Hydrogenation and Redox Isomerization of Allylic Alcohols Catalyzed by a New Water-Soluble Pd–tetrahydrosalen Complex

For
applications in aqueous media, sulfonated tetrahydrosalen (sulfosalan,
HSS) was synthesized by sulfonation of tetrahydrosalen in fuming sulfuric
acid. The Pd­(II) complex of this ligand, [Pd­(HSS)], showed outstanding
activity in hydrogenation and redox isomerization of allylic alcohols
in homogeneous aqueous solutions or in aqueous–organic biphasic
systems (for oct-1-en-3-ol TOF­(hydrogenation) = 1580 h–1, TOF­(redox isomerization) = 400 h–1). DFT calculations
revealed that H2 is activated heterolytically, resulting
in a Pd­(II)–hydride complex, [Pd­(H)­(HSS-Hphen)], in which one
of the phenolate oxygens is protonated. Both hydrogenation and redox
isomerization take place via concerted transfer of a proton and a
hydride from the hydrogenated catalyst to the allylic alcohol.