Deactivation of Secondary Amine Catalysts via Aldol Reaction–Amine Catalysis under Solvent-Free Conditions

Despite intense interest in amine-catalyzed stereoselective reactions, high catalyst loadings of ≥10 mol % are still common and either due to low reactivity or catalyst deactivation. Yet, few deactivation pathways are well understood. Here, we unraveled the deactivation of secondary amines by undesired aldol reaction. Mechanistic studies with peptide and prolinol silyl ether catalysts showed the generality of this so-far underappreciated catalyst deactivation pathway. The insights enabled conjugate addition reactions between aldehydes and nitroolefins on a multigram scale in the absence of solventconditions that are attractive as environmentally benign processeswith excellent product yields and stereoselectivities in the presence of as little as 0.1 mol % of a chemoselective peptidic catalyst.