Immobilization of Diphenylamine-Linked Bis(oxazoline) Ligands and Their Application in the Asymmetric Friedel-Crafts Alkylation of Indole Derivatives with Nitroalkenes

A diphenylamine-linked bis(oxazoline) ligand with trans-diphenyl substitution on the oxazoline rings has been immobilized onto one-to three-generation Frechet-type dendrimers and a C-3-symmetric core structure. The catalytic activities and enantioselectivities of these new ligands were tested in the asymmetric Friedel Crafts alkylation reactions of indole derivatives with nitroalkenes. The two types of immobilized ligands exhibited similar enantioselectivities and substrate compatibilities to the free ligand trans-DPBO we reported previously. No dendrimer effect was observed in the kinetic investigation of the Frechet-type dendrimer-immobilized ligands. The in situ recycling of the catalysts was also tested to illustrate the effect of reducing catalyst loading and the efficiency of our system.