asymmetric -functionalizations of prochiral pronucleophiles have been very successfully introduced by utilizing hypervalent iodine chemistry recently. Among the different classes of commonly used pronucleophiles, prochiral -ketoesters emerged as compounds of particular interest. Inspired by impressive recent reports, this short review therefore discusses different concepts using hypervalent iodine chemistry in asymmetric organocatalytic -functionalization reactions of prochiral -ketoesters.
We herein report an unprecedented strategy for the asymmetric α‐chlorination of β‐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 β‐keto esters under operationally simple conditions.
Dedicated to Prof. Alois Fürstner on the occasion of his 60 th birthday (thanks for giving me a chance to join your group, thanks for everything I learned from you, and thanks for all your support over all the years lieber Alois!!).Abstract: (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.
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