PEPPSI
PEPPSI is an abbreviation for pyridine-enhanced precatalyst preparation stabilization and initiation. It refers to a family of commercially available palladium catalysts developed around 2005 by Prof. Michael G. Organ and co-workers at York University, which can accelerate various carbon-carbon and carbon-heteroatom bond forming cross-coupling reactions. In comparison to many alternative palladium catalysts, Pd-PEPPSI-type complexes are stable to air and moisture and are relatively easy to synthesize and handle. Structure and synthesis. In the basic structure of Pd-PEPPSI, R can be a methyl (CH, Me), ethyl (CH, Et), isopropyl (CH, Pr), isopentyl  (CH, Pent), or isoheptyl (CH, Hept) group, and the resulting catalysts are thus labeled as PEPPSI-IMes, PEPPSI-IEt, PEPPSI-IPr, PEPPSI-IPent, and PEPPSI-IHept respectively, with or without "Pd-" added in front. Commonly used PEPPSI catalysts such as Pd-PEPPSI-IPr contain an unsubstituted imidazole core (R=H) and a 3-chloro substituted pyridine ligand (R=3-Cl). However, structural modifications of the imidazole backbone and pyridine ligand can profoundly affect the catalytic activity of these complexes. The synthesis and structure of Pd-PEPPSI catalysts were presented in 2005 and published in 2006. PEPPSI catalysts are organopalladium complexes  containing N-heterocyclic carbene (NHC) ligands. They can be obtained by reacting an imidazolium salt, palladium(II) chloride, and potassium carbonate in 3-chloropyridine as a solvent, under vigorous stirring at 80 °C for 16 hours in air. The yield of PEPPSI in this reaction is 97–98%. Contrary to other common palladium-based catalysts, such as tetrakis(triphenylphosphine)palladium(0), PEPPSI is stable to exposure to air and moisture. Even heating in dimethyl sulfoxide at 120 °C for hours does not result in significant decomposition or deactivation of PEPPSI catalysts. Properties and applications. PEPPSI can catalyze various palladium cross-coupling reactions including  Negishi coupling, Suzuki coupling, Sonogashira coupling, Kumada coupling, and the Buchwald–Hartwig amination as well as aryl sulfination and the Heck reaction. In Negishi coupling, PEPPSI promotes reaction of alkyl halides, aryl halides or alkyl sulfonates with alkylzinc halides, and the important advantage of PEPPSI over alternative catalysts is that the reaction can be carried out in a general chemical laboratory, without a glove box. PEPPSI contains palladium in the +2 oxidation state and is thus a "precatalyst", that is the metal must be reduced to the active Pd(0) form in order to enter the cross-coupling catalytic cycle. This  is usually achieved "in situ" in the presence of active transmetalating agents such as organo-magnesium, -zinc, -tin, or -boron reagents. Once activated, the NHC-Pd(0) species becomes rather air-sensitive.