Synthetic Approaches to New Redox-Active Carbene Ligands

The design of new redox-switchable molecules requires the development of simple and efficient synthetic approaches. This study demonstrates the possibility of ortho-lithiation of ferrocenecarboxaldehyde (aryl)imines (Ia–Ic) followed by the reaction with ketones to give 1,2-disubstituted ferrocenes (IIa–IIc). These products, in turn, can be cyclized by treatment with trimethylsilyl triflate to give the cationic precursors of ferrocene-containing N-heterocyclic carbenes (IIIa–IIIc), in which the heterocycle is annulated to one of the ferrocene cyclopentadienyl ring. Treatment of IIIa–IIIc with a base in the presence of a source of rhodium afforded rhodium carbene complexes (IVa, IVb) in which the carbene ligand resembled cyclic alkylaminocarbenes in the electron-donor properties. Compounds Ib and IVa were studied by X-ray diffraction (CIF file CCDC nos. 2000413 and 2000414, respectively).