Dihydrogen reduction of nitroorganics, alkenes, alkynes, and Schiff bases using polymer-anchored orthometalated Schiff-base complexes of palladium(II) as catalysts

Complexation of Schiff bases derived from the condensation of aminopolystyrene and carbonyl compounds, PhCOR (R=H, CH3, C6H5) with palladium(II) acetate results in the formation of acetatobridged, dinuclear orthopalladated complexes anchored to macroporous polystyrene beads. Hydrogen activation of the material al 80 degrees C in DMF suspension brings to the reduction of coordinated azomethine group with simultaneous replacement of the bridged acetate by H and DMF, producing the corresponding mononuclear secondary amine complexes. The activated species are highly active towards the dihydrogen reduction of aromatic nitro-and carbonyl compounds, alkenes, alkynes, and Schiff bases under normal pressure at ambient temperature in aprotic solvents like DMF, DMSO, ethyl acetate, and THF. The highest activity was observed with R=H in a DMF medium. Similar polystyrene-based orthometalated secondary amine complexes of palladium(II) prepared by alternative methods exhibit comparable catalytic activities, The same specimen of the catalyst can bs used repeatedly for the reduction of the same or different substrates under comparable reaction conditions and stored indefinitely without any loss of catalytic activity. A tentative reduction mechanism has been suggested on the basis of catalyst transformation, Identification of the intermediates at various stages of reaction, and kinetic studies. (C) 1998 Academic Press.