Cyclobutadiene iron tricarbonyl electrophilic substitution

In a series of classic studies, Pettit et al. reported the synthesis of (cyclobutadiene)iron tricarbonyl together with a variety of electrophilic substitution reactions of this aromatic system (Scheme S).25-27... 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

One of the most interesting properties of the complex concerns its reactions with electrophilic reagents. It is found that these reactions lead to substituted cyclobutadiene-iron tricarbonyl complexes and, in this sense, the complex is classified as aromatic just as ferrocene may be so classified. The substitution reactions which have been performed so far are summarized below. 

Pettit et al. (39a) have found that cyclobutadieneiron tricarbonyl (XVIII) undergoes electrophilic substitution under very mild conditions to give a variety of monosubstituted cyclobutadiene-iron tricarbonyls (CII R = D, COCH3, CHO, CHjCl, HgCl, CH2NMC2). The authors have suggested that the reactions proceed via cyclobutenyliron tricarbonyl cationic complexes (Cl). 

Similarly in electrophilic substitution of w-cyclobutadiene iron tricarbonyl, w-enyl (i.e. n-1 electron) intermediates have been postulated cf. p 195, e.g. 

In cyclobutadiene iron tricarbonyl, the cyclobutadiene ligand shows aromatic properties and readily undergoes electrophilic substitutions [48]. 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

Tricarbonyl(cyclobutadiene)iron undergoes similar, facile, electrophilic substitution reactions (Maitlis, 1966 Fitzpatrick et al 1965). 

---