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Pyrene-tagged dendritic catalysts noncovalently grafted onto magnetic Co/C nanoparticles: an efficient and recyclable system for drug synthesis.

The reuse of catalysts is highly desirable for economic and ecological reasons, and to this day constitutes an important challenge. Along these lines, magnetic nanoparticles (MNP) are increasingly recognized as appealing supports for catalytic systems in the development of more efficient and green processes. Contrary to conventional supports, such as polymers or silica, which require time-consuming precipitation and filtration steps, their separation can easily be achieved by magnetic decantation. Moreover, MNPs are mechanically robust and can be easily agitated during a reaction by application of an external magnetic field. Generally, MNPs are stabilized by coating the magnetic core with polymer or silica shells, which are used for the covalent immobilization of homogeneous catalysts. Magnetic carbon-coated nanoparticles have also been used as reusable supports for the covalent immobilization of catalysts; however, their graphene-like shell offers the unique possibility for non-covalent catalyst attachment by p–p stacking, a concept that was recently demonstrated with pyrene-tagged Pd monomeric complexes. Covalently immobilized dendronized catalysts, which possess an anchoring site and branches terminating with active sites, represent another promising strategy for recoverable catalysts, as they were shown to afford enhanced surface functionalization and better catalytic activity than corresponding monomeric catalysts. Combining both strategies, we planned to graft pyrene-tagged dendritic Pd-phosphine catalysts onto Co/C MNPs by p–p stacking, and to evaluate the activity and recyclability of the resulting composites in Suzuki reactions. Pyrene derivative 3 was prepared in high yield (91%) from commercial 1 and tyramine 2 (Scheme 1). 3 was allowed to react with N3P3Cl6 and Cs2CO3 to afford 4-G0 in 81% yield. The growth of this dendron was achieved by using the reactivity of the P Cl bonds towards phenolic group of 4-OH-