Investigation Of [3+3] And [4+2] Cyclization Reactions Catalyzed By N-heterocyclic Carbenes

Since the separation of stable N-heterocyclic carbene(NHC),as a new class of organocatalysts, NHC has been widely used in a variety of reactions. This thesis is mainly focused on the investigation of NHC catalyzed asymmetric [3+3] cyclization reactions of α, β-unsaturated carboxylic acid and 1,3-dicarbonyl compounds, forming chiral pyrone derivatives in good to excellent yields and enantioselectivities.Furthermore, a NHC-catalyzed [4+2] cyclization reaction of γ-fluorinated α,β-unsaturated aldehyde with diazoacetyl esters was also studied, and the desired pyridazinone derivatives were obtained in good to excellent yields.The main contents of this thesis are as follows:In the first chapter, we introduce the history of NHCs and elaborated the three important intermediates categories generated in NHC catalyzed organic reactions in detail, they are: 1. the homoenolate equivalent intermediates, 2. enolate equivalent intermediates, 3. α,β- unsaturated acyl oxazole intermediates.In the second chapter, a NHC catalyzed asymmetric [3+3] cycloaddition reactions of α,β-unsaturated carboxylic acid and 1,3-dicarbonyl compound was investigation. In this reaction, the α,β-unsaturated carboxylic acid was used instead of the corresponding unstable and expensive aldehydes and esters by using coupling reagent EDCI as activation reagent, which makes the reaction much easier and cheaper.At last, a NHC catalyzed [4+2] cycloaddition reactions of γ-fluorinated α,β-unsaturated aldehyde and diazoacetyl esters was investigated. We found that the F atom in γ-position of the unsaturated aldehyde could act as good leaving group in the presence of NHC catalyst, leading to a good nucleophilic intermediate which can react with diazoacetyl esters to form the desired pyridazinone derivatives in good to excellent yields. In this investidation, the traditional expensive oxidant could be avoid,which makes this methodology highly efficient and low cost.