Intramolecular Crossed Benzoin And Sulfa-michael Addition Reactions Catalyzed By N-Heterocyclic Carbenes

Firstly, based on the N-Heterocyclic carbene-catalyzed umpolung strategy with aldehyde, NHCs are successfully used to catalyze the intramolecular crossed benzoin reaction(aliphatic, aromatic carbonyl aldehydes) by asymmetry to desymmetrization with good yield and high enaotioselectivity which contains two quaternary centers. On the other hand, NHCs as a Br?nsted base which can also catalyze the sulfa-Michael addition and dithiolation reaction of α-haloenals and thiols. The thesis consists of the following three sections:1. Intramolecular Cross Benzoin ReactionsThree chiral and one achiral N-Heterocyclic Carbene precursors and 21 symmetric carbonyl aldehydes were synthesized. Based on asymmetry desymmetrization strategy, NHCs catalyze the Intramolecular Cross Benzoin Reactions which involves aliphatic, aromatic, cyclic and non-cyclic carbonyl aldehydes to produce19 polycyclic tertiary alcohols including two quaternary stereocenters with 48-99% yields, 20-99%enantioselectivity and > 20:1 diastereoselectivity.2. Sulfa-Michael Addition ReactionsIn the presence of 6 mol% NHC precursors and 5 mol% base, various thiols undergo conjugate addition to α,β-unsaturated ketone, ester, amide, sulfone to produce 27 β-sulfydryl products in 79% to99% yields. Finnaly, a proposed mechanism was given in which NHC acted as a good Br?nsted base..3. Dithiolation ReactionsBaesd on the sulfa-Michael addition study, the dithiolation reaction of α-haloenals and thiols was developed. In the presence of 5 mol% IMes·HCl precursors, 1.2 equiv triethylamine and 1.2 equiv lithium chloride, various primary, secondary and tertiary thiols couple with α-haloenals to give 11 kinds of2,3-dithioaldehydes in 23% to 99% yields. In this process, NHC acted as Br?nsted base to activate the thiols and undergo the conjugate addition and Baldwin’s cyclization to form sulfonium ion intermediate,then a second molecule of thiol attack the sulfonium ion to produce the desired product.In summary, N-Heterocyclic Carbene-catalyzed Intramolecular Cross Benzoin, Michael Addition and Dithiolation reactions allow to construct various C-C and C-S bond. The advantage of these reactions include mild reaction conditions, environment friendly and high catalytic efficiency which add new vitality for the development of N-Heterocyclic Carbene.