Gold-catalyzed Cycloisomerization Reactions within Guanidinium ML Nanospheres: the Effect of Local Concentrations.

Gold-catalyzed cycloisomerization reactions have been explored using guanidinium functionalized ML nanospheres that strongly encapsulate gold complexes functionalized with a sulfonate group through hydrogen bonds. As the ML nanospheres can bind up to 24 gold complexes, the effect of local catalyst concentration on the reaction outcome can be easily evaluated. Also, the guanidinium groups of the sphere can weakly interact with the carboxylic group of the substrates, facilitating the pre-organization of the substrate near to the catalytic active site. Both effects can influence the selectivity and rate of the gold-catalyzed transformation. Challenging acetate-containing substrates with internal acetylene functional groups can be cyclized efficiently within the ML nanospheres, where the pre-organization of the substrate plays a crucial role. For 2-alkynyl benzoic acids the selectivity of the reaction can be controlled by adjusting the local concentration of gold catalyst in the guanidinium functionalized ML nanosphere.