Fluoride-Free Hiyama and Copper- and Amine-Free Sonogashira Coupling in Air in a Mixed Aqueous Medium by a Series of PEPPSI-Themed Precatalysts

A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX2(pyridine)-type (X = Cl, Br) precatalysts of C4-C5 saturated imidazole- (1-4) and triazole-based (5 and 6) N-heterocyclic carbenes for the Hiyama and Sonogashira couplings under amenable conditions are reported. Specifically 1-6 efficiently catalyze the fluoride-flee Hiyama coupling of aryl halides with PhSi(OMe)(3) and CH2=CHSi(OMe)(3) in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H2O, 2:1. v/v). Along the same lines, these 1-6 precatalysts also promote the Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with phenylacetylene in air and in a mixed aqueous medium (DMF/H2O, 3/1 v/v). The complexes 1-6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl2, in pyridine in the presence of K2CO3 as a base. DFT studies on the catalytically relevant palladium(0) (NHC)Pd(pyridine) precursors 1a-6a reveal significant donation from the N-heterocyclic carbene lone pair onto the unfilled sigma* orbital of the trans Pd-pyridine bond. This weakens the Pd-bound "throw-away" pyridine ligand, and its dissociation marks the initiation of the catalytic cycle. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)