Vinylic Addition Polynorbornene as Support for N-Heterocyclic Carbene Palladium Complexes: Use as Reservoir of Active Homogeneous Catalytic Species in C-C Cross-Coupling Reactions

摘要

A vinylic addition polynorbornene (VA-PNB) functionalized with imidazolium groups can be used to synthesize a well-defined supported palladium carbene complex. This complex is a suitable precatalyst for Suzuki and Negishi cross-coupling reactions, and it can be reused. A close look at how the catalysis works reveals that palladium aggregates form on the polymer surface after the first catalytic reaction. However, they are not easily washed out and serve as a source of small amounts of homogeneous palladium active species (about 0.01% mol), which are responsible for the catalysis. The recovered polymer can be reused several times although increased induction and reaction times are observed in consecutive uses from one run to the next. The reaction was followed up by in situ IR spectroscopy by using the nu(C-Br) absorption band of the electrophile, so there is no need for an additional functional group to apply this technique.