Two substrates, three products: Reactions between ferrocene-carboxaldehyde and dioxaphospholene, characterization and crystal structures of oxygenated C3- and C4-chain-containing ferrocenes

Reactions of aldehydes RCHO with 2,2,2-trimethoxy-4,5-dimethyl-1,3,2-dioxaphospholene are known to form trimethylbiacetyl-aldehyde-phosphite adducts which upon methanolysis gave rise to the corresponding symmetrical 3-R-2,4-pentanediones (R = aryl, alkyl). Reactions between ferrocenecarboxaldehyde and the biacetyl-trimethylphosphite adduct, whatever the experimental conditions used, afforded three new oxygenated C3 and C4 side-chain containing ferrocenyl complexes, namely, 1-ferrocenyl-1,2-propanedione (1, Fc–C(O)C(O)CH3; Fc = (η5-C5H5)Fe(η5-C5H4)), 4-methoxy-4-ferrocenyl-3-hydroxy-3-methyl-2-butanone (2, Fc–CH(OCH3)C(CH3)(OH)C(O)CH3) as a single diastereoisomer, and the ferrocenyl enone 4-ferrocenyl-3-methyl-3-buten-2-one (3, Fc–CHC(CH3)C(O)CH3) with the enone portion adopting the s-trans conformation in the solid state. The compounds readily purified by silica gel packed column chromatography were isolated in 31–44% yields, and their constitution is in agreement with elemental analyses, IR and multinuclear NMR spectroscopy, and electrospray mass spectrometry. Their molecular identity and geometry have been confirmed by single-crystal X-ray diffraction.

Graphical abstractDepending on the experimental conditions used, the reaction between biacetyl trimethylphosphite adduct and ferrocenecarboxaldehyde gives rise to either 1-ferrocenyl-1,2-propanedione, or 4-methoxy-4-ferrocenyl-3-hydroxy-3-methyl-2-butanone (illustrated) as a single diastereoisomer, or 1-(3-oxo-2-methylbut-1-enyl)ferrocene; this behavior could originate from a common charge separated state with a carbonium ion stabilized by the ferrocene unit.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Reactions between biacetyl:trimethylphosphite adduct and ferrocenecarboxaldehyde. ► Synthesis of three oxygenated C3- and C4-chain-containing ferrocenes. ► 4-Methoxy-4-ferrocenyl-3-hydroxy-3-methyl-2-butanone diastereoselectively formed. ► s-Trans conformation of the carbonyl and ethylenic double bonds of ferrocenylenone. ► Structures of the three compounds were determined by X-ray diffraction.