Thermodynamically controlled chemoselectivity in lipase-catalyzed aza-Michael additions

Abstract

Chemoselective synthesis of N-protected β-amino esters involving lipase-catalyzed aza-Michael additions and α,β-unsaturated precursors is mainly hampered by the two electrophilic sites present on these compounds. In order to control the chemoselectivity a solvent engineering strategy based on the thermodynamic behaviour of products in media of different polarity was designed. This strategy allowed to obtain aza-Michael adducts from benzylamine and different acrylates with high selectivity. In almost all reactions carried out in n-hexane, a non-polar solvent, aminolysis was avoided while the corresponding Michael adducts were exclusively synthesized in 53–78% yields. On the contrary, in reactions carried out in a polar solvent such as 2-methyl-2-butanol the aminolysis products were favoured. Thermodynamic analyses of these processes using the COSMO-RS method helped to understand some of the key factors affecting chemoselectivity and confirmed that a reliable estimation of the thermodynamic interactions of solutes and solvents allows an adequate selection of a reaction media that may lead to chemoselectivity.