Английская Википедия:Cyclobutadieneiron tricarbonyl

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Cyclobutadieneiron tricarbonyl is an organoiron compound with the formula Fe(C₄H₄)(CO)₃. It is a yellow oil 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.

Preparation and structure

Cyclobutadieneiron tricarbonyl 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.

Properties

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:

Cyclobutadieneiron tricarbonyl Reactions

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:

Cyclobutadieneiron tricarbonyl EAS reactionmechanism

Related compounds

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.

History

In 1956, Longuet-Higgins and Orgel predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate e_g orbital of cyclobutadiene has the correct symmetry for π interaction with the d_xz and d_yz orbitals of the proper metal. The compound was synthesized three years after the prediction This case of theory before experiment.