COMMUNICATIONSthe rate of macrocyclic ring circumvolution increases and the process becomes rapid on the NMR timescale.Comparison with the [DJDMSO 'H NMR spectra of both 1 and 13 recorded at room temperature indicates that this hindrance to circumvolution is peculiar to 12 and therefore must be governed wholly by the electronic nature of the pyridine unit rather than by the steric requirements of the catenane components. The X-ray crystal structure of 1 suggests that circumvolution of benzylic amide catenanes is restricted primarily by the two sets of intramolecular, intermacrocyclic hydrogen bonds between the cisoid 1,3-diamide hydrogen atoms and the amide carbonyl groups (Fig. 4). Remarkably, even in a solvent that . Circumvolution of the catenanes involves the breaking and subsequent reforming of the intramolecular hydrogen-bonding networks between the two macrocyclic components. Rotation is achieved more easily when the 1.3-dicarhonyl moiety is attached to a benzene ring (e.g. I, top) rather than a pyridine unit (e.g. 12, bottom), since the lowest energy conformation of the isophthaloyl group has the diamide in a transoid orientation. which releases the hydrogen-honded carbonyl of the other macrocyclic ring. In the minimum energy conformation of the pyridine unit of I 2 the diamide is in a cisoid orientation. This can be explained by the hydrogen bonding of the amide hydrogens to the pyridine nitrogen and/or electrostatic repulsions between the pyridine lone pair and the carbonyl group in the pyridine-2.6-dicarbonyl system. This leads to strong bifurcated hydrogen bonds between the two rings, fixing them in position and preventing macrocyclic ring rotation can form hydrogen bonds such as DMSO the circumvolution of catenane 12 is slow on the NMR timescale at temperatures below lOO"C! Since catenanes 13, 1, and 11 do not display this slow rotation behavior in DMSO, the phenomenon can be attributed to the greater propensity for pyridine-2,6-dicarbamido units to adopt the cisoid conformation necessary for strong binding to a carbonyl group.[lol Conversely, in macrocyclic components of catenanes 1, 11, and 13 the lack of binding sites for strong bonds with carbonyl groups allows unhindered, and consequently rapid, circumvolution to occur. The synthesis of benzylic amide based catenanes is remarkably tolerant to significant variations in both the diamine and the dicarboxylic acid dichloride starting materials. The resulting catenanes display rotational properties associated with an intricate intramolecular network of hydrogen bonding interactions, which can be controlled both electronically and sterically. These fascinating molecules are clearly well suited for the construction of revolutionary nanoscale molecular devices. Experimental ProcedureThe optimum conditions for the catenane-forming reaction described in ref. [I 11 were worked out by systematic variations in reactant concentration, solvent, and temperature. The ideal concentration for catenane formation is ca. 0.02M; more dilute solutions decrease the yi...
By learning how to balance natural resource limitations and pollution prevention with economic growth, green chemistry will become the central science of sustainability. The elimination of persistent pollutants is vital for a sustainable civilization. To achieve this, the most important guiding concept is that the elemental composition of technology should be shifted toward the elemental composition of biochemistry. Oxidation chemistry is currently a prolific producer of persistent pollutants. Many arise from the use of chlorine, hypochlorite, or chlorine dioxide in large-scale oxidation processes. Oxidation chemistry can be greened by replacing these with catalyzed alternatives based on Nature's oxidizing agent, hydrogen peroxide. TAML® (TetraAmidoMacrocyclicLigand) iron catalysts, which were invented at Carnegie Mellon University, are widely patented and are being developed to activate H2O2 for commercial applications. TAML activators are water-soluble, easy to use, function well from neutral to basic pH, are not dominated by nonselective Fenton-like reactivity, are straightforward to synthesize, work effectively in minute concentrations, enable peroxide processes to occur at temperatures well below those of the processes targeted for replacement, and are amenable to modification for capturing novel selectivities. TAML activators are "dial-a-lifetime" catalysts: an activator can be chosen exhibiting a lifetime commensurate with the desired task.
The Design of Green Oxidants -[39 refs.]. -(COLLINS, TER-RENCE J.; GORDON-WYLIE, SCOTT W.; BARTOS, MICHAEL J.; HORWITZ, COLIN P.; WOOMER, CHRISTINE G.; WILLIAMS, STACY A.; PATTERSON, ROBERT E.; VUOCOLO, LEONARD D.; PATERNO, STEVEN A.; STRAZISAR, STEPHANIE A.; ET AL.; Green Chem. (1998) 46-71; Dep. Chem., Carnegie Mellon Univ., Pittsburgh, PA 15213, USA; EN)
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