Abstract
ABSTRACT 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-dimethyMH-pyrazole (L1), 3,5-di-ferf-butyl-1ZÏ-pyrazole (L2) and 5-ferrocenyl-1Zf-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). Keywords: Pyrazolyl catalysts, sorbic acid, hydrogenation, selectivity.
Highlights
Hydrogenation of α,β-unsaturated compounds has been widely employed in the vitamins, fragrances, pharmaceuticals, petrochemicals, agrochemicals, and cosmetics industries
Nickel9 and palladium10 complexes have in recent times gained considerable attention as efficient catalysts in hydrogenation reactions
We report on pyrazolyl nickel(II) and palladium(II) complexes as catalysts for the hydrogenation of 2,4-hexanoic acid, which is an α,β-unsaturated acid
Summary
Hydrogenation of α,β-unsaturated compounds has been widely employed in the vitamins, fragrances, pharmaceuticals, petrochemicals, agrochemicals, and cosmetics industries. One of the extensively used transition metal catalysts in these hydrogenation reactions is chlorotris(triphenylphosphine)rhodium(I), [RhCl(PPh3)3].2,3 The catalyst, RhCl(PPh3), catalyzes the chemo-specific hydrogenation of C=C bonds in the presence of other reduced groups, like nitro (NO2) or carbonyl (CHO), as well as terminal alkenes even when the substrate has internal alkenes. Other transition metals complexes have been extensively studied as heterogeneous and homogeneous catalysts in the catalytic hydrogenation of olefins and α,β-unsaturated compounds. Other transition metals complexes have been extensively studied as heterogeneous and homogeneous catalysts in the catalytic hydrogenation of olefins and α,β-unsaturated compounds. Among these metal complexes are ruthenium, rhodium, iridium, and platinum.. P∧C∧P palladium pincer complexes are highly active catalysts for the chemo-selective transfer hydrogenation of α,β-unsaturated ketones.. P∧C∧P palladium pincer complexes are highly active catalysts for the chemo-selective transfer hydrogenation of α,β-unsaturated ketones.13 These highly reactive and selective palladium pincer complexes afforded saturated ketones from α-enones.. We report on pyrazolyl nickel(II) and palladium(II) complexes as catalysts for the hydrogenation of 2,4-hexanoic acid (sorbic acid), which is an α,β-unsaturated acid. This study forms part of a bigger project on partial hydrogenation of biofuels from triglycerides
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