Cyclodimerization reactions of the β-ferrocenylvinyl(methyl)ketones in the presence of zinc-organic compounds

Trans-ferrocenyl-3-buten-2-one 1 undergoes cyclic dimerization with formation of 4-acetyl-3,5-diferrocenylcyclohexanone 5 and 5,8-diferrocenyl-1-hydroxybicyclo-[2.2.2.]octan-3-one 6 in the presence of cyclohexenes and 2-PyZnBr or Me2Zn. No adducts of the Diels–Alder reaction of compound 1 in its enolic form with cyclohexenes were observed. Under similar conditions, in the absence of cyclohexenes trans-ferrocenyl-3-buten-2-one 1 affords cyclodimer 5 and cyclotrimer – 6-acetyl-4,5,7-triferrocenyl-1,9-dehydro-2-decalone 7. 3-(Ferrocenylmethylidene)pentane-2,4-dione 2a and ethyl (E)-2-acetyl-3-ferrocenylacrylate 2b cyclodimerized analogously with formation of 3,5-diferrocenyl-1-hydroxy-2,4,4-triacetylcyclohexene 8a and diethyl 4-acetyl-3,5-diferrocenyl-1-hydroxycyclohexene-2,4-dicarboxylate 8b, respectively. The structures of the obtained compounds were established based on data from IR, 1H and 13C NMR spectroscopy, mass spectrometry and X-ray diffraction analysis. The mechanistic aspects of these reactions are discussed.