Catalytic hydrogenation of sorbic acid using pyrazolyl palladium(II) and nickel(II) complexes as precatalysts

We have prepared several pyrazolyl palladium and nickel complexes ([(L1)PdCl₂] (1), [(L2) PdCl₂] (2), [(L3) PdCl₂] (3), [(L1) NiBr₂] (4), [(L2) NiBr₂] (5) and [(L3) NiBr₂] (6)) by reacting 3,5-dimethyl-1H-pyrazole (L1), 3,5-di-tert-butyl-1H-pyrazole (L2) and 5-ferrocenyl-1H-pyrazole(L3) with [PdCl₂(NCMe)₂] or [NiBr₂(DME)] to afford mononuclear palladium and nickel complexes, respectively. These complexes were then investigated as pre-catalysts in the hydrogenation of 2,4-hexadienoic acid (sorbic acid). The active catalysts from these complexes demonstrate significant activities under mild experimental conditions. Additionally, the active catalysts show that the hydrogenation of sorbic acid proceeds in a sequential manner, where the less hindered C=C bond (4-hexenoic acid) is preferentially reduced over the more hindered C=C bond (2-hexenoic acid).