Inductive heating for organic synthesis by using functionalized magnetic nanoparticles inside microreactors.

Interest in magnetic nanoparticles has increased considerably lately, with diverse applications as magnetic liquids, in catalysis, in biotechnology and biomedicine, and in magnetic resonance spectroscopy. A principal problem associated with naked metallic nanoparticles is their high chemical reactivity, in particular oxidation by air. This drawback can be overcome by coating the nanoparticles with SiO2, metal oxides, gold, or carbon. Several applications of these nanoparticles for quasi-homogeneous catalysis have been disclosed. These particles are typically removed after the reaction by exploiting their magnetic properties. An unexploited and very important feature of magnetic materials is the possibility of heating them in an electromagnetic field. It has been demonstrated that isolated magnetic nanoparticles show magnetic behavior different from that in the bulk. These magnetic nanoparticles when coated with a silica shell can show superparamagnetic behavior. The silica coating prevents the magnetic cores from coupling, thereby preserving their superparamagnetic properties. These composites do not have a residual magnetization and their magnetization curves are anhysteretic. However, the susceptibility of a superparamagnetic material is almost as high as that of a ferromagnetic material. The concept of magnetically induced hyperthermia is based on specific properties of the magnetic nanoparticles upon exposure to a constantly changing magnetic field. Surprisingly, this property of magnetic nanoparticles has so far not been applied in chemical synthesis, although organic chemists are constantly testing new technologies such as microwave irradiation, solid-phase synthesis, and new reactor designs in their work with the goal of performing syntheses and workups more efficiently. Herein we disclose the first application of heating magnetic silica-coated nanoparticles in an electromagnetic field. We demonstrate that these hot particles can be ideally used inside a microfluidic fixed-bed reactor for performing chemical syntheses including catalytic transformations. Thus, besides conventional and microwave heating, magnetic induction in an electromagnetic field is a third way to introduce thermal energy to a reactor. Superparamagnetic materials like nanoparticles 1 can be heated in mediumor high-frequency fields. As the technical setup for the middle-frequency field (25 kHz) is simpler (see Figure 1b,c), we investigated the electromagnetic induction of heat in magnetic nanoparticles in this frequency range. In principal, the processes can be operated in a cyclic or a continuous mode. The inductor can accommodate a flowthrough reactor (glass; 14 cm length, 9 mm internal diameter), which is filled with superparamagnetic material 1. The reactor can be operated up to a backup