Negative templates

Scheme 10.20 Directed synthesis of a [2] catenane by use of a negative templating steric barrier as an auxiliary linkage.71...

The use of metal ion templates to achieve selective cyclisation reactions is one of the most successful applications of templating [18] and the concept has been extended to other cyclisations. In this context Sanders et al. [19] have described a negative template, a tetrapyridyl porphyrin. They have used this template to suppress formation of the otherwise favoured cyclic dimer por-... 

In the use of templates, Sanders and co-workers [47] differentiate between two modes of action that are complementary a positive template directs the approach of two reactive end groups in a molecule, thereby generally supporting the desired intramolecular cyclization. In contrast, a negative template effect holds the reactive groups apart, thereby suppressing the intramolecular cyclization and leading preferentially to the (undesired) intermolecular reaction. 

Scheme 1-18 Schematic illustration of positive and negative templating.

Figure 1-2. Schematic illustration of positive and negative template effects in directed synthesis of chemical species.

Negative templates Tlie templates discussed so far favour reaction between bound substrates they are positive templates. It is also possible for a template to disfavour reaction between bound substrates they are tlien negative templates. A negative template disfavours tlie formation of a given product not by accelerating a competitive reaction but by specifically disfavouring the formation of Uiat product. As we show... 

A more satisfying example of negative templating is tlie efficient one-step synthesis of cyclic tetramer 4 from linear dimer 5 templated by Py4Porph (Fig. 8).[6] This is currently our best route for production of cyclic tetramer. Here the template inhibits cyclic dimer formation by acting negatively, and positively accelerates cyclisation of the intennediate liiieai teti ainer 6. 

Figure 7. (Right) Positive templating brings reactive ends together negative templating keeps reactive ends apart...

Porous sol-gel foams have also been produced using the more traditional polymer foam reticulation [75], but the process offers little benefit over the sol-gel foaming method unless ordered mesoporosity is required [76]. Polymer foam reticulation involves using a polymer (usually polyurethane) foam as a negative template. The sol (or sol-gel particles) can be coated on the polymer foam, which is then burned out in the furnace. Reticulated foams often have hollow struts, which reduces their mechanical properties. 

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