Arene Iron Complexes in Organic Synthesis. New Routes to Benzophenones, Diaryl Selenides, Xanthones and Thioxanthones

Abstract: A multi-step route to unsymmetrically substituted benzophenones based on cationic hapto6-arene-hapto5-cyclopentadienyl iron complexes has been developed. The key features are nucleophilic aromatic substitution reactions performed on hapto6-2-chlorocarbomethoxybenzene-hapto5-cyclopentadienyl iron hexafluorophosphate with phenoxides, followed by intramolecular Friedel-Crafts ring-closure. The thus formed xanthone complexes are regioselectively opened, on the iron-coordinated aromatic ring, delivering benzophenone complexes. Regioselective cyanide addition/oxidative demetalation releases heavily substituted benzophenones. A formal total synthesis of the benzophenone appendage of the protein kinase C inhibitor balanol is presented. Xanthones and thioxanthones have been synthesised via nucleophilic aromatic substitution reactions, with phenoxides or thiophenoxides as nucleophiles, performed on hapto6-2-chloro-N,N-diethylbenzamide-hapto5-cyclopentadienyl iron hexafluorophosphate. Photolytic decomplexation followed by Directed remote Metalation delivers the heterocycles. A new synthetic method for the preparation of diaryl selenides based on nucleophilic aromatic substitution reactions performed on hapto6-chloroarene transition metal complexes has been developed. The preparation of hapto6-trifluoromethanesulphonylbenzene-hapto5-cyclopentadienyl iron hexafluorophosphate and its reactivity in SNAr reactions or palladium-catalysed trans-formations is described.