Non-covalent bond template

In the molecular imprinting technique, a cross-linked polymer matrix is formed around a target analyte (the template). The precursor mixture contains a functional monomer which can interact with the template molecule by covalent or non-covalent bonding. After the polymerisation process, the functional groups are held in position by the polymer backbone and the template molecule is removed. The residual binding sites are complementary to the target molecules in size and shape. 

Molecularly imprinted polymers (MIPs) may be prepared according to a number of approaches that are different in the way the template is linked to the functional monomer and subsequently to the polymeric binding sites (Table 5.1) (see Chapter 4). Thus, the template can be linked and subsequently recognised by virtually any combination of cleavable covalent bonds, metal ion coordination or non-covalent bonds. The first example of molecular imprinting of organic network polymers introduced by Wulff (see Chapter 4) was based on a covalent attachment strategy, i.e. covalent monomer-template, covalent polymer template [1]... 

For example, using non covalent bonding, Mossbach et al. have prepared a stationary phase for HPLC in order to resolve ( )-timolol [2). MIP s have fiinctional groups arranged in such a manner that they are complementary in shape and electronic features to the template. Therefore, Wulf et al. have selectively prepared L-Threonine with an enantiomeric excess of 36% by using a polymer which was imprinted with L-DOPA [3). 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

The template plays an important role in the formation of capsule F because no directional non-covalent bonds such as H-bonding and M-L interactions are involved in the self-assembly process. In addition, an external hydrophobic template is necessary because aqueous solvent does not template the formation of F. Capsule F consists of two octa-acid, deep-cavity cavitands with a pseudo-conical hydrophobic cavity (Figure 6.6a). The eight carboxylic acid groups are located at the periphery of the cavitand, thereby inducing... 

In chemistry, self-assembly is defined as a phenomenon by which molecules order themselves into a particular arrangement without external intervention. The process occurs under the guidance of numerous weak non-covalent bonding interactions that represent the core of supramolecular chemistry. It is not surprising that chemists engaged in the synthesis of topologically non-trivial structures are interested in utilizing self-assembly to direct the templation of chemical components into intermediates prone to cyclization, followed by the formation of covalent bonds. 

Nevertheless, self-assembling processes by nature rely on weak interactions and the isolation of chemically stable knots and links requires a final knotting step in which covalent bonds are formed. While irreversible reactions such as Williamson ether reactions" and ring closing methathesis are employed successfully to form knots and links, their template-directed syntheses have recently been enriched by the concept of dynamic covalent chemistry (DCC). DCC employs reversible covalent bond formation that allows equilibration of a system toward the most thermodynamically stable structures dictated by the sum of the non-covalent bonding interactions. 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

Few examples of covalent and non-covalent DCLs have been reported over the past few years, with only a small number of them making use of hydrogen-bonding templates. One of such examples is the barbiturate receptor 73 reported by... 

Two different techniques have been developed for MIP production, namely the covalent and the non-covalent approaches. The covalent way is based on the chemical derivatization of the template with molecules containing polymerizable groups using reversible covalent bonds. Different chemical reactions can be ap-... 

---