A practical heterogeneous catalyst for the Suzuki, Sonogashira, and Stille coupling reactions of unreactive aryl chlorides.

Transition-metal-catalyzed coupling reactions have contributed greatly to the straightforward and facile construction of carbon–carbon bonds. Significant progress in this area has been achieved with a variety of homogeneous palladium catalysts. However, homogeneous catalysis suffers from the problematic separation of the expensive catalyst from the product for re-use. Moreover, the homogeneous palladium catalysts tend to lose their catalytic activity because of palladium metal aggregation and precipitation. These problems are of particular environmental and economic concern in large-scale syntheses. Heterogenization of the existing homogeneous palladium catalysts could be an attractive solution to this problem. There has been considerable interest in the development of heterogeneous catalytic systems that can be efficiently re-used whilst keeping the inherent activity of the catalytic center. Aryl iodides and bromides have been widely employed as substrates in heterogeneous coupling reactions. From a practical point of view, the use of aryl chlorides is highly desirable because they are readily available and inexpensive. However, they are much more difficult to activate than aryl iodides and bromides. The activation of aryl halides is particularly challenging for heterogeneous catalysts, and although there have been many reports of heterogeneous reactions in the literature, successful examples using deactivated aryl chlorides are quite rare. Therefore, the development of high-performance catalysts for practical catalytic coupling reactions is of ongoing interest. Magnetite Fe3O4 nanoparticles have recently emerged as promising supports for immobilization because Fe3O4-supported catalysts can be separated from the reaction medium by an external permanent magnet. This circumvents time-consuming and laborious separation steps, and allows for practical continuous catalysis. In particular, Fe3O4 nanoparticles coated with a thin layer of silica have beneficial properties, such as invariant catalytic activity and stability. We have previously reported the use of a (b-oxoiminato)(phosphanyl)palladium complex as an efficient catalyst in coupling reactions. In this context, we have prepared triethoxysilyl-functionalized palladium complex 2, which can be anchored easily onto the surface of the silica. Commercially available Fe3O4 nanoparticles, with an average diameter of 20 nm, were coated with a thin layer of silica using a sol-gel process to give silica-coated Fe3O4 (3 ; SiO2@Fe3O4). [13] The silica shell has plenty of hydroxyl groups for potential derivatization with different functional groups, and also protects the magnetite core from abrasion under harsh shaking conditions. The silylated palladium complex 2 was successfully immobilized on the surface of robust SiO2@Fe3O4 (3) (Scheme 1). Schiff-base condensation of 2,4-pentanedione with (3-aminopropyl)triethoxysilane under microwave heating afforded 1 in only 3 min in quantitative yield. Deprotonation of 1 with EtOTl in tetrahydrofuran, followed by treatment with [Pd2(m-Cl)2Me2(PPh3)2], [14] led to the formation of 2. Magnetic-nanoparticle-supported (b-oxoiminato)(phosphanyl)palladium complex 4 was obtained by reaction