RESUMO
Using cinchona alkaloid-derived primary amines as catalysts and aqueous hydrogen peroxide as the oxidant, we have developed highly enantioselective Weitz-Scheffer-type epoxidation and hydroperoxidation reactions of α,ß-unsaturated carbonyl compounds (up to 99.5:0.5 er). In this article, we present our full studies on this family of reactions, employing acyclic enones, 5-15-membered cyclic enones, and α-branched enals as substrates. In addition to an expanded scope, synthetic applications of the products are presented. We also report detailed mechanistic investigations of the catalytic intermediates, structure-activity relationships of the cinchona amine catalyst, and rationalization of the absolute stereoselectivity by NMR spectroscopic studies and DFT calculations.
Assuntos
Alcaloides de Cinchona/química , Compostos de Epóxi/química , Peróxido de Hidrogênio/química , Alcenos/química , Aminas/química , Catálise , Ciclização , Cicloexanonas/química , Cromatografia Gasosa-Espectrometria de Massas , Hidrólise , Iminas/química , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Compostos de Amônio Quaternário/química , Espectrometria de Massas por Ionização por Electrospray , Estereoisomerismo , Relação Estrutura-Atividade , Ácido Trifluoracético/químicaRESUMO
Recently, the use of enantiomerically pure counteranions for the induction of asymmetry in reactions proceeding through cationic intermediates has emerged as an exciting new concept, which has been termed asymmetric counteranion-directed catalysis (ACDC). Despite its success, the concept has not been fully defined and systematically discussed to date. This Review closes this gap by providing a clear definition of ACDC and by examining both clear cases as well as more ambiguous examples to illustrate the differences and overlaps with other catalysis concepts.
RESUMO
Counteranion control enables the enantioselective, organo Lewis acid catalyzed Hosomi-Sakurai reaction of (hetero)aromatic aldehydes and allylsilanes using an easily handled disulfonimide precatalyst (see scheme). The key to the success of this system is to turn the usually undesired silylium ion catalysis into the desired catalytic regime and pair the cation with an enantiopure disulfonimide anion, thereby applying the concept of asymmetric counteranion-directed catalysis.
