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 CF2 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.