A Flow Process for Aminomethylations using Catalysts Supported on Magnetic Nanoparticles

Homogeneous catalysts offer several advantages over their heterogeneous counterparts; including the greater selectivity and controllability because their molecular nature ensures that only one type of active site is present. Furthermore, it is estimated that 85% of all chemical processes are run catalytically, with the ratio of applications of heterogeneous to homogeneous catalysis of ca. 75:25.However, continuous flow processes involving homogeneous catalysis present difficulties and many efficient systems in batch processes cannot be transferred to flow. A major problem is associated with separating the products from the catalyst. The group at Bath has recently prepared two types of catalyst consisting of either organometallic species or a metallic shell around superparamagnetic iron oxide cores. Preliminary results indicate that immobilized sulfonated phosphines or acetate ligands allow the coordination of rhodium or palladium complexes that efficiently catalyse (up to 100% conversion) the conjugate addition of boronic acids, and Suzuki and Heck coupling, as well as hydrogenation and dihydroxylation reactions. The catalysts retained activity after magnetic separation, in some cases even after 10 consecutive runs. In this proposal we wish to develop flow chemistry protocols for the palladium-catalysed coupling of aminoalkylboron reagents using new types of magnetically moveable and recoverable semi-homogeneous catalysts. Their size means that they operate in the same manner as homogeneous catalysts but they are easily recovered in a magnetic field. With a clear emphasis on developing methodology of broad application to the synthesis of medicinal compounds, we will focus on the catalytic aminomethylation of aryl/vinyl halides as a strategic alternative to reductive amination. Normally the magnetic properties of the nanoparticles have been used to facilitate separation from the reaction product(s). We wish to extend this by further exploitation of the magnetism to (i) entrap the nanoparticle catalyst within certain regions of a flow reactor and (ii) to apply alternating magnetic fields to manipulate and move the nanoparticles around the reactor, enhancing mass transfer. This new technology will offer a number of advantages, chiefly entrapment of the homogeneous catalyst in the reactor without necessity of separation from products.