Acetic anhydride ferrocene acylation

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

Roberts and Wells354 were the first to use triflic acid in the acylation of phospha-ferrocenes with acetic anhydride and benzoic anhydride to afford ketophosphaferro-cenes in good yields (62-78%). Recently, acyl trifluoroacetates in excess triflic acid have been shown to be even more effective reagents355 [Eq. (5.141)]. 

Ferrocene reacts with acetyl chloride and aluminum chloride to afford the acylated product (287) (Scheme 84). The Friedel-Crafts acylation of (284) is about 3.3 x 10 times faster than that of benzene. Use of these conditions it is difficult to avoid the formation of a disubstituted product unless only a stoichiometric amount of AlCft is used. Thus, while the acyl substituent present in (287) is somewhat deactivating, the relative rate of acylation of (287) is still rapid (1.9 x 10 faster than benzene). Formation of the diacylated product may be avoided by use of acetic anhydride and BF3-Et20. Electrophilic substitution of (284) under Vilsmeyer formylation, Maimich aminomethylation, or acetoxymercuration conditions gives (288), (289), and (290/291), respectively, in good yields. Racemic amine (289) (also available in two steps from (287)) is readily resolved, providing the classic entry to enantiomerically pure ferrocene derivatives that possess central chirality and/or planar chirality. Friedel Crafts alkylation of (284) proceeds with the formation of a mixture of mono- and polyalkyl-substituted ferrocenes. The reaction of (284) with other... 

Good synthetic results have been achieved in the acylation of ferrocene with acetic anhydride (AAN) in EtMeimCl-AlClg/toluene mixture. The monoacetylated product can be obtained in 80%-96% yield, ft is significant that no acetylation of toluene is observed under these conditions. 

The Friedel-Crafts acylation of ferrocene, which was mentioned above, has been studied in considerable detail. Substitution can be effected under very mild conditions, for instance with acetic anhydride containing phosphoric add as catalyst, to give mono-acetylferrocene. With aluminium chloride as catalyst, good yields of either mono- or l,l -diacetylferrocene can be obtained according to the proportions of reagents taken. 

Chiral n complexes of heterocycles with transition metals can serve as effective catalysts for an array of useful organic reactions.The most efficient nucleophilic catalysts previously used had planar structures, e.g., 4-(dimethylamino)pyridine, and therefore required an asymmetric environment in the vicinity of an sp -hybridized nucleophilic atom. In a recent paper, G. C. Fu et al describe a procedure, which they call a second-generation system for kinetic resolution, that employs an iron complex at a mole fraction of 2% as the chiral catalyst, and acetic anhydride as the acylating agent (Figure 9.7.1). The authors attached the 4-(dimethylamino)pyridine moiety to a chiral ferrocene analog. The lower portion of the ferrocene, coordinated to the iron atom is pentaphenylcyclopentadiene, as shown in Figure 9.7.1. 

The relative sensitivity of furan to electrophilic attack stands in the order benzene < naphthalene < ferrocene < furan as found from competition experiments with styrene palladium(II) acetate.441 Relative rales for acylation by trifluoroacetic anhydride (no catalyst) arc observed to be thiophene (1), selenophene (6.5), furan (1.4 x 102), pyrrole (5.3 x I07), and... 

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