Dynamic Covalent Chemistry

Dynamic covalent chemistry forms the key ingredient for a new approach towards organic synthesis. Dynamic covalent chemistry regards the synthesis of covalent organic molecules under thermodynamic control. It relies on the use of covalent bonds that can be reversibly formed under the experimental conditions. It combines the advantages typically associated with noncovalent synthesis (the formation of molecular structures using noncovalent interactions), such as spontaneous formation, error-correction and proof reading, with the robustness of covalent bonds. For many covalent bonds experimental conditions are known under which the reaction occurs reversibly, but the majority of systems relies on trans(thio)esterifications, disulfide or imine-type exchange reactions. These reactions occur rapidly under mild conditions, use readily accessible building blocks and can be turned off to obtain kinetically inert products. It is illustrated that dynamic covalent chemistry gives a rapid access towards organic structures of nanosized dimensions. Typically, a one-step, one-pot synthetic protocol is used involving in some cases also multiple dynamic covalent bonds in an orthogonal manner. The fundamental difference with covalent synthesis is that equilibrium reactions are used. This introduces a characteristic property, which is the possibility of the chemical system to adapt to changes in its environment according to Le Châtelier’s principle. This concept is referred to as dynamic combinatorial chemistry and has been successfully applied for the preparation of new receptors, guests, sensors, materials and so forth. Examples will be discussed together with the potential and limitations of this approach. The final section is dedicated to systems in which dynamic covalent bonds provide structural stability, but in which intramolecular noncovalent interactions determine the composition of the resulting equilibrium. It will be shown that this approach, dynamic covalent capture, is a very sensitive tool for the quantification of weak interactions, for instance in protein folding or catalysis.