PEPPSI

In the basic structure of Pd-PEPPSI, R1 can be a methyl (CH3, Me), ethyl (C2H5, Et), isopropyl (C3H7, iPr), isopentyl (C5H11, iPent), or isoheptyl (C7H15, iHept) group, and starting from the second in the row the resulting catalysts are thus labeled as 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 (R2=H) and a 3-chloro substituted pyridine ligand (R3=3-Cl). However, structural modifications of the imidazole backbone and pyridine ligand 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%. and moisture. Even heating in dimethyl sulfoxide at 120 °C for hours does not result in significant decomposition or deactivation of PEPPSI catalysts. iPEPPSI Examples of abnormal NHCs based on the mesoionic 1,2,3-triazol-5-ylidene structure have been used for palladium catalysis. In this manner, pyridine fused tzNHCs were prepared to yield palladium complexes with pyridine attached to the carbene core. With this ligand, air stable and highly active palladium complexes of iPEPPSI (as in internal PEPPSI) were synthesized. ==Properties and reactions==

PEPPSI catalyze various cross-coupling reactions including Negishi coupling, and the Buchwald–Hartwig amination 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. (a) and Buchwald-Hartwig reaction (b) can be activated by PEPPSI complexes. iPEPPSI (internal PEPPSI) type catalyse the copper-free Sonogashira reaction in aqueous solution. ==References==