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

Abstract

AbstractA 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‐free 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 σ* orbital of the trans Pd–pyridine bond. This weakens the Pd‐bound “throwaway” pyridine ligand, and its dissociation marks the initiation of the catalytic cycle.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)