Formation and structural characterization of amino-substituted [3]ferrocenophanes derived from intramolecular Mannich-type coupling reactions at the metallocene framework

Treatment of lithium(1-dimethylamino-ethenyl)cyclopentadienide (5a) with FeCl2 generated the corresponding substituted ferrocene (2a) that rapidly underwent cyclization in a subsequent Mannich-type condensation reaction to yield the unsaturated dimethylamino-substituted [3]ferrocenophane 3a. The treatment of 1,1′-diacetylferrocene (1) with dimethylamine, diethylamine or piperidine in the presence of TiCl4 resulted in the formation of the analogously structured functionalized [3]ferrocenophanes 3a–3c, respectively, by a related intramolecular enamine-condensation pathway (complexes 3a and 3b were characterized by X-ray diffraction). Catalytic hydrogenation of 3a–c in THF at Pd/C gave the corresponding saturated amino-substituted [3]ferrocenophane systems 6a–c. The catalytic hydrogenation proceeds trans-product selectively with trans-6–cis-6 ratios ranging between ca. 5:1 and 7:1 for these examples (the complexes trans-6a·HCl, trans-6b, and trans-6c were characterized by X-ray crystal structure analysis). Quaternization of 6a by treatment with methyliodide followed by the reaction with pyrrolidine led to amine exchange at the [3]ferrocenophane framework with the formation of 6d. The amine exchange proceeds stereoselectively with a predominant overall retention (the trans-6d–cis-6d ratio obtained is again ca. 7:1). The complex trans-6d was characterized by X-ray diffraction.