Catalytic Reactions in Cyclodextrin Cavities Aromatic Substitution

3 Catalytic Reactions in Cyclodextrin Cavities Aromatic Substitution 

In the previous reactions the cyclodextrin acted as a reactant, not a catalyst. However, there are some excellent examples in which true catalysis occurs with simple binding into a cyclodextrin cavity. Here we will describe the cases where the cyclodextrin has not been modified, while in later sections we will discuss cases in which additional catalytic groups have been added to the cyclodextrin, and mimics of metaUoenzymes and of enzymes with co-enzymes have been achieved. 

In a subsequent detailed study we saw that the chlorination of unbound anisole involved two molecules of HOCl, surely as their very reactive product CI2O, while the chlorination in the complex involved only a single molecule of HOCl. The obvious conclusion from this is that the HOCl reacts with a hydroxyl group of the cyclodextrin to make a hypochlorite that is able to donate a chlorine atom to the para position of bound anisole, which is the one accessible when the anisole is fully bound. Proximity makes up for the weak chlorinating ability of a hypochlorite. 

One extra point might be mentioned about these catalysts. There is an enzyme, chlo-rinase, that will also chlorinate anisole but with only the random 60/40 distribution between para and ortho chlorination. The enzyme obviously has not been optimized for this process, but it is fair to say that our very simple artificial enzyme, a-cyclodextrin, is actually better at selective chlorination of this substrate than the enzyme is. 

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