Cyclobutadieneiron tricarbonyl

Cyclobutadieneiron tricarbonyl or (C₄H₄)Fe(CO)₃ is an organometallic complex of cyclobutadiene and an iron metal carbonyl. The chemical compound is used in organic chemistry as a precursor for cyclobutadiene. It was first prepared in 1965 by Rowland Pettit starting from cyclooctatetraene ^[1] ^[2] ^[3] :

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Cyclooctatetraene is chlorinated to the [4.0.2]-bicyclic compound which reacts further with the alkyne dimethyl acetylenedicarboxylate in a Diels-Alder reaction followed by a reverse-DA reaction by pyrolysis at 200°C releasing cis-dichlorocyclobutene.

This compound reacts with di-iron nonacarbonyl (obtained from photolysis of iron pentacarbonyl) to Cyclobutadieneiron tricarbonyl.

Cyclobutadieneiron tricarbonyl displays aromaticity as evidenced by some of its reactions which can be classified as electrophilic aromatic substitution ^[4] :

It reacts in Friedel-Crafts acylation with acetyl chloride and aluminum chloride to the acyl derivative 2, with formaldehyde and hydrochloric acid to the chloromethyl derivative 3, in a Vilsmeier-Haack reaction with N-methylformanilide and phosphorus oxychloride to the formyl 4 and in a Mannich reaction to amine derivative 4

The reaction mechanism is identical to that of EAS:

References

^ Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes G. F. Emerson, L. Watts, R. Pettit; J. Am. Chem. Soc.; 1965; 87(1); 131-133. First Page
^ Cis-dichlorocyclobutene , Organic Syntheses, Coll. Vol. 6, p.422 (1988); Vol. 50, p.36 (1970) Article.
^ Iron, tricarbonyl (η4-1,3-cyclobutadiene)- R. Pettit and J. Henery Organic Syntheses, Coll. Vol. 6, p.310 (1988); Vol. 50, p.21 (1970) Link
^ Cyclobutadieneiron Tricarbonyl. A New Aromatic System J. D. Fitzpatrick, L. Watts, G. F. Emerson, R. Pettit J. Am. Chem. Soc.; 1965; 87(14); 3254-3255 Abstract