Janovsky reactions between cyclopentadienyliron complexes of substituted benzenes and carbanions derived from ketones

Janovsky reactions were carried out between various η6-substituted benzene-η5-cyclopentadienyliron cations containing at least one electron-withdrawing substituent in the complexed arene and the enolate-carbanion derived from acetone, butanone, or 3-pentanone. Highly colored adducts were obtained from reactions of the cyclopentadienyliron (CpFe) complexes of nitrobenzene, cyanobenzene, p-tolylsulfonylbenzene, benzoylbenzene, o-, p-, or m-nitrotoluene, and o-, p-, or m-dichlorobenzene with acetone in aqueous KOH, the products being derived from exo-addition of the acetonyl anion regioselectively at a position ortho to an electron-withdrawing substituent. For example, reaction of the η6-nitrobenzene-η5-cyclopentadienyliron cation (1a) with acetone in aqueous KOH gave (R,S)-(1-5-η5-1-nitro-6-exo-(2-oxo-1-propyl)-cyclohexadienyl)(η5-cyclopentadienyl)iron (2a), isolated as a purple solid. Similarly, reaction of the nitrobenzene complex 1a with 3-pentanone in aqueous KOH gave two diastereomeric adducts, (R,R-S,S)- and (R,S-S, R)-(1-5-η5 1-niro-6-exo-(3-oxo-2-pentyl)-cyclohexadienyl)(η5-cyclopentadienyl)iron (11a, b). Reactions of the CpFe complexes of chlorobenzene and o- or p-dichlorobenzene with butanone in aqueous KOH were also investigated. In each case, three Janovsky adducts, two being diastereomers, were obtained from addition of the two carbanions derived from loss of either α-proton of butanone. Potential synthetic applications via these Janovsky adducts are pointed out since their treatment with ammonium ceric nitrate could lead to demetallation–oxidation; for example, (2,5-dichlorophenyl)propanone, a new compound, was obtained from the adduct formed between the p-dichlorobenzene complex and the acetonyl anion.