Expanding the Toolbox of Heterogeneous Asymmetric Organocatalysts: Bifunctional Cyclopropenimine Superbases for Enantioselective Catalysis in Batch and Continuous Flow

A strategy for the immobilization of chiral 2,3-bisaminocyclopropenium salt (pre-catalyst) onto polystyrene and silica supports is presented together with a suitable procedure for the conversion into the corresponding cyclopropenimine superbase catalysts. The activity and recyclability of polystyrene- and silica-supported cyclopropenimines were initially tested under batch conditions in a model Michael addition detecting comparable efficiencies but a superior stability of the latter heterogeneous catalyst (5 cycles, accumulated TON of 27.1). The preferred silica-supported cyclopropenimine behaved very similarly to the soluble counterpart in the reaction of glycine imine with different Michael acceptors (48-92% yield; 60-98% ee) and it could be utilized as packing material for the fabrication of fixed-bed mesoreactors (pressure-resistant stainless-steel columns). Continuous-flow experiments were performed with satisfactory long-term stability (24 h on stream) with unaltered conversion efficiency and enantioselectivity.