Cyclobutadiene ligand, aromaticity

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... 

One final comment relating to the aromaticity of the cyclobutadiene ligand concerns the orientation effect of substituents towards introduction of a second substituent. To date, the only reaction bearing on this question which has been performed is the acetylation of methylcyclobutadiene -Fe(CO)3 (XVIII), which was prepared by reducing the chloromethyl complex (IX). Acetylation of Complex XVIII produces a mixture of 2-and 3-acetyl-1-methylcyclobutadiene-iron tricarbonyl complexes with the latter isomer (XIX) predominating ( 2 1). This is not the orientation... 

In its reactions, coordinated cyclobutadiene exhibits aromatic character, undergoing electrophilic substitution, e.g. Friedel-Crafts acylation. A S5mthetic apphcation of (r " -C4H4)Fe(CO)3 in organic chemistry is as a stable source of cyclobutadiene oxidation releases the ligand making it available for reaction with, for example, alkynes as in scheme 23.117. 

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

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character-... 

Cyclobutadiene, C4H4, is anti-aromatic (i.e. it does not have 4n + 2 TT-electrons) and readily polymerizes. However, it can be stabilized by coordination to a low oxidation state metal centre. Yellow crystalline (ri -C4H4)Fe(CO)3 is made by reaction 23.115 and its solid state structure (Figure 23.25a) shows that (in contrast to the free ligand in which the double bonds are localized) the C—C bonds in coordinated C4H4 are of equal length. 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

---