Ferrocenyl Migrations and Molecular Rearrangements: The Significance of Electronic Charge Delocalization

Abstract

The enhanced stabilization of a carbocationic site adjacent to a ferrocenyl moiety was recognized within a few years of the discovery of sandwich compounds. While a detailed understanding of the phenomenon was the subject of some early debate, researchers soon took advantage of it to control the ease and direction of a wide range of molecular rearrangements. We, here, discuss the progress in this area from the pioneering studies of the 1960s, to more recent applications in chromatography and analytical detection techniques, and currently in the realm of bioactive organometallic complexes. Several classic reactions involving ferrocenyl migrations, such as the pinacol, Wolff, Beckmann, and Curtius, are discussed, as well as the influence of the ferrocenyl substituent on the mechanisms of the Nazarov, Meyer-Schuster, benzoin, and Stevens rearrangements. The preparation and isomerizations of ferrocenyl-stabilized vinyl cations and vinylcyclopropenes, together with the specific cyclization of acetylcyclopentadienyl-metal derivatives to form 1,3,5-substituted benzenes, demonstrate the versatility and generality of this approach.

Rearrangements and Migrations along the Ferrocene Periphery: On the Way to Planar‐Chiral and (Multi)Substitution Patterns

“…In 1956 Weliky and Gould reported on the synthesis of 1,2‐diferrocenyl‐1,2‐diphenylethane‐1,2‐diol 131 and its treatment with HCl g to give pinacolone Fc 2 PhC−C(O)Ph 132 showing that Fc was favored over Ph migration (Scheme 36). [32,34] Later on, it was demonstrated that the Fc movement is also preferred over alkyl transfers [32] . The authors mention that the appropriate pinacol rearranges with unexpected ease by migration of Fc and it is very straightforwardly converted into the respective ketone.…”

“…In 2020 a review discussing ferrocenyl migrations and molecular rearrangements based on electronic charge delocalization focused on some cationic reorganizations, however, focus was put on the groups in close proximity, and only a few examples of rearrangements occur at the ferrocenyl's periphery directly [32] . Hence, within this Section ferrocenyl‐mediated cationic rearrangements, for example, Pinacol, Beckmann, retro S E Ar (retro Friedel‐Crafts alkylations and acylations), remote Fries, and internal alkyne‐vinylidene rearrangements, as well as movements including α ‐ferrocenyl carbo cations will be discussed.…”

“…[33] In 1956 Weliky and Gould reported on the synthesis of 1,2-diferrocenyl-1,2-diphenylethane-1,2-diol 131 and its treatment with HCl g to give pinacolone Fc 2 PhCÀ C(O)Ph 132 showing that Fc was favored over Ph migration (Scheme 36). [32,34] Later on, it was demonstrated that the Fc Scheme 34. General process for a hypothetical anionic homo phospho-Fries rearrangement (top) and unsuccessful attempts.…”

“…[30] movement is also preferred over alkyl transfers. [32] The authors mention that the appropriate pinacol rearranges with unexpected ease by migration of Fc and it is very straightforwardly converted into the respective ketone. Furthermore, it was shown that a carbenium ion achieved by an elimination of a hydroxyl group α to Fc is exposed to special stabilization.…”

Five shades of green: substituent influence on the (spectro-) electrochemical properties of diferrocenyl(phenyl)methylium dyes

“…The shift diଏerence ̀δ between carbon atoms C 5 and C 4 /C 6 has been identified as a good indicator for the dipolar character of a tritylium-type ion. 4,36,48 A smaller value ̀δ correlates with a lesser resonance stabilization of the positive charge at the trivalent carbon atom, a smaller polarization of the attached aryl/ferrocenyl substituent and a larger contribution of a pentafulvene-type resonance structure with a positive charge at the Fe ion (cf. structure at the top of Fig.…”