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Unprecedented Formation of π‑Copper Complexes during Sonogashira Coupling: Synthesis of a Unique, Recyclable, Ethynyl Ferrocene Derived Cu(I) Specific Ligand

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posted on 05.04.2016, 11:35 by Mayukh Deb, Dheeraj Kumar, Jatinder Singh, Anil J. Elias
During the synthesis of a chiral oxazolinyl-derived ethynyl ferrocene {Fc-CC–C6H4-o-(4-iPr-2-Ox)} (Fc = ferrocenyl; Ox = oxazolinyl), we observed an unprecedented formation of highly air stable and monomeric π-copper­(I) complexes 4a and 4b, which were structurally characterized. CuI was used as a cocatalyst in this Sonogashira coupling. By the reaction of 4a with aqueous NH3/DMF, the CuI was removed from the complex and the metal-free compound 5a was obtained. This was found to be an excellent ligand for selectively binding Cu­(I) halides. Analogous 4-Ph-substituted oxazoline-based ligand 5b and its Cu–I complex 4c were also isolated and characterized. The possible role of these complexes in explaining the copper cycle proposed for Sonogashira coupling has also been discussed.