Science/Technology thesis to the progression from building houses brick by brick and board by board to joining prefabricated units. "I think we'll see chemists in many laboratories espousing these principles with inorganic, as well as organic, compounds across the whole face of structural chemistry." Stoddart made the catenane in work with postdoctoral fellow Neil Spencer, graduate student Mark V. Reddington of Sheffield, and exchange student Cristina Vicent from the Institute of General Organic Chemistry at the [Consejo Superior de Investigation Cientificas] in Madrid, Spain [Angew. Chem. Int. Ed. Engl, 28, 1396 (1989)]. Mass spectroscopist Peter R. Ashton verified the linked nature of the molecule by fast atom bombardment mass Georgia Tech worker experiments with new, eye-safe lidar spectrometry at Sheffield. Further verification came from search scientist in Georgia Tech's the eye and so could not be focused school of geophysical sciences. "We on the retina where it could do dam- x-ray single-crystal studies by struccan get a sensitive lidar that is use- age. Thus it can be operated at much tural chemistry professor David J. ful for visibility measurements, and higher power levels than lidars op- Williams and graduate student Alexandra M. Z. Slawin at Imperial Colfor infrared bands of interest with erating in visible light wavelengths. various sensor systems/' In field tests, the eye-safe lidar lege, London. Spencer used dynamCarbon dioxide lidars, which are has measured clouds 11 km in alti- ic nuclear magnetic resonance to exeye safe, have existed for years. But tude and atmospheric aerosols as plore interactions of the linked rings they are bulky and require frequent much as 4 km away. Patterson be- at Sheffield. And physical chemmaintenance, the Georgia group lieves that range would be great istry professor Angel E. JKaifer and notes. Detection equipment used to enough for a number of applica- graduate student Timothy T. Goodmeasure atmospheric backscatter tions, among them the continuous now studied the linked environment from these lasers must be cooled monitoring of pollution sources such in electrochemical work at the Uniwith liquid nitrogen. The Georgia as factories to detect the type and versity of Miami, Coral Gables, Fla. One reactant, di-l,4-phenylene-34Tech device, by contrast, uses com- amounts of chemicals released. Such mercially available components that monitoring would require an abili- crown-10, also served as a template. ty to scan the lidar, an operation In cyclic crown ether notation, "34" operate reliably at room temperature. Light from the eye-safe lidar has that would be dangerous without denotes ring size and "10" is the number of heterocyclic oxygens. a wavelength that would cause it to the eye-safe device. The other reactant was a dication James Krieger be absorbed by the aqueous humor of with a 1,4-phenylenedimethyl group bonded to one nitrogen of each of two 4,4 / -bipyridyl units. The two reactants formed a deep yellow complex in acetonitrile in which the Chemists at Sheffield University, En- Stoddart finds that synthesis of large gland, have succeeded in making a assemblies is often easier than that open-chain tetrapyridinium molecule was threaded through the "eye" [2]catenane in a one-step reaction of the components. "Synthetic chemistry is ready to of the crown ether. in 70% yield. Catenanes are comThe Sheffield chemists then closed pounds whose cyclic components undergo a fairly dramatic change," are intertwined like chain links. The he says. "We've become exceeding- the second ring by reaction with n u m b e r in brackets denotes the ly adept at synthesizing beautifully l,4-bis(bromomethyl)benzene. Prenumber of links. large molecules with many chiral cipitation of the red catenane as the This a c c o m p l i s h m e n t d e m o n - centers through the whole game of hexafluorophosphate salt was comstrates what organic chemistry pro- stitching atoms together and ma- plete after 48 hours. Spencer and graduate student fessor J. Fraser Stoddart calls struc- nipulating functional groups in a Chris Brown in Stoddart's group ture-directed synthesis. "Informa- very elegant way. "I think what we see this cate- have recently made a [3]catenane. tion" is "preprogramed" into the reactants for efficient assembly of nane as a prototype for is the build- The approach to this compound was large molecules. No catalyst or out- ing of large assemblies molecule by to make a larger tetrapyridinium side reagent is needed. In fact, molecule." Stoddart likens such syn- ring, again in the presence of excess
