Tetra-substituted olefin synthesis using palladium-catalysed C-H activation

Abstract

In an effort to obtain more efficient and greener chemical transformations, a substantial amount of research interest has been directed towards the use of arene C-H bonds as functional groups. Hydroarylation of alkynes through direct functionalisation of C-H bonds has been studied in recent years leading to the development of high-yielding metal-mediated processes. The main aim of the current work is the addition of a third component in the hydroarylation of alkynes trough C-H activation, in order to achieve a second C-C bond formation. Attempts at palladium-catalysed three-component reaction of unactivated indoles with alkynes and aryliodides are described. The three-component reaction was studied in the intermolecular mode with both aryliodides and the more reactive diaryliodonium salts. These latter regents are reactive arylating and oxidising agents and have been used in the direct arylation of indoles under mild conditions through a PdII-PdIV catalytic cycle. In both cases the three-component product was not obtained. The intramolecular version of the reaction using alkyne-tethered indoles and diaryliodonium salts is also described. In this case the tandem process was successful, especially when using ethynylbenzyl indole derivatives, the Z-tetrasubstituted olefins could be selectively obtained under mild conditions. Finally, a low-yielding synthesis of chromenes from propargylaryl ethers and diaryliodonium salts is also discussed.