Synthesis and structure of planar chiral ferroceno[d]pyridazinones, the first representatives of a novel class of fused metallocenes

A series of planar chiral ferroceno[d]pyridazinones was prepared by hydrazine-mediated cyclisations of (Sp)-2-formylferrocenoyl fluoride obtained in two steps from (2S,4S)-4-(methoxymethyl)-2-ferrocenyl-1,3-dioxane. Further derivatives based on the novel heterocyclic scaffold were resulted by means of the facile N-alkylation reactions carried out on the easily accessible fused product derived from the hydrazine-mediated cyclisation. Upon treatment with hydrazine (Sp)-2-formylferroce carboxylic acid, the precursor of the acid fluoride with enhanced reactivity, afforded a dimeric azine avoiding cyclisation. The spectacular difference between the experimentally observed reactivity of the bifunctional formylferrocenes was disclosed by comparative DFT analysis of the assumed hydrazone- and hydrazide intermediates with different values of LUMO energy associated with different conformation. The structures of the new compounds were established by IR and NMR spectroscopy, including HMQC, HMBC and DNOE measurements.

Graphical abstractA series of novel planar chiral ferroceno[d]pyridazinones were prepared by hydrazine-mediated cyclisations of (Sp)-2-formylferrocenoyl fluoride obtained by the fluorination of the appropriate carboxylic acid. Further derivatives based on the heterocyclic scaffold were resulted by N-alkylation reactions of the easily accessible fused lactam containing NH group.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of 2-formylferrocenecarbonyl fluoride, an easy-to-handle intermediate. ► Construction of ferroceno[d]pyridazinone, a novel planar chiral metallocene scaffold. ► DFT modelling of the intermediates predisposed for cyclisation.