Imines self-templating

Imine metathesis has continued to be a popular exchange reaction for DCLs. Various groups have found novel systems in which the reaction can be applied, as well as interesting ways to halt the equilibration. For example, Wessjohann and coworkers have demonstrated that Ugi reactions can efficiently halt equilibration of an imine DCL, combining an irreversible diversification process with areversible library selection [24]. Xu and Giusep-pone have integrated reversible imine formation with a self-duplication process [25], and Ziach and Jurczak have examined the ability of ions to template the synthesis of complex azamacrocycles [26]. The mechanistically related reactions of hydrazone [27] and oxime [28] exchange have also been explored as suitable foundations for DCL experiments. 

Scheme 4.14 Formation of a dynamic imine library and Cu+ templated self-sorting.

An intriguing example of a structurally very simple self-replicating systems utilizes amidinium carboxylate salt bridges to enhance the catalytic condensation of an aniline structure with a benzaldehyde derivative (Figure IS). " Several structures with various substituents in para-position to the recognition sites have been investigated. For instance, autocatalysis was established in the formation of imine 14 from aniline 12 and aldehyde 13. Adding preformed template to the reaction mixture increased the initial rate of product formation, and the obtained experimental data was in accordance with the square root model. 

As a representative example, the solid state structure of the corresponding ethylene-bridged complex is shown in Scheme 2.2 and exhibits the special role of one lithium counter ion that stabilizes the helicate in concert with two molecules of water as a template. Later, bisimine-bridged ligands were also used to examine this class of coordination compounds. In this case, the helicates can be formed by simply mixing a diamine, 2,3-dihydroxybenzaldehyde, a source of titanium(IV) ions, and an alkali metal carbonate as a base. This procedure in principle resembles a hierarchical process with an initial imine condensation followed by helicate self-assembly, although the order of events cannot be predicted definitively. 

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