Rearrangements and dimerizations of congested ferrocenyl allyl alcohols

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.

From Calcium Interaction to Calcium Electrochemical Detection by [(C5H5)Fe(C5H4COCHCHC6H4NEt2)] and Its Two Novel Structurally Characterized Derivatives

[(C(5)H(5))Fe(C(5)H(4)COCH=CHC(6)H(4)NEt(2))] (1) has been electrochemically evaluated toward different cations in solution. Calcium sensing by this compound and its two new derivatives [(C(5)H(5))Fe(C(5)H(4)CO(CH=CH)(2)C(6)H(4)NMe(2))] (2) and [(C(5)H(5))Fe(C(5)H(4)CH=CHCOCH=CHC(6)H(4)NEt(2))] (3) that exhibit a conjugated link between the ferrocene unit and the nitrogen atom has been thoroughly examined. Compounds 2 and 3 have been structurally characterized by single-crystal X-ray diffraction studies. The three related protonated species [1H][BF(4)] (4), [2H][BF(4)] (5), and [3H][BF(4)] (6) have been isolated in a good yield. NMR experiments clearly established that calcium interaction occurs in the vicinity of the carbonyl group, and mass spectrometry studies confirmed that this interaction, which involves several ligand-Ca(2+) adducts, is complex. A combination of electrochemical and NMR experiments highlighted an original salt influence on the electrochemical calcium sensing result.

The retro-Nazarov reaction

Treatment of 2-bromo-4-t-butoxy-2-cyclopentenone with an amine base in refluxing trifluoroethanol afforded a ring-opened product in moderate yield. The mechanism of the reaction has been formulated as a retro-Nazarov reaction in which an oxyallylic cation undergoes ring-opening to a dienone. Several other examples of the reaction have been established through a protocol involving the conjugate addition of an organocuprate to 2-bromo-4-t-butoxy-2-cyclopentenone followed by treatment of the adducts with base in refluxing trifluoroethanol.