Dynamic Covalent Chemistry

Abstract

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of “error checking” and “proof‐reading” into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self‐assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self‐assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more‐complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical “balancing acts” revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re‐adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

Mechanically bonded macromolecules

“…In order to construct the mechanically interlocked dendritic structure, a tripod molecule 16Á3PF 6 , with three dialkylammonium unit containing arms, was used as the dendrimer core. 1 41 we have developed a practical way of making mechanically bonded dendrimers, in which the thermodynamically controlled seven-component self-assembly process proceeds quantitatively by overcoming the massive steric hindrance of three dendrons as large as the [G3] Fre´chet-type wedge-shaped dendrons.…”

Porous organic molecular solids by dynamic covalent scrambling

“…We exploit here the dynamic nature 35,36 of the imine bonds in cages 1 and 3 to form new cage molecules with mixed vertex functionalities. This can be achieved in three different ways.…”

Intrinsic self-healing polymers with a high E-modulus based on dynamic reversible urea bonds

“…[3][4][5] In contrast, intrinsic self-healing materials can recover their properties due to the presence of specific reversible chemical bonds, which is beneficial due to the possibility of multiple healing steps at the same location. 6 These binding motifs can be composed of either dynamic covalent bonds, [7][8][9] for example, the Diels-Alder reaction [10][11][12] or radical-based systems, 13 or supramolecular interactions, such as hydrogen or halogen bonds, [14][15][16] ionic interactions, 17 π-π interactions, 18 host-guest interactions 19 or metal-ligand interactions. [20][21][22][23] Often there is a trade-off between the mechanical properties and the healing efficiency (Figure 1).…”