Bulky-alkyl-substituted bis(trifluorovinyl)ferrocenes: redox-autocatalytic formation of fluorinated ferrocenophanes

Diastereomerically pure 1,1′-diiodoferrocenes with bulky alkyl groups in 3,3′ positions are obtained by a dilithiation strategy of 1,1′-di-tert-butylferrocene (1a) and 1,1′-bis(1-ethyl-1- methylpropyl)ferrocene (1b). The 3,3′-dilithiated species are converted to tri-n-butyltin derivatives (3a and 3b), which smoothly undergo a tin-iodine exchange reaction, yielding 1,1′-diiodo-3,3′-di-tert-butylferrocene (4a) and 1,1′-diiodo-3,3′-bis(1-ethyl-1-methylpropyl)ferrocene (4b). Negishi-type coupling reactions with trifluorovinylzinc chloride afforded the highly reactive 1,1′-bis(trifluorovinyl)ferrocenes with tert-butyl groups (5a) or 1-ethyl-1-methylpropyl groups (5b) in 3,3′-positions in excellent yields. 5a and 5b undergo under redox-conditions cyclizations to ferrocenophanes by a redox-autocatalytic mechanism. The main product four-carbon handle ferrocenophanes consist of CF ₂ moieties in β-positions and one carbonyl function and one C(F)(H) unit (6a and 6c) or two carbonyl functions (6b and 6d) in α-positions. A ferrocenophane with a highly fluorinated handle bearing seven fluorine atoms (6e) was isolated as a byproduct. Formation of cyclic ether derivatives by unusual intramolecular substitution of fluorine is demonstrated for the case of 6a. Several new fluorinated ferrocenes were isolated and analyzed, including ¹H, ¹³C, and ¹⁹F nuclear magnetic resonance spectroscopies. Molecular structures of five fluorinated ferrocenophanes by tert-butyl groups were elucidated with X-ray single-crystal diffraction. Influences of the electron-withdrawing fluorous substituents and the electron-donating alkyl groups on the redox behavior of the iron center were investigated by cyclic voltammetry.