Synthesis of palladium(II)-N-heterocyclic carbene via transmetaliation of silver(I) complex and its catalytic activity in epoxidation of alkene

Abstract

A series of new silver(I)- and palladium(II)-N-heterocyclic carbene complexes of 1-benzyl-3-ethylbenzimidazolium iodide, 1-benzyl-3-(2’-nitrilebenzyl)benzimidazole bromide, 1-benzyl-3-(2’-methyl)propylbenzimidazolium bromide, 1-benzyl-(3’-methyl)butylbenzimidazolium bromide, 1-benzyl-3-hexylbenzimidazolium bromide were successfully synthesized. The respective N-heterocyclic carbene (NHC) precursors were prepared by the reaction of 1-benzylbenzimidazole with ethyl iodide, 2-bromomethylbenzonitrile, 2-methylpropylbromide, 3-methylbutyl bromide, and n-hexyl bromide in dioxane at C90? . Further, these halide salts were converted into their hexafluorophosphate counterparts by the salt metathesis reaction using KPF6 in methanol. Bis-NHC silver(I) complexes having hexafluorophosphate counterion were yielded (48.4-67.2%) by the reaction of NHC precursors with silver(I) oxide in acetonitrile/methanol at room temperature. Subsequent reactions of the silver(I) hexafluorophosphate complexes with [PdCl2(CNCH3)2] in dichloromethane/methanol afforded the bis-NHC palladium(II) complexes in good yield (48.3-76.4%) via carbene transfer method. All new complexes were fully characterized by various techniques such as CHN analysis, single crystal X-ray diffraction and spectroscopic methods such as FTIR and 1H and 13C NMR. Additionally, in case of silver(I) complex, the bis-[1-benzyl-3-(2’-nitrilebenzyl)benzimidazole] silver(I) hexafluorophosphate was characterized by single crystal X-ray diffraction technique. Preliminary catalytic studies show that the nitrile-functionalized palladium(II)-NHC complex is highly active in the oxidation of 1-octene as well as styrene in aqueous hydrogen peroxide as oxidizing agent at C70? . All the palladium(II) complexes oxidized both the aforementioned olefins efficiently to their corresponding oxidized products with 47 to 58% conversions. However, selectivity towards epoxy products was relatively low in the range of 7-10%. Major product in the oxidation of 1-octene was the 1,2-octanediol, while 2-octanone is the minor product.