Retrocyclization reactions of gem-dibromo(ferrocenyl)cyclopropanes

Retrocyclization of 1-methyl-, 1-isopropyl-, 1-cyclobutyl-, 1-phenyl-, 1-tert-butyl-1-ferrocenyl-, 1-ferrocenyl-3-methyl- and 1,1-diferrocenyl- 2,2-dibromocyclopropanes (2a–g) under the action of tBuOK in DMSO, which occurs in parallel with reduction and dehydrobromination, is studied. Cyclic dimers of 2-ferrocenylpropene, 2-ferrocenyl-3-methylbut-1-ene, and 1-cyclobutyl-1-ferrocenylethene were obtained upon retrocyclization of compounds 2a–c, respectively, while compounds 2d,e gave linear dimers of 1-ferrocenyl-1-phenylethene and 2-ferrocenyl-3,3-dimethylbut-1-ene upon retrocyclization. Retrocyclization of 2f,g afforded trans-1-ferrocenylpropene and 1,1-diferrocenylethylene, respectively. The action of tBuOK in DMSO on the dibromide 2a in the presence of 1,3-diphenylisobenzofuran resulted in the Diels–Alder adducts derived from 2-ferrocenylpropene and 3-ferrocenyl-3-methylcyclopropene. The structures of 1,2-(1-ferrocenyl-1,3,3-trimethylpropane-1,3-diyl)ferrocene and exo-1,5-diphenyl-3-anti-ferrocenyl-3-syn-methyl-6,7-benzo-8-oxatricyclo[3.2.1.02,4]oct-6-ene were confirmed by X-ray diffraction analysis.