Extending Alkyne Versatility

A collaborative academic-industry research team in the U.K. has devised a method to convert the terminal triple bond of an alkyne into a diester with the two ester groups at opposite ends of the molecule. This unusual reaction, made possible by a cascade of double-bond isomerization steps, provides new versatility in preparing chemical intermediates from alkynes, which are popular chemical feedstocks.

A team including A. Alberto Núñez Magro and David J. Cole-Hamilton of the University of St. Andrews, in Scotland, and Graham R. Eastham of acrylic polymer producer Lucite International, in Wilton, England, made the discovery when exploring methoxycarbonylation reactions (Chem. Sci., DOI: 10.1039/c0sc00276c). In these reactions, carbon monoxide and methanol are added sequentially to a carbon-carbon double or triple bond to form an ester.

Cole-Hamilton’s group found that the palladium catalyst used for the reaction is highly selective for producing linear esters rather than branched esters, which are the usual products of alkyne carbonylations. Lucite also uses this catalyst, which features a customized phosphinomethylbenzene ligand, to make methyl propanoate from ethylene as part of a commercial route to methyl methacrylate monomer.

In the new reaction, the first step is the methoxycarbonylation of an alkyne to form an α,β-unsaturated ester, Cole-Hamilton explains. The ester’s carbon-carbon double bond then isomerizes stepwise to each carbon along the molecule’s chain, a process his group has observed before with unsaturated esters. When the double bond reaches the end of the chain—the thermodynamically least-favored position for the double bond—it’s trapped by a second methoxycarbonylation step, thereby forming an α,ω-diester.

Although alkoxycarbonylation reactions to prepare esters are well-known, the new one boasts “very high linear selectivity, as well as the unprecedented second step to give α,ω-diesters,” says Chao-Jun Li of McGill University, in Montreal, who specializes in alkyne-based coupling reactions. “This work provides a nice alternative to the less-atom-economical Wittig reaction and Heck-type reactions,” Li notes. “Furthermore, the diesters are highly useful, for example, in polymer synthesis.”