Selective N-alkylation catalyzed by polymer gels with crosslinked domains containing iridium complexes

Abstract

Polymer gel catalysts are attractive due not only to their recyclability but also to the unique reaction environment in the internal space of the network structure. Appropriate design of the nanostructure around catalytically active sites in the gel network is particularly important. In this work, we aimed to control the activity and selectivity of the iridium-catalyzed N-alkylation of amine substrates with alcohols by incorporating an iridium complex into the crosslinked domain (CD) nanostructure of amphiphilic gels. A variety of gels with homogeneously dispersed CD structures containing iridium complexes with various crosslinking densities were prepared by the reversible addition–fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide and an iridium complex monomer using a poly(N,N-dimethylacrylamide) macro-chain transfer agent (CTA) in the presence of a divinyl crosslinker. The resulting CD gel showed catalytic activity for the N-alkylation of aniline with benzyl alcohol, and importantly, the steric effect of the CD structure allowed the selective formation of a secondary amine product by controlling the access of the substrate to the iridium complex. Thus, we demonstrated selectivity control through the design of the nanospace surrounding the catalytic center using a nanostructured amphiphilic gel. To develop a novel gel catalyst system for a selective reaction, we prepared a variety of gels with homogeneously dispersed crosslinked domain (CD) structures containing iridium complexes with various crosslinking densities. The designed CD gel catalyst catalyzed the N-alkylation of aniline with benzyl alcohol, and the steric effect of the CD structure allowed the selective formation of the secondary amine product by controlling the access of the substrate to the iridium complex.