Alkyne hydroarylations with chelating dicarbene palladium(II) complex catalysts: Improved and unexpected reactivity patterns disclosed upon additive screening

Abstract

Palladium(II) complexes with a ligand set made from a chelating N-heterocyclic dicarbene ligand and two weakly coordinating anions (generally introduced in situ upon addition of 2 equiv. of a suitable silver salt) were found to be very active and selective catalysts for the room-temperature hydroarylation of alkynes at low catalyst loading (0.1 mol-%). Moreover, the screening of various strong acids as reaction promoters revealed that both the strength of the acid and the coordinating ability of its conjugated base influence the catalytic performance. Most: remarkably the use of HBF4 together with a dicarbene Pd complex catalyst: provides a dramatic change in the selectivity of the reaction, with the prevalent formation of a product stemming from the insertion of two molecules of alkyne into the aromatic C-H bond. The results presented herein highlight the fact that: the dicarbene ligand, apart from stabilising the catalyst, is also able to enhance catalytic activity and, most notably, to steer the reaction selectivity towards novel products. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.