Study On Substituted Reactions Catalyzed By

N-Heterocyclic carbenes (NHCs), as a kind of versatile organocatalysts, can be employed to construct carbon-carbon bonds. NHCs are mainly used for the umpolung of aldehydes, in which addition of the NHC to an aldehyde results in the generation of an acyl anion equivalent, then reacts with the other molecule of aldehyde or Michael acceptors. The two most well-known addition reactions are the benzoin condensation and the Stetter reaction. In recent years, important advances in NHC catalyzed reactions have been achieved in this field. However, compared with the successfully developed NHC-catalyzed addition reactions, the utilization of NHCs for the nucleophilic substitution reactions is still lacking; only a few examples have been explored. As one of the most fundamental reaction types in organic chemistry, the SN2’ reaction catalyzed by NHC has not been reported.This dissertation consists of two parts. The first part reviewed the development of NHCs as organocatalyst in recent years, including the benzoin condensation, the Stetter reaction and nucleophilic substitution reactions.In the second part, the first NHC-catalyzed intramolecular SN2’ substitution reaction of aldehyde with allylic electrophiles is described. A variety of a, β-unsaturated chromanones were obtained through a domino SN2’ reaction and isomerization. We optimized the reaction by screening various NHCs、bases、solvents and temperature systematically and found the optimal conditions. Having identified the optimized conditions, we examined this new method in a range of substrates with different substitution patterns of the aromatic ring and different leaving groups. Overall, a variety of a, β-unsaturated chromanones were successfully prepared smoothly. Finally, mechanistic studies revealed that this reaction does not follow an addition-elimination mechanism, but a SN2’ type reaction.