E Alkenes From Alkynes

A practical method for making E (trans) alkenes from alkynes has been developed by chemists in Germany (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201205946). As organic chemistry students are taught, making alkenes with Z (cis) stereochemistry from alkynes is a simple matter of semihydrogenation with a poisoned catalyst. But until now, there has been no broadly applicable way to directly turn alkynes into E alkenes. Karin Radkowski, Basker Sundararaju, and Alois Fürstner of the Max Planck Institute for Coal Research report that a ruthenium-based catalyst is capable of performing just such a transformation. The ­hydrogenation proceeds in good yields and with high E-selectivity. The transforma­tion tolerates a range of functional groups, including esters, amides, carboxylic acids, ketones, primary alcohols, methyl and silyl ethers, and an elimination-prone primary tosylate. Reducible sites, such as a nitro group, an alkyl bromide, the N–O bond of a Weinreb amide, an aromatic nitrile, and a terminal alkene also remain intact under the reaction conditions. Adding silver triflate also improves the reaction. “The new method constitutes the first practical, efficient, functional-group-tolerant, broadly applicable, and highly E-selective semihydrogenation protocol for alkynes,” the researchers note.