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

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‐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)

Synthesis of an extremely sterically shielding N-heterocyclic carbene ligand

“…Under standard reaction conditions reported in the literature, 21 an isolated yield of 27% diphenylacetylene product suggests that the Sonogashira coupling does not benefit from the steric demand of the IPr** system. Scheme 3.…”

Synthesis, structure and catalytic activity of dicarbene dipalladium complexes with different substituents

“…151.3-152.5 ppm for these complexes. The resonance of carbene carbon in these C4-C5 unsaturated palladium complexes is significantly upfield compared to that in those C4-C5 saturated complexes (184.2-186.3 ppm) [22], which means that the carbene in unsaturated complexes is more electron rich than that in their saturated analogues. The 1 H NMR of complex 2 showed that only one signals for six of CH 3 of t-butyl group and the 1 H NMR patterns of four CH 3 groups of 1,6-substituents on phenyl 14 ppm for 4) as well.…”

Palladium‐Catalyzed α‐Ketone Arylation under Mild Conditions

“…[8] Thus, we synthesized different types of Pd-NHC-Py complexes 1-3 (Scheme 1) according to a literature procedure [9] and investigated their catalytic activities in the α-ketone arylation of aryl halides. To understand whether the supporting pyridine ligand affects the catalytic activity in the reaction, we also tested its 3-chloropyridine analogue 4, which is commercially available.…”