Fischer-Hafner synthesis

The Fischer-Hafner synthesis of sandwich compounds (33) does not permit functional groups to be incorporated into the arenes because of side reactions with the Lewis acid catalyst (Friedel-Crafts reducing conditions). This is not the case when metal atoms are used directly. Many metal-arene complexes have been identified that contain F, Cl, CH30, R2N and C02R substituents. It is reasonable to assume that polymer-bound phenyl substituents containing these functional groups will yield similar sandwich complexes. 

The correct identification of the sandwich structure of ferrocene led Fischer to consider the possibility of arenes acting as hexahapto 6VE ligands. By simple arithmetic, a neutral bis(arene) sandwich complex of a zerovalent Group 6 element, e.g. chromium (Figure 6.71), was anticipated, a line of reasoning which led Fischer to develop the synthesis of dibenzenechromium. His approach resulted in the comparatively general Fischer-Hafner synthesis (1955), which is applicable to many metals and arenes (devoid of Lewis-basic substituents, Figure 7.34). 

Bis(arene) complexes of ruthenium (99), first prepared in relatively low yield by the Fischer-Hafiier synthesis (see Fischer-Hafner Synthesis) (MCb/AlCb/arene) are now readily available through solvento cations [(arene)RuS] + (S = acetone, nitromethane), which in turn are made from the dimeric half-sandwich halides [(arene)RuCl2]2 (Section 6.3) with Ag+ with the appropriate arene as solvent (Scheme 23). This method allows a whole range of functionalities to be incorporated to at least one of the arenes. The dications (99) + are of relatively high solvolytic stability and are decomposed by DMSO only to form [(arene)Ru(DMSO)3] +. 

Benzene and its derivatives are among the better known of the many r 6-arene complexes. The best known of these is dibenzenechromium, (C6H6)2Cr. Like other compounds of formula (C6H6)2M, dibenzenechromium can be prepared by the Fischer-Hafner synthesis, using a transition metal halide, aluminum as a reducing agent, and the Lewis acid A1C13 ... 

Recent important developments consist in the synthesis of the unsubstituted pyrylium cation by Klagcs and Trager, the preparation of pyrylocyanines by Wizinger, the development of simple syntheses for alkyl-substituted pyrylium salts by Balaban and Nenitzescu, Praill, Schroth and Fischer, Schmidt, and Dorofeenko, the discovery of a variety of reactions by Dimroth and Hafner, and the study of physical properties by Balaban. 

Fischer and Hafner reported an alternate synthesis of bis-ir-benzene chromium using a reductive Friedel-Crafts method which probably also proceeds via a [Pg.231]

Organochromium species containing r-bonded ligands were first prepared by Hein as early as 1919, but were not definitively characterized for another 35 years. Hein reported on the reaction of PhMgBr with CrCH to give Chromorganoverbindungen this synthesis was reproduced by Zeiss and Tsutsui in 1954, at which time they identified the products as bis (arene) chromium cations. Shortly thereafter, Fischer and Hafner published their route to neutral bis(benzene)chromium. This early work has been chronicled, most recently by Seyferth. ... 

The reduction of a metal salt by aluminium and aluminium trichloride in the presence of an aromatic hydrocarbon, which is illustrated in Table XIII, provides the most general method known for the preparation of bis-ff-arene complexes. It was first used by Fischer and Hafner for the synthesis of bis-w-arene chromium complexes (Fischer s reducing Friedel-Crafts proceditfe). 

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