Kinetic and Spectroscopie Studies of the [Palladium(Ar-bian)]-Catalyzed Semi-Hydrogenation of 4-Octyne

摘要

The kinetics of the stereoselective semi-hydrogenation of 4-octyne in THF by the highly active catalyst [Pd{(m,m'-(CF_3)_2C_6H_3)-bian}(ma)] (2) (bian=bis(imino)acenaphthene; ma=maleic anhydride) has been investigated.The rate law under hydrogen-rich conditions is described by r=k[4-octyne]~(0.65)[Pd]-[H_2],showing first order in palladium and dihydrogen and a broken order in substrate.Parahydrogen studies have shown that a painwise transfer of hydrogen atoms occurs in the rate-limiting step.In agreement with recent theoretical results,the proposed mechanism consists of the consecutive steps: alkyne coordination,heterolytic dihydrogen activation (hydrogenolysis of one Pd-N bond),subsequent hydro-palladation of the alkyne,followed by addition of N-H to palladium,reductive coupling of vinyl and hydride and,finally,substitution of the product alkene by the alkyne substrate.Under hydrogen-limiting conditions,side reactions occur,that is,formation of catalytically inactive palladacycles by oxidative alkyne coupling.Furthermore,it has been shown that (2)-oct-4-ene is the primary reaction product,from which the minor product (E)-oct-4-ene is formed by an H_2-assisted,palladium-catalyzed isomerization reaction.