Cyclophane metal binding

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

Supramolecular metallocatalysts consist in principle of the combination of a recognition subunit (such as a macrocycle, a cyclodextrin, a cyclophane, etc.) that selects the substrate(s) and of a metal ion, bound to another subunit, that is the reactive site. Complexed metal ions presenting free coordination positions may present a variety of substrate activation and functionalization properties. Heterotopic coreceptors such as 70 bind simultaneously a substrate and a metal ion bringing them into proximity, thus potentially allowing reaction between them. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

The scope of the tether-directed remote functionalization has been expanded from Cgo to the higher fullerene C70, and the described reactions are completely regioselective, featuring, in the case of C70, the kinetically disfavored addition pattern. The crown ether is a real template, since it can be readily removed by transesterification, giving a much-improved access to certain bis-adducts that are not accessible by the direct route. Cation-binding studies by CV reveal that cyclophane-type crown ethers derived from C60 and C70 form stable complexes with metal cations, and a perturbation of the fullerene reduction potentials occurs because the cation is tightly held close to the fullerene surface. This conclusion is of great importance for future developments of fullerene-based electrochemical ion sensors. 

Schwabacher et al. (1972) prepared a cyclophane-type structure with metal ions coordinated into the walls of the macrocycle (70). This system was first reported in 1992 and shown to transport neutral aromatic hydrocarbons through an aqueous membrane. Even though this receptor has a net anionic charge, it has been illustrated to bind indole and naphthalene units functionalized with... 

In a completely different approach to metal-templated receptors, Schwa-bacher et al. have prepared a series of bis(amino add) derivatives like (57) which, on addition of transition metals such as mckel(II) or cobalt(II), dimerize to form cyclophane-like macrocycles (58) that are capable of substrate recognition in water [68]. These designs have the advantage that the metal can provide electrostatic binding to polar substituents on the substrate in addition to the primarily hydrophobic interactions [69]. 

The internal cavity of a cyclophane is endobasic if functional groups are present that are basic or electron donating the most obvious groups include the ethers, pyridines, amines, and phosphorus-based donors. Cyclophanes in this category would be expected to bind metal ions and also promote H-bonding interactions within the cavity. Hence, in this section, crown ethers and azamacrocycles could easily be included. Under this heading, we can also... 

Adding aUcaU metal iodide salts as templates induced modest changes in product distribution [36]. The most significant shift in library composition was induced by sodium iodide, which doubled the concentration of tetramer and pentamer in the reaction mixture. Binding constants were not measured however, the relative affinities of the amplified cyclophane macrocycles for the different metal alkali were studied by electrospray ionization-mass spectrometry (electrospray ionization ESI-mass spectrometry MS), finding good agreement with the main amplification observed. The yield of cychc tetramer was increased by the addition of sodium in the cases of the MEM and di(p-methoxybenzyl) monomers, and these were found to bind sodium preferentially in the ESl-MS study [109]. 

Protonation of azacycloalkanes and -cyclophanes is a process that competes with metal ion coordination and gives rise to the formation of positively charged ammonium species that may function as anion receptors. For this reason, the knowledge of the protonation properties of these ligands is paramount for both metal cation and anion coordination studies, in particular, when the variation of a ligand property with pH is followed to determine the complex species formed in solution and the relevant stability constants (see Binding Constants and Their Measurement, Techniques). 

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