Iron, tricarbonyl acylation

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

Azepines do not undergo electrophilic substitution reactions and introduction of substituents at the 3-or 4-position is a difficult problem. Acylation at C-3 can be achieved via the iron tricarbonyl complex as shown in Scheme 22.42 The N-ethoxycarbonyl group of the acylated product can be removed with methoxide methanol and the resulting 1H compound can then be methylated. 

Gibson nde Thomas, and Tustin have reported the formation of a number of iron carbonyl complexes of a,/J-unsaturated acyl silanes226. Propenoyl trimethylsilane did not give a stable complex, but the iron tricarbonyl complexes of cinnamoyl silanes were very stable. 

Cyclobutadiene-iron tricarbonyl itself is unique in that it undergoes many aromatic-type ring substitution reactions, such as Friedel-Crafts acylation. 

Although few examples of acylations of 1,3-butadienes have been described, Friedel-Crafts acylations of diene complexes, in particular iron tricarbonyl derivatives, can give synthetically useful yields. In acylations of iron tricarbonyl complexes with the Perrier reagent from acetyl chloride and aluminum chloride, acylation occurs only at unsubstituted terminal carbons (Scheme 18). ° The primary product is... 

The iron tricarbonyl unit has been described as a protecting group for a 1,3-diene, as in the acetylation of the complex of myrcene at low temperature (Scheme 20). The usual combinations of a,p- and p,"y-un-saturated ketones were formed. At higher temperatures, some acylation at the terminus of the diene complex was also observed. In an interesting extension, reaction of the complex with oxalyl chloride resulted in cyclization of the acid chloride initially formed by reaction at the alkene. ... 

Iron carbonyl complexes are obtained by irradiation of iron pentacarbonyl with vinylcyclopropanes. The principal modes of reaction are (i) metal insertion into a strained bond with hydrogen migration to give diene-Fe(CO)3 complexes, (ii) metal carbonyl insertion to give a n-allyl-complexed acyl iron tricarbonyl, and (iii) cycloinsertion of CO across the homodiene to give a cyclohexenone. Similar types of... 

The electron-donating properties of the diene-iron tricabronyl group is also indicated by the fact that phenylbutadiene-iron tricarbonyl (XVI) can be acylated with CH3COCI and SnCl4 under mild conditions to give the corresponding para-substituted complex 32). 

Other hypochlorites, and in particular tert-butyl hypochlorite [114], have also been used. For example, the latter reagent was apphed in the oxidation of a (cy-clobutadiencarboxaldehyde) iron tricarbonyl complex (9), which afforded ester 10 in 90% yield [Eq. (5)]. No oxidation of the metal center occurred. The reaction path involves the aldehyde oxidation by tert-butyl hypochlorite leading to the corresponding acyl chloride, which is then converted into the methyl ester by reaction with... 

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