Research Of The Umpolung Reaction Of α, β-Unsaturated Aldehydes Prompted By The Chiral N-heterocyclic Carbene

N-heterocyclic carbene is a recently developed nucleophilic organocatalyst. Itcan efficiently catalyze the umpolung reactions of aldehydes via the “Breslowintermediate”, which is formed through the nucleophilic additon of N-heterocycliccarbene and aldehyde. This reactive intermediate can be trapped by various ofelectrophilic species. The Breslow intermediates demonstrate different propertieswhen different aldehydes were employed. It can be transformed into variousimportant intermediates such as enolate, homoenolate, acyl azolium andα,β-unsaturated acyl azolium, etc., which further result in a series of reactions.Moreover, it can also be used as a chiral catalyst in the asymmetric synthesis.This work was divided into two parts. In the first part, we have synthesized aracemic and five chiral triazolium N-heterocyclic carbene precursors according tothe reported methods. Next, we use these chiral triazolium presursors for theannulation reaction of ynals with1,3-dicarbonyls. Moreover, we optimized thereaction conditions and investigated the substrate scope under this optimizedcondition. We found that this reaction has a wide substrate scope and moderate toexcellent yield, as well as excellent enantioselectivities which were up to98%. Theabsolute configuration of product was comfirmed by deriving method.In the second part, we investigated the reaction of α,β-unsaturated aldehyde,trifluoromethyl sulfonium salt and alcohol. Unfortunately, no desird product wasobtained. Accordingly to this protocol it was failed. Then we try to investigate thereaction for CF3-containing substrates. At first, we use the racemic N-heterocycliccarbene precursors to catalyze the reaction of β-trifluoromethyl cinnamaldehydewith alcohol. Fortunately, the desired β-trifluoromethyl carboxylic acid ester wassuccessfully obtained in high yield. Then we used these chiral triazolium salts as theasymmetric synthesis. The primary experimental results shown that the ee value ofthe product was up to28%.