Picking O over N

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Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1163
DOI: 10.1126/science.319.5867.1163b

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In general, amines react more rapidly with carbonyl electrophiles than do alcohols. Lipase enzymes manage to invert this tendency and efficiently catalyze ester formation even in the face of a nearby nitrogen group striving to form an amide. Artificial catalysts have been less successful for this purpose though, and laboratory amino ester syntheses therefore often require wasteful N protection and deprotection steps. Ohshima et al. have now prepared a tetranuclear zinc cluster that bucks this trend and affords selectivity along the same lines as the enzymes. At loadings as low as 1.25 mole percent, the oxophilic catalyst gives 82 to 99% yields of the ester in reactions of methyl benzoate with a range of terminal alkyl amino alcohols. Similarly, ester selectivities higher than 90:1 are observed when equal concentrations of various amines and alcohols compete intermolecularly. The authors posit a mechanism that entails dual activation of the alcohol and electrophile by cooperative Zn centers. — JSY

J. Am. Chem. Soc. 130, 10.1021/ja073578i (2008).

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