Cyclophane pockets

An example of the modular preparation of the cyclophane 3 from the substituted bipyridine 2 and a general tripeptide 1 is shown in Scheme 3-3. The host molecule 3 contains a pre-organized binding pocket. The overall basicity of such molecules also facilitates their intercalation within the lamellas of acidic zirconium phosphate, thus making this chemistry well suited for the desired application. 

Another indication of steric interference is seen in the reaction of the tetracoordinated heme cyclophane with CO in dry benzene. Under conditions (1 atm CO) where deuteroheme exists as an approximately equal mixture of the mono-CO and di-CO complexes (37), the cyclophane heme shows the distinct spectrum of the pure mono-CO complex. Thus distal-side residues in heme pockets should be capable of greatly altering ligation of CO, and probably of 02 as well. 

As the formation of five-coordinate complexes with nitrogenous bases largely depends on the size of the host cavity, single-face-hindered porphyrins have been developed pocket [40], cyclophane [41-44], bridged [45], crowned [46], capped [47-53], strapped [54-60] and hybrid [61], In these compounds, the size and the chemical nature of the superstructure can be conveniently varied by changing the number of methylene groups and the aromatic substituent incorporated. 

With the idea of creating a more rigid chiral bis-NHC ligand, Veige and coworkers linked the wingtips of independent NHCs to a tra s-9,10-dihydroetha-noanthracene backbone [59]. The dibenzimidazolium cyclophane precursor exhibited a fluxional process in solution, but was static once bounded to Pd. Evidence of spatial distinction in the chiral pocket was the observation of a 1 4 ratio of endolexo conformers in both solid state and solution. 

Synthetic molecular receptors command widespread interest as mimics of membrane transport agents and enzyme active sites.[1,2] The development of new host systems for the selective complexation of organic and inorganic cations and anions has mushroomed in recent years however, examples of solution phase coordination of neutral organic molecules are few. Most of these examples involve inclusion of an aromatic hydrocarbon into the hydrophobic pocket of a water soluble cyclodextrin or cyclophane receptor.[3-6] A smaller subset of synthetic receptors possess lipophilic exteriors and hydrophilic cavities for the association of polar substrates in nonpolar media. [7-9] We report here the synthesis and characterization of a new system of the latter type designed to bind polar guests in chloroform solution. 

The hexafluorophosphate salts of 5 and 6 were crystallographically characterized and are depicted in Figures 2 and 3, respectively. The silver atoms of 5 are bonded to the NHCs from each of the two cyclophanes. The bonding of the carbenes to the silvers is essentially linear. The tetranuclear silver conq)lex, 6, is similar to 5 but has Ag atoms (Ag2 and Ag2A) inserted into the pocket of the cyclophane. The Ag atoms form a planar cluster that has Ag-Ag bonding distances (Agl-Ag2) similar to that of metallic silver. The pocket silvers are stabilized by the pyridines of the cyclophanes as well as die N-heterocyclic... 

Corrole and sapphyrin analogs are exanqples of how the pocket sizes of imidazolium cyclophanes can be altered to accommodate different sized metals into the pockets. TTie corrole analog 16 would be best suited for first row transition metals because of its decreased size. The pocket cavity of the sapphyrin analog 17 is larger and would be potentially better suited for coordinating both second and third row transition metals. The synthesis of 16 and 17 is outlined below (72). Thermal ellipsoid plots of conq)ounds 16 and 17 are shown in Figure 6 and 7, respectively. 

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