Green catalysts: Solid-phase peptide carbene ligands in aqueous transition-metal catalysis

A series of functionalized imidazolium ions containing a pyridine moiety and carboxylic acid functionality has been synthesized in solution. These compounds serve as N-heterocyclic carbene precursors and were attached to a dipeptide on solid support by means of standard peptide coupling techniques. Treatment with base generated the corresponding carbenes, which were directly complexed to palladium(II) and subsequently studied by mass spectrometry and NMR spectroscopy. The supported monocarbene catalyst 7 was successfully applied in Sonogashira and Suzuki cross-coupling reactions, and the cross-coupling products were isolated in excellent yield. Bis(carbene) catalyst 8 was successfully applied in solution in high-yielding Suzuki cross-coupling reactions. Furthermore, the catalysts proved to be stable in aqueous media, which allowed the Suzuki cross-coupling reactions to be performed in water. No loss of catalytic activity was observed when supported catalyst 7 was recycled and subjected to repetitive cycles of Suzuki cross-coupling reactions in water.