Asymmetric Michael Addition between Nitroolefins and Sulfur Ylides Catalyzed by a Thiourea Organocatalyst

Abstract

Using density functional theory calculations, we have studied the 1-(2-chlorophenyl)-2-thiourea catalyzed reaction of nitrostyrene with a typical sulfur ylide to understand the Michael addition mechanism. Transition state structures for the C―C bond-forming step controlling the stereoselectivity of the reaction have been identified and their relative stabilities evaluated. The role of the catalyst in the reaction has also been determined. The calculated results show that the formation of the anti-product is energetically more favorable than that of the syn-product. Furthermore, the catalyst (proton donor) promotes the reaction by forming a double hydrogen-bonded complex with nitrostyrene (proton acceptor), where the charge transfer between the donor and acceptor increases the eletrophilicity of β-C atom of the nitrostyrene, favoring the nucleophilic attack of the sulfur ylide.