Polymer-Bound Halogen Bonding Organocatalysis

Noncovalent organocatalysts utilizing halogen bonding have emerged in the last decade as efficient alternatives to more established hydrogen bonding donors. However, currently they often cannot be recycled and, in some cases, even stoichiometric amounts need to be used. Herein, we present the first generation of recyclable, polymer-bound halogen bonding catalysts which are able to catalyze a halide abstraction reaction. To this end, bis(benzimidazolium)-based halogen bonding units were attached to a polystyrene polymer, yielding insoluble catalysts. These could be recycled up to three times with little loss in catalytic activity. The overall performance of the catalysts is comparable to that of a free (non-polymer-bound) halogen bond donor. Interestingly, catalysts featuring a preorganized and a non-preorganized halogen bond donating unit showed comparable activity, in contrast to the situation of the free donors. This proof-of-principle study provides the basis for further steps towards more sustainable halogen bonding organocatalysts.