Highly convenient, clean, fast, and reliable Sonogashira coupling reactions promoted by aminophosphine-based pincer complexes of palladium performed under additive-and amine-free reaction conditions

Article abstract of DOI:10.1002/adsc.200900112

Sequential addition of 1,1',1″-phosphine-triyltripiperidine and 1,3-diaminobenzene or resorci-nol to toluene solutions of (cyclooctadiene) palladium dichloride [Pd(cod)(Cl)₂] under nitrogen in "one pot" almost quantitatively yielded the aminophos-phine-based pincer complexes {[C₆H₃-2,6-(XP{piperi-dinyl}₂) ₂]Pd(Cl)} (X = NH 1; X = O 2). Complex 1 (and to a minor extent 2) proved to be efficient Sonogashira catalysts, which allow the quantitative coupling of various electronically deactivated and/or sterically hindered and functionalized aryl iodides and aryl bromides with several alkynes as coupling partners within very short reaction times and low catalyst loadings. Importantly, in contrast to most of the Sonogashira catalysts, which either are both air-and moisture-sensitive and/or require the addition of co-catalysts, such as copper(I) iodide [Cul], for ex- ample, or a large excess of an amine, the coupling reactions were carried out without the use of amines, co-catalysts or other aditives and without exclusion of air and moisture. Moreover, the desired products were exclusively formed (no side-products were detected) without employing an excess of one of the substrates. Ethylene glycol and potassium phosphate (K₃PO₄) were found to be the ideal solvent and base for this transformation. Experimental observations strongly indicate that palladium nanoparticles are not the catalytically active form of 1 and 2. On the other hand, their transformation into another homogeneous catalytically active species cannot be excluded.