摘要：

A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX<SUB>2</SUB>(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)<SUB>3</SUB> and CH<SUB>2</SUB>=CHSi(OMe)<SUB>3</SUB> in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H<SUB>2</SUB>O, 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/H<SUB>2</SUB>O, 3/1 v/v). The complexes 1 - 6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl<SUB>2</SUB> in pyridine in the presence of K<SUB>2</SUB>CO<SUB>3</SUB> 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 <IMG src="http://www3.interscience.wiley.com/giflibrary/12/sigma.gif" border=0>* orbital of the trans Pd-pyridine bond. This weakens the Pd-bound <IMG src="http://www3.interscience.wiley.com/giflibrary/12/ldquo.gif" border=0>throwaway<IMG src="http://www3.interscience.wiley.com/giflibrary/12/rdquo.gif" border=0> pyridine ligand, and its dissociation marks the initiation of the catalytic cycle.( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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