Gold(I) NHC catalysts immobilized to amphiphilic block copolymers: a versatile approach to micellar gold catalysis in water

Abstract

Fifteen gold(I)-NHC-functionalized amphiphilic block copolymers that differ in the type of linker (ethyl, pentyl, octyl and benzyl) that attaches the gold(I) NHC catalyst to the block copolymer backbone, as well as, the substitution pattern of the NHC ligand (i. e. mesityl, methyl, 2,6-diisopropylphenyl and n-hexyl) were synthesized by a reversible addition and fragmentation transfer (RAFT) polymerization process. Micelle formation of the gold(I) NHC polymers was analyzed by electron microscopy and dynamic light scattering and revealed spherical and rod-like particles from 12 to 96 nm. In the micellar, gold(I) catalyzed cycloisomerization of an allene to the corresponding dihydrofuran, linker flexibility and substitution pattern of the NHC-ligand showed a strong effect on the catalytic activity. Best results were obtained were obtained for gold(I) NHC catalysts bound to the polymer backbone by pentyl linker whereas the rather stiff benzyl linker gave lowest catalyst conversion. Moreover, the polymer catalyst could be recycled in four consecutive runs and gave activities from 35 to 84 % in the fourth run and underscores the importance of fine tuning structural parameters to achieve high conversion under micellar reaction conditions.