Catenane synthesized in one step in 70% yield
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November 13, 1989 C&EN
Proton signals of the methylene of the crown ether. They enlarged the tetrapyridinium ring by joining groups attached to the "inside" and the bipyridyl groups with 4,4'-biphe- "alongside" bipyridyl moieties only nylenedimethyl units. It fascinates separate into singlets of equal inStoddart that the [3]catenane was tensity at - 6 5 ° C Thus the tetramuch easier to make than the ex- pyridinium ring revolves through panded dipyridinium ring compo- the relatively stationary crown ether nent. "It is an intriguing prospect ring with an activation energy of that it is easier to make an assembly 12.2 kcal per mole. than one component. The difficult Stoddart notes that t h e lowsteps are at the beginning. There temperature revolution of the tetramay be even more cooperativity to pyridinium ring still preserves combe had as the assemblies grow." plexation of the bipyridinium groups "There may be a deep message to the "inside" 1,4-phenylenedioxy there," he continues. "This is a group. The high-temperature revomechanism which is undoubtedly lution of the crown ether ring breaks up all the noncovalent bonding in used by biological systems." Like all chemists, Stoddart is the molecule. The difference between "inside" thrilled at the esthetic value of such molecules, as well as any useful ap- and "alongside" bipyridyl groups plications. He dreams of making a also shows up in electrochemical [4]catenane, which is the logo of studies of Kaifer and Goodnow at Audi automobiles. And [5]catenane Miami. Tetrapyridinium rings are tetracations with positive charge is the Olympic symbol. In the [2]catenane, the noncova- spread over the pyridinium nuclei. lent bonding that complexed the The Coral Gables workers followed t e t r a p y r i d i n i u m chain t h r e a d e d successive reductions and oxidations through the crown ether ring lives among tetracations, neutral species, on. Spencer's solution NMR spec- and intermediate radical cations by tra, for example, distinguish pro- cyclic voltammetry. Because the tetrapyridinium ring tons of the bipyridyl unit inside the crown ether ring from the one is a tetracation, a total uptake of alongside it. NMR spectra also dif- four electrons is expected. In the ferentiate 1,4-phenylenedioxy units tetrapyridinium compound alone, inside and alongside the tetrapyr- not bound up in a catenane, Kaifer and Goodnow recorded two twoidinium ring. The two NMR proton signals of electron processes. They conclude " i n s i d e " and "alongside" 1,4-phe- that the two bipyridinium dications nylenedioxy groups coalesce into are in like environments, so each one at 81 °C. Thus the crown ether takes up one electron at one potenring revolves through the center of tial and one more at a different the tetrapyridinium ring with an potential. For the catenane, the Miami chemactivation energy of 14.0 kcal per ists observe two one-electron promole. cesses and one two-electron process. They suggest that this is because Preprogramed reactants the "inside" bipyridyl dication is key to efficient final step complexed by two electron-donating 1,4-phenylenedioxy groups and the "alongside" dication by only one. Thus the "alongside" dication may take up one electron at one potential. Then the "inside" dication takes up one at a second potential. With each reduced to a radical cation, there is not so much difference in the effects of one versus two 1,4phenylenedioxy groups. So each bipyridyl radical cation appears to take up one electron at about the same third potential. Stephen Stinson
Accountable indeed. A t Siegfried, we believe a custom manufacturer of pharmaceuticalfinechemicals should be held fully accountable for the confidentiality of customer data, from formulations to process engineering. L here is only one thing we do share freely with all our customers: a degree of process know-how ranging from bromination to catalytic hydrogénation, chloromethylation and cyanacetic ester synthesis, Friedel-Crafts acylation, Grignard reaction, phase transfer catalyzed reactions, phosgenation, sulfonation (including amide formation) and cyanidation, that may well be unique in the industry. Why not get in touch with one of our marketing chemists? CH-48UU Zotingen Switzerland Phone (062) 50 1111 Telefax (062) 52 27 04 Telex 982 444 sgfch November 13, 1989 C&EN
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