Macrocycles architectures

Fig. 12-9 is a collage of some of the trigonal-based macrocyclic architectures (15-20) that have been realized by the cyclization of the appropriate PASs. Complex topologies, such as the macrobicycles 16 or macrotetracycle 17, are available by double cyclization of appropriate PASs as described below. 

If the dye ligands contain their own metal centers, they are able to offer both electron lone pairs and Lewis acidic sites for the self-assembly process. Once the orientation between the ligand and the metal is appropriate, the intermolecular complementary coordination in a head-to-tail manner might lead to macrocyclic architectures. Indeed, quite a few supramolecular macrocycles were produced from metalloporphyrin derivatives through the metal-directed self-cycHzation approach. 

Excellent applications of these cyclizations were reported in consecutive papers from Still [23] and Schreiber [24], The example from Still s group is shown in Scheme 4.5. Macrocyclic triepoxide 23 formed with high stereocontrol due to the conformational constraints of the macrocyclic architecture was saponified (conditions unspecified) and worked up with HOAc to generate tricycle 24 in high yield. The stereochemical outcome in this reaction implies that the cascade commences with the acid-mediated addition of the carboxylic acid into the proximal epoxide. 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Architectural control of transition metal-directed assembly to construct well-arranged metallo-macrocycles is one of the current research areas to create organized nanostructures for advanced materials.510-513... 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

Further comments. The preceding discussion outlines typical syntheses for simple polyether crown rings. It needs to be noted that a considerable number of other types of crown derivatives, displaying a variety of molecular architectures, has also been synthesized. Many of these types parallel the structurally developed macrocycles (which incorporate mainly donor atom types other than ether oxygen) discussed in Chapter... 

Porphyrazines (pz), or tetraazaporphyrins, are compounds that can be viewed as porphyrin variants in which the meso carbon atoms are replaced with nitrogen atoms, as Fig. 1 shows (1). This difference intrinsically gives porphyrazines discrete physiochemical properties from the porphyrins. In addition, despite their similar molecular architecture, porphyrazines are prepared by an entirely different synthetic route than porphyrins—by template cyclization of maleonitrile derivatives, as in Fig. 2, where the open circle with the A in it represents the peripheral substituent of the pz—rather than by the condensation of pyrrole and aldehyde derivatives (1). The pz synthetic route allows for the preparation of macrocycles with chemical and physical properties not readily accessible to porphyrins. In particular, procedures have been developed for the synthesis of porphyrazines with S, N, or O heteroatom peripheral functionalization of the macrocycle core (2-11). It is difficult to impossible to attach the equivalent heteroatoms to the periphery of porphyrins (12). In addition, the preparation and purification of porphyrazines that bear two different kinds of substituents is readily achievable through the directed cocyclization of two different dinitriles, Fig. 3 (4, 5, 13). 

This method exclusively yields macrocyclic polyesters without any competition with linear polymers. Furthermore, the coordination-insertion ROP process can take part in a more global construction set, ultimately leading to the development of new polymeric materials with versatile and original properties. Note that other types of efficient coordination initiators, i.e., rare earth and yttrium alkoxides, are more and more studied in the framework of the controlled ROP of lactones and (di)lactones [126-129]. These polymerizations are usually characterized by very fast kinetics so as one can expect to (co)polymerize monomers known for their poor reactivity with more conventional systems. Those initiators should extend the control that chemists have already got over the structure of aliphatic polyesters and should therefore allow us to reach again new molecular architectures. It is also important to insist on the very promising enzyme-catalyzed ROP of (di)lactones which will more likely pave the way to a new kind of macromolecular control [6,130-132]. 

Cytochromes serve as electron donors and electron acceptors in biological electron transfer chains, and with >75,000 members (53) they provide the bulk of natural heme proteins in biology. Cytochromes may be fixed into place within an extended electron transfer chain, such as the membrane-bound 6l and 6h of the cytochrome bci complex, or may be soluble and act as mobile electron carriers between proteins, for example, cytochrome c (54). In either role, the cytochrome may be classified by the peripheral architecture of the porphyrin macrocycle. Figure 1 shows the dominant heme types in biological systems, which are hemes a, b, c, and d, with cytochomes b and c being most prevalent. The self-association of a protein with heme via two axial ligands is a... 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

Other more complex conjugated MSP architectures have also been investigated. One of the architectures that has received some attention is polyrotaxanes, in which a polymeric rodlike component is threaded through a macrocycle... 

Metal-directed Self-assembly of Complex Supramolecular Architecture Chains, Racks, Ladders, Grids, Macrocycles, Cages, Nanotubes and Self-intertwining Strands (Helicates)... 

I would also like to reemphasize the noncyclic character of these molecules, which I believe provides the way to circumvent the kinetic limitations on the rates of complexing and decomplexing such strongly hydrated ions as Na+, Li+, Ca2+, which I suspect has stood in the way previously of translating the complexing selectivity of macrocyclic com-plexones into effective mediators for the selective permeation of these ions across the bilayer membrane. This may already offer a clue to the architecture of the Na+ selective sites of the cell membrane. 

Fujita, M. and Ogura, K. (1996) Transition-metal-directed assembly of well-defined organic architectures possessing large voids from macrocycles to [2]catenanes. Coord. Chem. Rev, 148, 249-264. 

A large variety of ligands form bonds with transition metals via one or more pnictide atoms. These range from simple ER3 (E = P, As, Sb, Bi R = alkyl, aryl, alkoxy, aryloxy, halogen and their combinations/permutations) molecules to ligands of elaborate architecture such as the macrocycles described in Section 14.2.1.7. 

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