Esters with Ease

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Science  05 Aug 2005:
Vol. 309, Issue 5736, pp. 853
DOI: 10.1126/science.309.5736.853c

Organic esters are widely used as fragrances and in the synthesis of pharmaceutical compounds. Among the many synthetic routes to esters, the oxidative dimerization of alcohols is direct and involves the endothermic liberation of a dihydrogen equivalent from each alcohol; efficient reactivity therefore requires another component, a stoichiometric H2 acceptor.

Zhang et al. have developed a homogeneous ruthenium catalyst to couple primary alcohols into esters in the absence of any extra reagents. The reaction occurs in toluene (115°C) at 0.1 mole % catalyst loading; continuous purging of H2 from the system drives the equilibrium to >90% yield of the ester for butanol, hexanol, and benzyl alcohol. The key component of the catalyst is a tridentate ligand, consisting of diethyl-amino and di-tert-butylphosphino coordinating groups appended to a pyridine ring, and the low kinetic barrier may be due to lability of the diethylamino arm at the Ru center. Preliminary studies of the mechanism support the initial oxidation of one alcohol to the aldehyde, followed by addition of the second alcohol to form a hemiacetal, which in turn loses H2 to give the ester. — JSY

J. Am. Chem. Soc. 10.1021/ja052862b (2005).

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