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Methylation of tertiary amines by esters first appeared in the literature in 1902. Except for certain special cases, the reaction is slow, taking up to days even at temperatures above 100 °C. Now, Scripps Research Institute chemists Byron W. Purse, Pablo Ballester, and Julius Rebek Jr. have dramatically sped it up using molecular recognition principles. The work offers a simple model for catalyzing reactions—that is, by spatial positioning of reactants and eliminating solvent molecules from the reaction center through a supramolecular assembly. To accelerate the reaction, the researchers use a molecule that self-assembles into a bowl with a carboxyl group extending from the rim and pointing into the bowl’s cavity. The cavity can host tertiary amines, such as quinuclidine, that are held in place by nonbonding interactions. Treatment with diazomethane converts the dangling carboxyl group into a methyl ester group. An amine that displaces solvent molecules in the bowl rapidly captures the dangling methyl group at room temperature [J. Am. Chem. Soc., published online Nov. 8, http://dx.doi.org/10.1021/ ja036595q]. Commenting on the work, François Diederich, a chemistry professor at the Swiss Federal Institute of Technology, Zurich, says, “There are other examples of synthetic hosts that display recognition-controlled reactivity, but none has demonstrated such potent activation of an otherwise reluctant reaction.” And according to Steven C. Zimmerman, a chemistry professor at the University of Illinois, Urbana-Champaign, the bowl “appears to accelerate the reaction through strategies attributed to enzymes: binding-induced proximity, precise alignment of functional groups, and differential solvation of the transition state. All this leads to facile methyl transfer. The next step would be to build in turnover.” |
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