Dynamic covalent chemistry

Dynamic covalent chemistry is a strategy in supramolecular chemistry with the aim to synthesise large complex molecules. In it a reversible reaction is under thermodynamic reaction control and a specific reaction product out of many possible reaction products is captured. Because all the components in the reaction mixture are able to equilibrate quickly, (according to its advocates) some degree of error checking and proof reading is enabled. The concept of dynamic covalent chemistry was demonstrated in the development of specific molecular Borromean rings.

The concept is also demonstrated in a reaction sequence involving polyacetal macrocycles. The cyclophane C2 can be prepared by the irreversible highly diluted reaction of a diol with chlorobromomethane in the presence of sodium hydride. The dimer however is part of series of equilibria between polyacetal macrocycles of different size brought about by acid catalyzed (triflic acid) transacetalization. This particular type of transacetalization goes by the name of formal metathesis because it is reminiscent of olefin metathesis but then with formaldehyde. Regardless of the starting material, C2, C4 or a high molar mass product, the equilibrium will eventually produce an identical product distribution. In this system it is also possible to amplify the presence of C2 in the mixture when the catalyst is silver triflate because the silver ion fits ideally and irreversibly in its cavity.