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.
Highlights
The ability of ofthe theferrocenyl ferrocenylmoiety moiety stabilize an adjacent carbocationic center has been longThe ability to to stabilize an adjacent carbocationic center has long been attributed to delocalization of the electron deficiency onto ironatom atom [1]. [1]
The X-ray attributed to delocalization of the electron deficiency onto thetheiron
24, closure of the six-membered attack by the allyl cation on the diene to form the ferrocenyl-stabilized intermediate 24, closure of the ring to generate a different cationic site adjacent theadjacent other ferrocenyl moiety, as in 25, and six-membered ring to generate a different cationictosite to the other ferrocenyl moiety, as in ringand closure onto ferrocene, thusferrocene, yielding the observed
Summary
The ability to to stabilize an adjacent carbocationic center has long been attributed to delocalization of the electron deficiency onto ironatom atom [1]. The X-ray attributed to delocalization of the electron deficiency onto thetheiron. Tendency of the cationic α-carbon atom to lean towards crystallographically established [2,3] tendency of the cationic α-carbon atom to lean towards the the metal. 1, was was originally originally interpreted interpreted in vacant 2p orbital on carbon with metal as as in in 1, in terms terms of of an an overlap overlap of of the the vacant. 3d orbital orbitalof ofsuitable suitablesymmetry symmetryononiron iron [4]. 5 5 unit, as depicted in 2 (Figure 1), but system as a neutral fulvene ligand coordinated to a [(C5H5)Fe].
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