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

Abstract

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 ν(C−Br) absorption band of the electrophile, so there is no need for an additional functional group to apply this technique.

α‐Diimine–Palladium Complexes Incorporated in Vinylic‐Addition Polynorbornenes: Synthesis and Catalytic Activity

“…We have previously reported the behavior of polymeric complexes in which palladium is coordinated to VA‐PNB‐N‐heterocyclic carbenes; in that case, the nature of the palladium moiety changes dramatically after the first Suzuki reaction, and palladium aggregates can be clearly seen by electron microscopy. In contrast to the system described here, the activity of those polymers steadily decreases in every run …”

“…In the last few years, we have been using vinylic‐addition polynorbornenes (VA‐PNBs) (Figure a) as supports of catalysts or reagents in several organocatalytic and palladium‐catalyzed processes , . In contrast to the materials resulting from ring‐opening metathesis polymerization of norbornene (ROMP‐PNB) (Figure b), these VA‐PNB materials have an aliphatic backbone that keeps the bicyclic structure of norbornene.…”

“…Thus, encouraged by the stability that the aliphatic and robust backbone of VA‐PNB has shown in former catalytic applications, and its suitable bicyclic structure for supporting a chelating diimine fragment, we report here on the preparation of such a VA‐PNB–diimine ligand (Figure c) and the performance of the palladium complexes derived from it as a catalyst in the polymerization of olefins and as a recyclable catalyst in Suzuki couplings.…”

Trispyrazolylborate Ligands Supported on Vinyl Addition Polynorbornenes and Their Copper Derivatives as Recyclable Catalysts

“…We have previously used VA‐PNB‐based polymers as supports for catalysts bearing other donor ligands such as N‐heterocyclic carbenes . We also have supported organotin reagents and used them in palladium‐catalyzed Stille couplings, and radical reactions, with good recyclability and the additional advantage of leaving a very low tin contamination in the products.…”

Vinylic Addition Polynorbornene in Catalysis

“…The polymeric imidazolium salts 2 are the precursors of well‐defined palladium carbene complexes as shown in Eq. 2 . Complexes 7 were characterized by solid state MAS‐NMR and show the characteristic Pd‐C carbene 13 C NMR signal at 180 ppm.…”