Template Effects and Templated Synthesis
1. What is a template?
Templates can be described as molecular matrices with the following features and functions:
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Recognition:
A template interacts with complementary binding sites of reaction partners, thus involving selectivity -
Organisation:
A template organizes the reaction partners in space, thus modifying their reactivity -
Information transfer:
Information stored in a template, such as spacial arrangement and recognition pattern, is inherited to the reaction product
In a strict sense, templates must be removable from the reaction product, thus distinguishing them from simple reactants. But in supramolecular chemistry, this point is often neglected, since the terms “template” and “templated synthesis” are also widely accepted for those reactions in which the template becomes part of the reaction product.
2. How to classify templates?
We classify templates according to their topography . Another criterion which is appropriate for a classification is the type of interaction between template and reaction partners (covalent, non-covalent, hydrogen bond, metal-ligand-coordination, hydrophobic effects etc.). In the following images, all components serving as templates are orange colored.
2.1 Convex templates
Convex templates have been pioneered early on when crown ethers and other macrocycles were synthesized with the help of a metal cation suitable in size and coordination geometry. This approach is schematically shown in Figure 1. Around the convex surface of cations, more complex species such as rotaxanes or catenanes can be built. One of the examples from the Schalley group is the formation of hexameric pyrogallarene capsules around a templating tris-bipyridine ruthenium(II) dication.
Figure 1: Schematic representation of a convex template (orange) around which a macrocycle is formed (blue).
Example 1: Catenane synthesis has been done by metal ion templation, utilizing the tetrahedral metal-ligand coordination geometry of copper(I) (C. O. Dietrich-Buchecker, J.-P. Sauvage, J.-P. Kintzinger, Tetrahedron Lett. 1983, 24, 5095; C. O. Dietrich-Buchecker, J.-P. Sauvage, Chem. Rev. 1987, 87, 795).
Example 2: The following rotaxane synthesis through a convex anion template utilizes hydrogen bonding between chloride and amide protons and shows that convex templates do not necessarily involve cations (J. A. Wisner, P. D. Beer, M. G. B. Drew, M. R. Sambrook, J. Am. Chem. Soc. 2002, 124, 12469).
2.2 Concave templates
Concave templates organize reaction partners within a cavity, where they usually are bound by non-covalent bonds. Spatial orientation as well as a high local concentration of the partners provoke the desired bond formation (Figure 2).
Figure 2: Schematic representation of a concave template (orange) which brings togehter two reacting subunits (blue) inside its cavity.
Example 3: A cinnamic acid derivative can be regioselectively photodimerized through a cavitand template, utilizing the hydrophobic effect between cucurbituril and its aromatic guest. Without templation, only the cis-isomer of the monomeric cinnamic acid is obtained (>M. Pattabiraman, L. M. Kaanumalle, A. Natarajan, V. Ramamurthy, Langmuir 2006, 22, 7605).
Example 4: Rotaxane synthesis can be brought about by a macrocyclic tetralactam template, utilizing hydrogen bonding between the amide protons and a stoppering phenolate (G. M. Hübner, J. Gläser, C. Seel, F. Vögtle, Angew. Chem. 1999, 111, 395; Angew. Chem. Int. Ed. Engl. 1999, 38, 383). Another anion-templated rotaxane synthesis has been developed in our group. The shuttling motion of the wheel along the axle can be controlled by protonation and deprotonation.
2.3 Linear templates
An excellent example for a linear template (Figure 3) from nature is single-stranded DNA, along which the complementary strand is synthesized. In artificial systems, linear templates transferring sequence information are rather scarce. A number of examples is known, where the fibers of gels have been used in mineralization experiments. The fiber templates the formation of tunnel-shaped hollow spaces inside the resulting solid after removal of the organic template.
Figure 3: Schematic representation of a linear template (orange) combining two building blocks (blue).
Example 5: Regioselective photodimerization of trans-1,2-bis(4-pyridyl)ethylene is possible through a linear template, utilizing hydrogen bonding between resorcinol-OH and pyridine-N atoms (L. R. MacGillivray, J. L. Reid, J. A. Ripmeester, J. Am. Chem. Soc. 2000, 122, 7817).
2.4 Planar templates:
A planar template is usually a surface which binds molecules specifically and enables the self-assembly of these molecules into ordered architectures (Figure 4). One example from our group is the deposition of metallosupramolecular squares on chloride-covered Cu(100) surfaces.
A nice example for a planar template is the electropolymerisation of 3,4-ethyldioxythiophene on the liquid crystalline phase serving as template (J. F. Hulvat, S. I. Stupp, Angew. Chem. 2003, 115, 802; Angew. Chem. Int. Ed. 2003, 42, 778).
3. Summary
As shown in some of the examples, templates plays a major role in supramolecular chemistry as well as in nature and opens a pathway to complex molecular architectures - in particular, when combined with self-assembly processes. Understanding template effects is a key to sophisticated supramolecular design. The great variety of possible interactions between templates and reaction partners, encompassing covalent and non-covalent bonds, hydrogen bonds, metal-ligand-coordination and hydrophobic effects etc., makes templates a versatile tool in supramolecular synthesis which is applicable in a broad spectrum of reaction conditions.