Key Role of Water in Copper‐ and Base‐free Sonogashira Coupling in Ethanol with [{Pd(µ‐OH)Cl(IPr)}2] as a Highly Effective Precatalyst

Aryl bromides and 4‐chlorotoluene as an example aryl chloride in the presence of N‐heterocyclic carbene (NHC) palladium hydroxo dimers of the type [{Pd(µ‐OH)Cl(NHC)}2] (where NHC = IPr, SIPr, IMes, SIMes) undergo an efficient and selective Sonogashira cross‐coupling with (hetero)arylacetylenes. The procedure allows the high‐throughput and selective synthesis of a broad spectrum of 1,2‐diarylacetylenes using 10 ppm of [{Pd(OH)Cl(IPr)}2] as precatalyst. For the coupling of 4‐chlorotoluene with phenylacetylene, TON = 560000 was achieved. Hydrogen chloride was observed as a product of the Sonogashira cross‐coupling reaction. The formation of the active Pd(0) from the Pd(II) complex was found to proceed via ethanol oxidation. Mechanistic studies showed that water plays a key role in the reaction.