Selective Hydrogenation of Conjugated Dienes Catalyzed by Ethylbis(2, 2'-bipyridyl)cobalt( I)

Abstract

Ethylbis(2, 2'-bipyridyl)cobalt( I ), [Co(Et)(bpy)₂], was prepared by dissolving diethylbis (2, 2'-bipyridyl)cobalt (III) tetraethylaluminate in polar ethers (THF, 1, 2-dimethoxyethane etc. ). Conjugated dienes were selectively hydrogenated to the corresponding monoenes by [Co(Et)(bpy)₂] where internal cis-olefins were the main products (>80%). The inductio n period disappeared by treating the cobalt( I ) complex with dienes for 30 min. The hydrogenation of 1, 3-butadiene was of first order in the cobalt concentration, of first order in the hydrogen pressure and independent of the butadiene concentration. The rate of hydrogenation increased with the increasing concentration of pretreated butadiene. The increase was more pronounced as the cobalt concentration became lower. The lower stability of the cobalt( I ) complexes with internally methyl-substituted 1, 3-butadienes caused a slight decomposition to metals and a decrease in selectivity. Nortricyclene was initially formed as the main product in the hydrogenation of 2, 5-norbornadiene with [Co(Et)(bpy)₂]but the selectivity of nortricyclene decreased with the elapse of time due to its isomerization into norbornene. The hydridocobalt( I) formed by the dissociation of ethylene from [Co(Et)(bpy)₂] is thought to react with butadiene to give anti-[1-3-η-(1-methylallyl)]cobalt which then is hydrogenated to afford mainly cis-2-butene.