Self-directed synthesis

Cryptophanes are related to hemicarcerands - they are composed of smaller and shallower cavitand bowls, such as cyclotriveratrylene (CTV) molecules that have been used for both anion and neutral molecule recognition (Section 2.5.2). The first cryptophane was synthesised in 1981 via a self-directed synthesis. This small cage is named, chronologically, cryptophane-A (2.126) (Figure 2.31). Space-filling models of cryptophane-A show a closed surface container, with... 

Template-directed synthesis has been the object of much recent attention in the field of material science [169]. Using self-assembled aggregrates as a tern-... 

BINOL and its derivatives have been utilized as versatile chiral sources for asymmetric catalysis, and efficient catalysts for their syntheses are, ultimately, required in many chemical fields [39-42]. The oxidative coupling of 2-naphthols is a direct synthesis of BINOL derivatives [43, 44], and some transition metals such as copper [45, 46], iron [46, 47] and manganese [48] are known as active metals for the reaction. However, few studies on homogeneous metal complexes have been reported for the asymmetric coupling of 2-naphthols [49-56]. The chiral self-dimerized V dimers on Si02 is the first heterogeneous catalyst for the asymmetric oxidative coupling of 2-naphthol. 

There are two distinct approaches to catenane synthesis the statistical approach, and approaches relying on self-assembly, so-called directed synthesis . The statistical approach relies on the small chance that macrocyclisation may occur while a linear precursor is threaded through a macrocyclic component. Because this is a rather unlikely eventuality, it naturally results in low yields of interlinked product and is chiefly of historical interest. It was this kind of statistical approach that resulted in the first synthesis of a [2] catenane by Wasserman in 1960 (10.64), from cyclisation of the long-chain diester 10.65 while threaded through the annulus of a deuterated C34 cycloalkane 10.66 (Scheme 10.11), 57 Although the overall yield of the catenation reaction was less than 1 %, the existence of the catenane was firmly established. The relatively polar [2] catenane product, along with other polar macrocyclisation reaction products and... 

Another field of research is concerned with the one-pot synthesis of this system from readily available starting materials. In Scheme 12, reaction of phenyl azide and malononitrile in the presence of sodium ethoxide leads, via the intermediate triazole (139), to the self-condensation product (140), itself a triazolopyrimidine. In the presence of other nitriles the preferred formation of the more useful compounds (141) occurs in low to moderate yields with small amounts of dimer <87JHC997>. Direct synthesis of the 5,7-dione derivatives (143) (60-90%) is achieved by the reaction of the readily available aminopyrimidinedione (142) with an azide iminium salt in dichloromethane at moderately high dilution <87JHC1493>. 

As a step in this direction, a one pot self-assembly synthesis has been described of hexagonal mesoporous silica fibers sensitized with a dye that functions as a laser (Fig. 2.11) [89]. In this example, large mesopores and process-... 

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

Template-directed synthesis has also been exploited for combinatorial purposes in which a reversible reaction and the use of thermodynamic templates have been used. Two different processes have been envisaged and validated, both of which consider the dynamic optimization of a receptor-ligand interaction where one of the partners is the template that drives the self-assembly of a reversible library of other partners from which the best binder for the template is selected (Fig. 8.53). If the receptor is a template, a library made using a reversible reaction is incubated with the receptor and... 

There are now many self-assembled supramolecular systems in which metal ions have played an important, if not essential, role in the assembly process. In this chapter, the use of metal-directed synthesis to achieve assembled systems is discussed. 

Stoddart and co-workers transferred the strategy of template-directed synthesis to the formation of the first molecular system consisting of five interlocked rings in a linear array (Structure 51). They suggested that this [5]catenane should be called olympiadane [64]. Recently they reported the self-assembly of a new type of rotaxane, in which three side chains are linked directly to a single central core, producing a dendritic-type structure [65]. 

The classical thermochemical approach has also already been applied to special template-assisted chemical reactions like the template-directed synthesis of oligonucleotides [260], for which a detailed system of kinetic elementary steps has been derived and solved. Other examples are the thermochemical model for the assessment of cooperativity in self-assembly processes proposed in Ref. [261, 262] and for the quantitative description of multicomponent self-assembly processes of polymetallic helicates [263, 264]. More thermochemical approaches applied within particular fields of supramolecular chemistry will be mentioned in the following. 

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