Bimolecular reactions are highly dependent on the concentration of substrates. Therefore, when a supramolecular container encapsulates both reactants within its small cavity, the effective local concentration of the reactants is increased and, as a result of an entropic effect, the rate of the reaction is accelerated. That is to say an intramolecular reaction is faster than its corresponding intermolecular reaction.
Although high raise in effective concentration is observed, molecules that employ this mode of catalysis have tiny rate acceleration compared to that of enzymes. A proposed explanation is that in a container the substrates are not as tightly bound as in enzyme. The reagents have room to wiggle in a cavity and so the entropic effect might not be as important. Even in the case of enzymes, computational studies have shown that the entropic effect might also be overestimated.
Examples of molecules that work via this mechanism are Rebeks tennis ball and Fujitas octahedral complex.