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

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.

An easily accessible isospiropyran switch

In the presence of SiCl4, three molecules of 5'-bromo-2'-hydroxyacetophenone underwent an unexpected tandem aldol condensation to give a novel isospiropyran switch (69%), with X-ray crystallography confirming its structure. The strong Brønsted acid CH3SO3H turned the colorless isospiropyran into its protonated and open form possessing red color. This process was reversed using the Et3N base, with the acid/base toggling repeatable for at least six times (UV-Vis). When printed on a silica plate, however, the isospiropyran formed a blue-colored product due to, as posited, its stabilization by hydrogen bonding (HB) to silica. An exposure to HB-competing ethyl acetate temporarily "erased" the print only to be brought back by subjecting the plate to a higher temperature for evaporating the solvent. The isospiropyran described here is an easily accessible, chromic, modular and switchable compound that one can incorporate into dynamic materials or use for building chemosensors, molecular machines and organic electronic devices.

Three-Component Synthesis of a Library of m-Terphenyl Derivatives with Embedded β-Aminoester Moieties

The three-component reaction between alkyl- or arylamines, β-ketoesters and chalcones in refluxing ethanol containing a catalytic amount of Ce(IV) ammonium nitrate allowed the construction of a large library of highly substituted dihydro- m-terphenyl derivatives containing β-alkylamino- or β-arylamino ester moieties. This process generates three new bonds and one ring and proceeds in high atom economy, having two molecules of water as the only side product. Another domino process, in which the original MCR was telescoped with a subsequent aza Michael/retro-aza Michael sequence, allowed the one-pot preparation of a library of compounds with a N-unsubstituted β-aminoester fragment. Finally, to extend the structural diversity of these libraries, we also examined the aromatization of the central ring of our compounds in the presence of dichlorodicyanoquinone. This reaction sequence did not affect the integrity of a stereogenic center belonging to the amino component.

An investigation of (C5H5)Fe(C5H4–C(OBF3)–CH3),

Acetylferrocene readily forms an adduct with BF3. The product is a neutral model for protonated acetylferrocene, a species that is a crucial intermediate in the formation of 1,3,5-trisferrocenyl-benzene and other conjugated heterometallic derivatives by cyclocondensation. The title compound’s structure was determined in the solid state by X-ray diffraction and its spectroscopic properties were examined in detail, particularly the electronic excitation spectrum, which confirms the previous identification of the in situ generated protonated ketone.

Synthesis of a new class of carborane-containing star-shaped molecules via silicon tetrachloride promoted cyclotrimerization reactions

Symmetrical star-shaped molecules with carborane clusters on the periphery have been synthesized in good yields via silicon tetrachloride mediated cyclotrimerization reactions of 9-benzyl derivatives of carboranes with acetyl group substitution on the benzene ring. Facile functionalization of these symmetrical core structures with 1-iodoheptane and trivinylchlorosilane produce compounds which could be used as liquid crystalline substances and precursors for synthesis of higher order carbosilane dendrons.

Birch reduction of hexaphenyl- and pentaphenylbenzene and an X-ray crystallography and NMR spectroscopy study of cis- and epi-1,2,3,4,5,6-hexaphenylcyclohexane and of 2,3,5,6-tetraphenyl-1,1'-bicyclohexylidene: Cannizzaro's conundrum revisited

The Birch reduction of hexaphenylbenzene yields two isomers of 1,2,3,4,5,6-hexaphenylcyclohexane. The X-ray crystal structure of the all-cis isomer, 1, reveals that the severe steric crowding among the three axial phenyls is alleviated by a marked splaying out of those three aryl substituents relative to the positioning in a conventional chair structure. A second product, 2, was identified crystallographically and by NMR spectroscopy as the 1,3-diaxial-2,4,5,6-tetraequatorial (epi) isomer of hexaphenylcyclohexane, in which only five of the six additional hydrogen atoms are positioned on the same face of the C(6)Ph(6) precursor. A variable-temperature NMR study of the all-cis isomer 1 yielded a chair-to-chair inversion barrier of approximately 19 kcal mol(-1), which is somewhat higher than the previously reported values for all-cis-1,2,3,4,5,6-C(6)H(6)R(6) in which R=Me or CO(2)Me. The possible relevance to Cannizzaro's 1854 report of a product with the formula (C(7)H(6))(n) is discussed. By contrast, Birch reduction of pentaphenylbenzene led to the formation of 2,3,5,6-tetraphenyl-1,1'-bicyclohexylidene.

Triferrocenes built on a C3-symmetric ligand platform: entry to redox-active pseudo-triphenylenes via chelation-driven stereoselection of triple Schiff bases

An expedient tandem deprotonation-trapping protocol was employed to prepare a tris(difluoroboronyl) complex of a triferrocenyl ligand that is geometrically analogous to substituted triphenylenes. A triple Schiff base condensation reaction between 1,3,5-triformylphloroglucinol and aminoferrocene afforded the tris(N-salicylideneamine) adducts 5a + 5b in ca. 1:1 ratio. The keto-enamine tautomeric core of this isomeric mixture could be converted to a common enolate-imine intermediate. Subsequent trapping with BF3.Et2O cleanly afforded the tris(difluoroboronyl) adduct 6 in essentially quantitative yield. The electronic and structural properties of this new class of ferrocene compounds were investigated using various methods including UV-vis, cyclic voltammetry (CV), differential pulse voltammetry (DPV), and X-ray crystallography. In CH2Cl2-CH3CN, 6 displayed a reversible three-electron oxidation process at E1/2ox = +210 mV (vs Fc/Fc+). Despite the sharing of a common [pi,pi]/[n,pi]-conjugated core, no significant electronic communication was observed among the three ferrocenyl units in 6 under either CV or DPV conditions. On the other hand, the broad oxidation wave of 5a + 5b at E1/2ox = +60 mV in CH2Cl2-CH3CN was comprised of at least two major components at +20 and +90 mV, which collapsed to become a single peak in DMF electrolyte, despite that the ratios between the two isomers 5a,b remained essentially invariant to the change in solvent.