Dynamic covalent bond

DCC depends on a reversible connection process under mild conditions and an adequate time scale for the spontaneous production of library members consisting of basic components. Recently, the dynamic covalent bond having both reversibility and stability against the external environment has attracted much attention, as it can possibly be used in the development of novel materials in which the components can be reconstructed, deconstructed, or reshuffled, triggered by external stimuli [16], Therefore, the dynamic covalent bond is advantageous for establishing a DCL, and for developing novel materials with a reversible nature. 

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

Amamoto, Y Higaki, Y Matsuda, Y Otuska, H. Takahara, A. Programmed thermodynamic formation and structure analysis of star-like nanogels with core cross-linked by thermally exchangeable dynamic covalent bonds. J. Am. Chem. Soc. 2007,129, 13298-13304. 

The most recent method developed for the nA —> An approach relies on dynamic covalent bond formation using a metathesis reaction. In this case, reactions are typically under thermodynamic control, providing the potential for increased selectivity in product formation. The initial examples using alkyne metathesis toward the formation of SPMs were reported by Adams, Bunz, and coworkers using the precatalyst [Mo(CO)6] [27,28], but rather low yields of the desired products (4) limited general applicability (Scheme 6.2). Recent efforts by Moore and coworkers using a Mo(VI)-alkylidyne catalyst, however, have refined this process such that precipitation-driven reactions now provide moderate to excellent results (see Scheme 6.24) [29]. 

The small-molecule-based machine conceived by von Delius, Geertsema, and Leigh [45] is a linear (for reviews, see [46], [100]) motor based on dynamic covalent chemistry [19-24] (forming, breaking, and reforming of dynamic covalent bonds with relatively fast equilibration in response to stimuli), namely on acyl-hydrazone and disulfide exchanges. The motor consists of a track that has four functional groups disposed alternately aldehyde-thiol-aldehyde-thiol which are the positions 1,2, 3, and 4 of the track, a walker NH2-NH-CO-(CH2)5-SH which has the feet A (hydrazide or acyl-hydrazine) and B (thiol), and a placeholder with a foot C of type thiol (Fig. 10). 

This molecular motor is an example that illustrates how the changes in configuration can have an influence on the exchanges of dynamic covalent bonds like -C=N- from acyl hydrazones and -S-S- (disulfides). 

Maeda T, Otsuka H, Takahara A (2009) Dynamic covalent polymers reorganizable polymers with dynamic covalent bonds. Progr Polym Sci 34 581-604... 

Kawai H, Umehara T, Fujiwara K, Tsuji T, Suzuki T (2006) Dynamic covalently bonded rotaxanes cross-linked by imine bonds between the axle and ring inverse temperature dependence of subunit mobility. Angew Chem Int Ed 45 4281 1286... 

The preparation of [6]cycle 14 is illustrative of the need for dynamic covalent bond formation in the ADIMAC process (Scheme 6.7). First reported by Staab in 1974, a... 

The attraction of exchangeable covalent bonds is beyond any doubt their stabiUty in the absence of an exchange stimulus, be it heat, acid, or base, or other catalytic species [2, 48, 49]. The still somewhat Umited Ust of dynamic covalent bonds is... 

Organic cage compounds formed by covalent bonds alone have been synthesized with the help of dynamic covalent bond formation. Cage compounds through formation of imine bonds (hemicarcerands, cavitands, adamantoid nano-cages and chiral nano-cubes based on chiral... 

The successful synthesis of large molecular structures brings an intrinsic joy to chemists. The ability to rationally create well-defined architectures has risen to great heights, and in many cases the obtained structnres can be considered miniature pieces of art. " " It is also the area that most abundantly employs the complete toolbox of chemical bonds available, covering the entire range of chemical stabilities. Here, only a narrow selection of structures that are entirely organic in nature (i.e., without the use of metal ions) and that exclusively rely on dynamic covalent bonds for their formation is discussed. The focus is on the dynamic synthesis of these structures, rather than on their applications which are extensively discussed elsewhere in this series. 

The use of imines as the dynamic covalent bond significantly augmented the versatility of the thermodynamic approach toward the formation of interlocked systems. Stoddart et al. reported on the synthesis of [2]rolaxanes by the clipping protocol, in which a dialdehyde was used to clip the macrocycle under thermodynamically controlled conditions (Figure 6). In this system, the driving force for the formation of rotaxane 10 is the stabilizing interaction between ammonium ions and crown ethers, which was widely exploited by Stoddart and coworkers. In the first instance, a library of macrocyclic and linear compounds was prepared on condensation of tetraethylene glycol bis(2-aminophenyl)ether 6 and 2,6-diformylpyridine 7. 

The conceptual simplicity of DCC gave it a jumpstart in terms of research groups involved. Having mastered the skill to use noncovalent and dynamic covalent bonds at will, researchers found DCC the obvious area for the application of supramolecular systems. In fact, the examples shown in the previous section are evidence of this potential. However, an evaluation of the literature over the past 15 years leads to some interesting observations. First, the number of DCLs that have displayed spectacular amplifications are rather limited. Second, apart from some exceptions (see later), nearly all DCLs are rather small (<25 components), especially in comparison to the huge covalent libraries prepared in the conventional manner. These are indicators that the application of DCC may not be as straightforward and general as it appears. In this section we discnss some of these critical issnes. 

DCC is associated with the principle of survival of the fittest and gives appreciable responses only when strong noncovalent interactions occur. Sensibility toward much weaker noncovalent interactions can be introduced when a combination of dynamic covalent bonds and noncovalent interactions occur in the same molecule. Apart from a way to fine-tune the dynamic synthesis of organic structures, it allows the application of DCC in areas where small energetic differences are crucial. 

The examples discussed in this chapter illustrate that the dynamic covalent bond is now forming an integral part of the toolbox available to organic chemists. It also illustrates that the classical distinction between covalent and noncovalent bonds in terms of kinetic and thermodynamic stabilities is losing relevance. This, in turn, points to the important role supramolecular chemistry is playing in all areas of chemistry. 

This trend is the same as for the synthesis of pillar[5]arene in the template solvent of dichloroethane. Another interesting aspect is that the reaction is reversible between oligomers and the macrocyclic compounds. Nierengarten et al. investigated the dynamic covalent bond formation using a linear trimer... 

On the basis of the dynamic covalent bond formation, we investigated the dynamic interconversion between pillar[5]arene 2.20 and pillar[6]arene 2.21 (Figure 2.6). Pillar[6]arene converted to pillar[5]arene with BF30Et2 in... 

Imato K., Nishihara M., Kanehara T., Amamoto Y., Takahara A. and Otsuka H. (2012), Self-heahng of chemical gels cross-linked by diarylbibenzofirranone-based trigger-free dynamic covalent bonds at room temperature, Angen . Chem. /nr.FA, 51,1138-1142. 

Mastalerz M (2010) Shape-persistent organic cage compounds by dynamic covalent bond formation. Angew Chem Int Ed 49 5042-5053... 

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