Diastereoselectivity in the triethylamine-catalyzed sulfa-Michael addition of thiols to nitroalkenes: kinetic and thermodynamic control

Abstract

The diastereoselectivity in the triethylamine-catalyzed sulfa-Michael addition of nitroalkenes and thiols was investigated. The sulfa-Michael addition is kinetic control at the beginning and thermodynamic control at the end for less bulky reactants. Thus, kinetic and thermodynamic-controlled adducts can be obtained as major products by controlling the reaction time in those cases. Linear nitroalkenes generally produce anti-adducts as major kinetic products due to favorable steric and stereoelectronic effects, but the diastereoselectivity decreases obviously with steric increase of the substituent located in the vicinal olefinic carbon to the nitro group, even leading to syn-adducts as major kinetic products. 1-Nitrocyclohexene gives rise stereospecifically to kinetic cis-adduct, which epimerizes into more stable trans-adduct as major product through the thermodynamic equilibrium. However, the Michael additions involving bulky reactants are generally slow, resulting in the direct generation of thermodynamic adducts.