Nucleophilic Aromatic Substitution Reactions for the Synthesis of Ferrocenyl Aryl Ethers

A range of ferrocenyl
aryl ethers of type Fc–O–Ar
(Fc = Fe­(η5-C5H5)­(η5-C5HnX4–n); n = 2–4; X = H, P­(O)­(OEt)2, SO2CF3; Ar = 2,4-(NO2)2-C6H3, 4-NO2-C6H4) have been successfully prepared by using the nucleophilic
aromatic substitution reaction (SNAr) of Fc–OLi (1a–Li) and
electron-deficient aryl fluorides, enabling a new pathway to this
rarely described family of organometallic compounds. Initial studies
of 1a–Li and o-phosphonato-substituted
hydroxyferrocenes (1b–Li) have also been performed,
indicating a low nucleophilicity of the oxygen atom. The SNAr reaction protocol tolerates ortho substituents, e.g. phosphonato
and sulfonyl groups, resulting in 1,2-X,O (X = S, P) ferrocenyl ethers
that can be obtained in a one-pot synthesis procedure
including 1,3-O → C anionic phospho- and thia-Fries rearrangements.
Within these studies, the first 1,3-diortho-functionalized ferrocenyl
aryl ether could be synthesized. An electrochemical study of the redox
potentials of the obtained compounds allows conclusions on the reactivity
of the varying electronic properties and the electrophilicity of different
aryl fluorides and the nucleophilicity of the functionalized hydroxyl
ferrocenes, which is directly reflected by the potential of the first
ferrocene related redox processes. Chiral-pool-based
phosphonates rearrange to the aryl ethers with diastereomeric excesses
of 74 and 81% after the anionic Fries rearrangement. The usage of
aryl 1,2-/1,3-difluorides resulted in the synthesis of the first examples
of bis­(FcO)-substituted benzenes. Their electrochemical investigations
reveal a splitting of the two redox processes of the ferrocenyls that
decreases from 155 mV (1,2-substitution) to 130 mV (1,3-substitution)
on the basis of electrostatic interactions.