The Chemistry of Diene–Iron and Dienyl–Iron Complexes

Abstract

Abstract (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.