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Chemical Shifts.

What's new in chemistry research? This new column, compiled by Cathleen Crudden, MCIC, will appear bimonthly and offer readers a concentrated look at the latest developments in chemistry from Canadian researchers. Have you heard of any new and exciting research that readers may welcome? Contact Crudden at cruddenc@unb.ca with your news.

The development of novel organic materials for use in optical devices and erasable memory applications is just one of the research interests of Neil Branda, MCIC. This recent recipient of a Canada Research Chair at Simon Fraser University in Burnaby, BC is putting his knowledge of self-assembly to work in the area of data storage. Molecules which have two thiophene species linked by an alkene can be photochemically cyclized to give two different states. Although these systems have the advantage that they are thermally robust and fatigue resistant, the UV/Vis spectroscopy used to read the information is not significantly different from the light that is used to erase information, which leads to a loss of information during the read-out process. Measuring changes in optical rotation between two states of a molecule is a promising alternative, since detecting the optical rotation doesn't cause any loss of information. In order to do this, one needs to have a clean reaction that produces only one stereoisomer an d there needs to be a significant difference in the optical rotation between the two states. In a recent communication, Branda and his co-workers Elisa Murguly and Tyler Norsten have described an elegant method for the preparation of a thiophene-derived optical storage device that relies upon the self-assembly of two chiral bisthiophenes in the presence of copper. In a nice twist, chiral, optically pure oxazolines on the periphery of the thiophenes control the self-assembly to give dimers of only one helical chirality, and this helical chirality controls the photodimerization to give essentially only one diastereomer (see opposite). In the absence of Cu, the bisozaxoline-modified bisthiophene cyclizes without any control of stereochemistry, giving equal mixtures of the (S,S)-(R,R) and (S,S)-(S,S) diastereomers. In the presence of Cu at carefully controlled concentrations, up to 98% selectivity for the (S,S)-(S,S) isomer is obtained. Finally, the Branda team showed that there are several spectral regions where the cyclized and uncyclized compounds have significantly different optical rotatory power, permitting differentiation during read-out. For more details, see the ninth issue of Angew. Chemie., Int. Ed. Engl., p. 1752.

Although catalytic reactions can often be sped up by the addition of a larger amount of the catalyst, this isn't always the case. Lisa Rosenberg, MCIC, Colin W Davis, and Junzhi Yao have shown through a careful study of the dehydrocoupling of silanes that less is more. On p. 5120 of this year's JACS, Rosenberg, then at the University of Manitoba, now at U. Vic., showed that adding more catalyst without controlling other variables actually decreased the efficiency of the reaction she was studying (eq. 1). This reaction has potential applications in the preparation of polysilanes, which are important industrial materials because of their unique optical and electronic properties. The trick is to promote the dehydrocoupling, and prevent a competing redistribution reaction (eq. 1), both of which are catalyzed by rhodium. Rosenberg and co-workers found that the equilibrium between [Ph.sub.2][SiH.sub.2] and the desired product favoured the starting material. However, removal of hydrogen could be used to drive the e quilibrium to the right. Under optimized conditions, Wilkinson's catalyst gave the dehydro-coupling product almost 100 times faster than previous Ti-based catalyst systems!

Richard Puddephatt, FCIC and coworkers Christopher Fraser, MCIC and Michael Jennings, MCIC at the University of Western Ontario have recently reported the first example of an organometallic polyrotaxane. A variety of different organic polyrotaxanes have been reported over the last 10 years or so, and are generally of the linear 'beads-on-a-string' type. Other architectures are possible, and if the polymer chains also contain rings, 2-D sheets can be obtained. The Puddephatt work describes a new type of 2-D sheet in which repeating interpenetration of the polymer chains occurs. To prepare these unique molecules which may have applications ranging from drug delivery vehicles to sensor devices, organometallic chemistry, co-ordination chemistry and self-assembly are used. Although these techniques have all been employed in different ways in the past, putting them together in one synthesis is unique. As shown below, oxidative addition of [alpha]-bromo-4-toluic acid to a Pt-bipy complex is followed by treatment wi th [AgPF.sub.6] and a tris pyridine species.

The triacid thus prepared self assembles into rotaxanes by hydrogen bonding between the carboxylic acid groups. Remarkably, two different chains interpenetrate producing the polyrotaxane structure (see above). For more details on this chemistry, see the original publication in Chemical Communications, p. 1310.

Synthetic chemists have a lot to learn from enzymes. They can do incredible transformations, quickly, efficiently and in water. One thing enzymes are not good at, is fluorination, though clearly some enzymes are able to introduce fluorine into organic molecules since 13 fluorinated natural products are known. What is not known is how they do it. Chemists introduce fluorine into organic or inorganic compounds using electrophilic, radical, or nucleophilic sources. But nature has primarily insoluble mineral forms of fluoride to work with. It is perhaps because of this that there are no known enzymes with fluorinating ability that is, no natural enzymes.

Stephen Withers, ECIG and colleagues David Zechel, AGIG, Stephen Reid, Oyekanmi Nashiru, Christoph Mayer, Dominik Stoll, David L. Jakeman, and R. Antony Warren at UBC have recently developed mutant enzymes that catalyze the formation of carbon-fluorine bonds. These mutants of retaining glycosidases have had the catalytic glutamate nucleophile replaced with alanine, glycine, or serine, preventing the usual double displacement mechanism for glycosidic bond cleavage. If F is added, the activity of the enzyme is 'rescued' since fluoride takes the place of the missing nucleophile. The product of this reaction is a glycosyl fluoride, which is also a substrate for the enzyme, so it is not isolated, but it can be observed by a variety of techniques.

Not satisfied with finding a mutant that synthesizes [alpha]-glycosyl fluorides, Withers and his team identified another mutant that makes [beta]-glycosyl fluorides. The general acid-base mutant of Man2A (E429A) has a nucleophile present so the initial bond formation can take place, but the second step, deglycosylation, is prevented and so the covalent intermediate builds up and effectively shuts down the enzyme. Once again, fluoride comes to the rescue. In the presence of F, the covalent bond is cleaved and the activity of the enzyme restored.

For this work, see p. 4350 in issue 18 of JACS, and for other fun and games with fluorine, see more work from the Withers group in Angew. Chemie., Int. Ed. Engl., 2001, p. 417.

Cathleen Crudden, MCIC, is a university research professor at The University of New Brunswick.
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Author:Crudden, Cathleen
Publication:Canadian Chemical News
Date:Sep 1, 2001
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