Mechanistic investigation of Heck reaction catalyzed by new catalytic system composed of Fe3O4@OA–Pd and ionic liquids as co-catalyst

Abstract

A simple and efficient protocol has been developed for the Heck coupling reaction catalyzed by a two component catalytic system composed of palladium deposited oleic acid coated-Fe3O4 nanoparticles (Fe3O4@OA–Pd) and thermo-regulated molybdovanadate-based ionic liquid salts (H5PMo10V2O40-IL, Mo10V2-IL) as a novel and recyclable catalytic system. In this system, low amounts of Mo10V2-ILs are used as an efficient co-catalyst, which reduce cost and increase efficiency in the Heck coupling reaction. These thermo-regulated co-catalysts are thermo-responsive, and could reversibly transform to precipitate with a decrease in temperature. In this study, the catalytic activities of various types of Mo10V2-IL including Mo10V2-(3-sulfonic acid) propylpyridine (Mo10V2-PyPS), Mo10V2-(4-sulfonic acid) butylpyridine (Mo10V2-PyBS), Mo10V2-(4-sulfonic acid) butyltrimethyl amine (Mo10V2-TMABS), Mo10V2-(4-sulfonic acid) butyltriethyl amine (Mo10V2-TEABS) and Mo10V2-(4-sulfonic acid) butyltributyl amine (Mo10V2-TBABS) were investigated in the Heck reaction. It was shown that all of the mentioned co-catalysts are efficient. It is apparent from cyclic voltammetric measurements that, these catalytic systems are electroactive and undergo reversible redox transitions between palladium nanoparticles and Mo10V2-IL species, also, it contains strong acid sites and mobile protons which can activate Ar–X and Pd–X bonds in oxidative addition and reductive elimination steps. As evidenced from mechanistic investigations, the electron transfer properties and Brönsted acidity of Mo10V2-ILs have been most pronounced upon application of the Fe3O4@OA–Pd and Mo10V2-IL system in the Heck coupling reaction. Fe3O4@OA–Pd and Mo10V2-PyPS exhibited excellent activities and the methodology is applicable to diverse substrates providing good to excellent yields of desired products. Moreover, this catalytic system was recycled for four consecutive cycles without any significant loss in activity.