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Cited by 10 publications
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(η 4 ‐Diene)iron complexes and η 5 ‐dienyliumiron complex salts represent intriguing structures with planar π‐donating C 4 or C 5 ligands attached to the central iron atom. In many cases, they are rather stable compounds due to an 18‐electron count at the iron center. Tricarbonyl(η 4 ‐diene)iron complexes are formed from cyclic and acylic 1,3‐dienes and a tricarbonyliron source. Under certain conditions 1,4‐dienes can be employed with concomitant rearragement of the double bonds. The tricarbonyliron source can be a binary carbonyliron complex or a tricarbonyliron transfer reagent. (η 4 ‐1‐Azabutadiene)tricarbonyliron complexes have been proven to transfer the tricarbonyliron unit efficiently to the diene system. 1‐Azabutadiene ligands can even be employed in catalytic amounts. Chiral 1‐azabutadiene ligands enable the catalytic asymmetric synthesis of planar chiral (η 4 ‐diene)iron complexes. The tricarbonyliron fragment in (η 4 ‐diene)iron complexes is shielding one side of the diene system. Thus, it can be utilized to control stereoselective reactions in its proximity. Moreover, due to their stability (η 4 ‐diene)iron complexes can be employed as protected 1,3‐dienes. η 5 ‐Dienyliumiron complexes are generated from (η 4 ‐diene)iron complexes by two principal methods. Either by hydride abstraction at the α‐position of the diene ligand using triphenylcarbenium salts or by elimination of leaving groups adjacent to the diene system. η 5 ‐Dienyliumiron complexes are versatile electrophiles which react with a large variety of nucleophiles leading to functionalized (η 4 ‐diene)iron complexes. In most cases, nucleophilic attack at the terminus of the co‐ordinated dienylium system is preferred. Especially with C‐nucleophiles, the reaction is extremely useful for the construction of natural products or advanced synthetic precursors. Employment of substituted anilines as nucleophiles has paved the way to the total synthesis of a large number of biologically active carbazole alkaloids. After exploiting the activating, protecting or stereodirecting effect of the iron complex, the organic ligand is disengaged from the metal. A variety of mild demetalation procedures has been developed for this purpose. In addition to the stoichiometric reactions, first examples of catalytic applications of (diene)iron complexes are dicussed in this chapter.
(η 4 ‐Diene)iron complexes and η 5 ‐dienyliumiron complex salts represent intriguing structures with planar π‐donating C 4 or C 5 ligands attached to the central iron atom. In many cases, they are rather stable compounds due to an 18‐electron count at the iron center. Tricarbonyl(η 4 ‐diene)iron complexes are formed from cyclic and acylic 1,3‐dienes and a tricarbonyliron source. Under certain conditions 1,4‐dienes can be employed with concomitant rearragement of the double bonds. The tricarbonyliron source can be a binary carbonyliron complex or a tricarbonyliron transfer reagent. (η 4 ‐1‐Azabutadiene)tricarbonyliron complexes have been proven to transfer the tricarbonyliron unit efficiently to the diene system. 1‐Azabutadiene ligands can even be employed in catalytic amounts. Chiral 1‐azabutadiene ligands enable the catalytic asymmetric synthesis of planar chiral (η 4 ‐diene)iron complexes. The tricarbonyliron fragment in (η 4 ‐diene)iron complexes is shielding one side of the diene system. Thus, it can be utilized to control stereoselective reactions in its proximity. Moreover, due to their stability (η 4 ‐diene)iron complexes can be employed as protected 1,3‐dienes. η 5 ‐Dienyliumiron complexes are generated from (η 4 ‐diene)iron complexes by two principal methods. Either by hydride abstraction at the α‐position of the diene ligand using triphenylcarbenium salts or by elimination of leaving groups adjacent to the diene system. η 5 ‐Dienyliumiron complexes are versatile electrophiles which react with a large variety of nucleophiles leading to functionalized (η 4 ‐diene)iron complexes. In most cases, nucleophilic attack at the terminus of the co‐ordinated dienylium system is preferred. Especially with C‐nucleophiles, the reaction is extremely useful for the construction of natural products or advanced synthetic precursors. Employment of substituted anilines as nucleophiles has paved the way to the total synthesis of a large number of biologically active carbazole alkaloids. After exploiting the activating, protecting or stereodirecting effect of the iron complex, the organic ligand is disengaged from the metal. A variety of mild demetalation procedures has been developed for this purpose. In addition to the stoichiometric reactions, first examples of catalytic applications of (diene)iron complexes are dicussed in this chapter.
Catalysis is an important field in both academic and industrial research because it leads to more efficient reactions in terms of energy consumption and waste production. The common feature of these processes is a catalytically active species which forms reactive intermediates by coordination of an organic ligand and thus decreases the activation energy. Formation of the product should occur with regeneration of the catalytically active species. The efficiency of the catalyst can be described by its turnover number, providing a measure of how many catalytic cycles are passed by one molecule of catalyst.For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions.Many transition metals have been applied as catalysts for organic reactions [1]. So far, iron has not played a dominant role in catalytic processes. Organoiron chemistry was started by the discovery of pentacarbonyliron in 1891, independently by Mond [2] and Berthelot [3]. A further milestone was the report of ferrocene in 1951 [4]. Iron catalysis came into focus by the Reppe synthesis [5]. Kochi and coworkers published in 1971 their results on the iron-catalyzed crosscoupling of Grignard reagents with organic halides [6]. However, cross-coupling reactions became popular by using the late transition metals nickel and palladium. More recently, the increasing number of reactions using catalytic amounts of iron complexes indicates a renaissance of this metal in catalysis. This chapter describes applications of iron complexes in organic chemistry and thus paves the way for an understanding of iron catalysis.
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