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] +. 

A prototype for the new cyclization reaction is the Ziegler-Hafner synthesis of azulene (J)4 by heating the fulvenoid decapentaene (/). Woodward28 was the first to argue that a thermal, electrocyclic [rr 10 s] ring-closure could be involved in this thermolytic process. Of course it seems likely that the nonisolable intermediate (2) may be formed by an intramolecular electrophilic attack of atom 10 to the electron-rich position 1 in compound (1). Indeed a distinction between the two different reaction modes is mechanistically irrelevant in this case. But only its interpretation as an electrocyclic process allows the generalization to [ir 6 s] cyclizations too. 

This mode of cyclization opened a new access74 to the stable, tetracyclic peri-condensed, nonbenzenoid aromatic hydrocarbon dicyclopenta(e,f—k,l)heptalene (.159), also named73 azupyrene. The 4,5-cyclopentenoazulene (156) needed was formed in 28% yield by a Hafner synthesis from the pentamethinium salt (155)75 and sodium cyclopentadienide ... 

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 ... 

The Fisher-Hafner synthesis is the convenient synthetic method for various kinds of ( -(arene))2Cr type compounds, since it yields a single product different from the Grignard synthesis. However, precautions should be taken on compounds on which transalkylations or isomerization tends to proceed [16]. For example, mesitylene is reactive as described above in that a transalkylation partly proceeds to afford xylene and tetramethylbenzene. The transalkylation such as the reaction with... 

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