Continuous Flow Asymmetric Hydrogenation with Supported Ionic Liquid Phase Catalysts Using Modified CO₂as the Mobile Phase: from Model Substrate to an Active Pharmaceutical Ingredient

Abstract

The continuous flow asymmetric hydrogenation of (hetero)aromatic enamides has been realized using a Rh-Quinaphos catalyst immobilized in a supported ionic liquid phase (SILP) and employing supercritical CO2 modified with toluene (modCO2) as the mobile phase. This approach allows expansion of the scope of the original SILP/scCO2 system to nonvolatile substrates with poor solubility in pure CO2. The potential of a SILP catalyst in combination with modCO2 was demonstrated for an industrial case study using the continuous flow hydrogenation for the synthesis of a key intermediate of an active pharmaceutical ingredient (API) from AstraZeneca’s portfolio. Toluene was selected as the most promising modifier, and the influence of the ratio of modifier to CO2 was evaluated in detail. The catalyst support was found to play a major role for maintaining constant performance and the use of hydrophobic fluorous reverse-phase silica (FRP-SiO2) instead of dehydroxylated silica strongly enhanced the long-term stability under continuous flow operation. Virtually a single enantiopure product was obtained over a prolonged time-on-stream of 90 h (quantitative single-pass conversion, ee > 99%) reaching a total turnover number of 10 300 at a space–time yield (STY) of 24 g L–1 h–1. No metal contamination was detected in the product solutions, indicating effective catalyst retention.