Ferrocene acylation carboxylic acids

While metallocenes are usually acetylated with acid halides, acid anhydrides, or carboxylic acids, a number of other acylating agents have been reported. The reaction of ferrocene with various isocyanates and aluminum chlorides leads to N-substituted ferrocenecarboxamides (IX) (89). Use of ruthenocene in place of ferrocene leads to analogous results (88). The preparation of V-phenyl-ferrocenecarboxamide from phenyl isocyanate in this manner has been used as a proof of structure for the product obtained from the Beckmann rearrangement of benzoylferrocene oxime (124). 

A useful synthetic technique involves the acylation of ferrocene with di-phenylcarbamyl chloride and aluminum chloride (50). The product, diphenyl-carbamylferrocene (X), is readily hydrolyzed to ferrocenecarboxylic acid (XI). Acylation of X followed by hydrolysis produces good yields of l -acylferrocene-carboxylic acids. 

An efficient and ecofriendly procedure for the small-scale acylation of ferrocene with carboxylic acids is based on the in situ production of the mixed carboxylic-triflic anhydride (Table 4.15 see also Chapter 3). The reaction is simply performed by adsorbing ferrocene on the surface of activated alumina (preheated at 150°C for 3 h) and adding a mixture of carboxylic acid and trifluoroacetic anhydride (TFAA) at room temperature for a selected time. Products 40 are recovered in 55%-98% yield simply by elution with diethyl ether. 

Table 4.15 Ferrocene acylation with carboxylic acid-TFAA mixtures promoted by alumina...

Reaction of the acid chloride of furan-2-carboxylic acid with ferrocene in a Friedel-Crafts acylation gave the expected ketone. 

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