E-Selective semi-hydrogenation of alkynes via a sulfur-radical mediation over cyclodextrin-modified nickel nanocatalyst

A nickel-based catalytic system, including simple NiCl2 center dot 6H(2)O, sodium borohydride and mercapto-beta-cyclodextrin (CD-SH), is developed for the semi-hydrogenation of alkynes to E-alkenes. This system features a high efficiency with good E-selectivity at room temperature. The high efficiency is not only due to the highly active H delta- (NaBH4), H delta+ (methanol) and in situ formed Ni nanoparticles, but also because of the host-guest interaction between CD-SH and Z-alkene intermediate. Kinetic and mechanistic studies demonstrate that the E-alkene is obtained via an efficient Z-selective semi-hydrogenation, followed by a sulfur-radical mediated Z-to-E isomerization. This study provides a cheaper, efficient and practical strategy for the activity and selectivity improvement of E-selective semi-hydrogenation of alkynes.