The activity of silica immobilized palladium N-heterocyclic carbene complexes toward Mizoro-Heck reaction and their characterization

Abstract

C-C bond formation reactions are among the important processes in chemistry.A palladium-catalyzed reaction, commonly known as the Mizoroki-Heck (M-H) reaction, is a powerful method in construction of C-C coupling of olefins with aryl and vinyl halides.This thesis describes, mainly development and applications of novel silica anchored saturated molecular Pd-N-heterocyclic carbene (Pd-NHC) catalysts toward the M-H reactions of aryl halides.Characterization of the novel Pd-NHC complexes were performed using elemental analysis, NMR and X-ray crystallography techniques. The synthesized complexes were anchored over an amorphous silica surface through their triethoxy silane groups.On the basis of the results, the silica supported saturated Pd-NHCs were found to be highly active catalyst precursors for Mizoroki-Heck reactions of iodo- and bromoarenes with styrene and butyl acrylate olefins, when reactions were performed at 140 C in the presence of 1.5 molar equivalent Na2CO3 base. The catalyst preserved its original activity for 4-iodoanisole, 4-bromoacetophenone, 4-iodoacetophenone, iodobenzene and 4-iodotoluene reagents at least for 8 uses when Pd concentration, with respect to the aryl halide, was 0.5% in the reaction medium.The higher turnover frequencies were realized at the lower Pd concentrations. The AAS analyses revealed the presence of dissolved Pd species within the reaction medium at the early stages of reaction. Along with this finding, determination of Pd agglomerates onto silica surface by TEM examination indicates that the catalyst acted as a Pd reservoir and reactions were catalyzed homogeneously. In the course of reaction, Pd is liberalized to the solution involved with the reaction cycle and deposited onto silica surface to lead to form less active Pd agglomerates. That no trace of dissolved Pd was detected at the end of reaction by AAS is a positive finding for the purity of product.