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Concept
Cyclobutadieneiron tricarbonyl
Summary
Cyclobutadieneiron tricarbonyl is an organoiron compound with the formula Fe(C4H4)(CO)3. It is a yellow solid that is soluble in organic solvents. It has been used in organic chemistry as a precursor for cyclobutadiene, which is an elusive species in the free state.
It was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and diiron nonacarbonyl:
C4H4Cl2 + 2 Fe2(CO)9 → (C4H4)Fe(CO)3 + 2 Fe(CO)5 + 5 CO + FeCl2
The compound is an example of a piano stool complex. The C-C distances are 1.426 Å.
Oxidative decomplexation of cyclobutadiene is achieved by treating the tricarbonyl complex with ceric ammonium nitrate. The released cyclobutadiene is trapped with a quinone, which functions as a dienophile.
Cyclobutadieneiron tricarbonyl displays aromaticity as evidenced by some of its reactions, which can be classified as electrophilic aromatic substitution:
It undergoes Friedel-Crafts acylation with acetyl chloride and aluminium chloride to give 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 5.
The reaction mechanism is identical to that of EAS:
Several years before Petit's work, (C4Ph4)Fe(CO)3 had been prepared from the reaction of iron carbonyl and diphenylacetylene.
(Butadiene)iron tricarbonyl is isoelectronic with cyclobutadieneiron tricarbonyl.
Official source
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