Integrating membrane separation with gold‐catalyzed carboxylative cyclization of propargylamine and catalyst recovery via organic solvent nanofiltration

Abstract

AbstractBACKGROUNDTransition metal catalysis has, over recent decades, developed into one of the most important methods of constructing molecules. However, although efficient in the construction of complex molecules, these catalysts can be expensive as they are often based upon second‐ and third‐row transition metals which have, for the most part, low terrestrial abundance. Present removal and recovery techniques for homogeneous catalysts tend, at best, to focus on recovering the metal center and to consider ligands as single‐use components. Recovery of the whole catalyst would be more beneficial from economic and environmental points of view.RESULTSThe integration of a membrane separation protocol with gold‐catalyzed carboxylative cyclization of propargylamine was investigated. Filtration conditions were identified in membrane screening experiments with the dinuclear catalyst [Au2Cl2(L)] (Au‐1) and the mononuclear catalyst [Au(IPr)Cl] (Au‐2). Recovery of the whole catalyst was then investigated. Catalyst Au‐1 proved unstable and led to metal recovery for reprocessing upon completion of the process. However, the membrane methodology does allow recovery of the catalyst within the reaction mixture which increases its efficiency. On the other hand, catalyst Au‐2 proved more stable and can be recovered along with its ancillary ligands.CONCLUSIONSFor both catalysts a membrane‐based recovery protocol was successfully demonstrated. This protocol was either internal or external in that the Au complex along with its ligands can be recovered after reaction. This initial work shows the possibility that ligands do not always need to be considered as single‐use components. © 2021 Society of Chemical Industry (SCI).