Reversible boroxine formation

Fig. 9 (a) Reversible synthesis of a bisiminoboronate-guanosine derivative from guanosine and a bisiminoboronic acid, (b) Dynamic polymeric networks based on reversible boroxine formation, (c) Formation of a macrocycle in a [4 + 4 + 2] condensation via simultaneous reversible formation of imine bonds and boronic esters... 

Scheme 1.12 Templated synthesis of a Cs-symmetric [4]rotaxane 26 generated through reversible boroxine formation [43].

Reversible bond formation can, in principle, be used for the selfcorrection of product structures. For example, the formation of Schiff bases, boroxines and boronates has been applied to on-surface polymerizations, but has so far met with limited success. Linderoth and Gothelf employed trialdehyde 62 and diamine monomers 63 towards 2-D polymer synthesis on Au(lll) under UHV (Figure 28.28a) [134], but unfortunately obtained only branched or irregularly networked stmctures, a situation that was ascribed to the flexible monomer structure employed. In addition, UHV conditions caused an irreversible loss of water, the presence of which was essential to bring about the back-reaction. When Abel and coworkers synthesized boroxine/boronate networks on Ag(lll) under UHV [135] they were unable to achieve the expected periodic order this contrasted with the findings of Yaghi et al., who employed the solution approach (see Section 28.5.2). This difference was most hkely a consequence of irreversible bond formation. 

Boroxine formation, through the cyclotrimerization of three boronic acid units, is another reversible reaction recently applied in dynamic covalent synthesis (Figure 1.3u). The forward reaction is entropicaUy driven by the release of water molecules upon condensation and is favored where electron-donating groups in the para position are used [42]. To date, this reaction has only been used in the designed thermodynamic synthesis of a Cj-symmetric [4]rotaxane 26, apparently under thermodynamic control (Scheme 1.12) [43]. However, it could be envisaged that... 

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