Chapter 15, Problem 40DSP

Cyclobutadiene and (Cyclobutadiene)tricarbonyliron

As we saw in Section $12.17$, cyclobutadiene is antiaromatic and exceedingly difficult to prepare and study. Its successful preparation by Rowland Pettit (University of Texas) in $1965$ demonstrated how transition-metal organometallic chemistry can provide access to novel reactions and structures. His approach was to prepare cyclobutadiene as a transition-metal complex, then destabilize the complex to trigger its dissociation. The sequence for cyclobutadiene begins with the reaction of $\text{cis-3,4- dichlorocyclobutene}$ with diironnonacarbonyl $\left[{\text{Fe}}_2{\left(\text{CO}\right)}_9\right]$. The resulting iron–cyclobutadiene complex satisfies the $\text{18-electron}$ rule, is stable, and undergoes a variety of reactions. Most importantly, oxidation with ceric ammonium nitrate (a source of ${\text{Ce}}^{4+}$) lowers the electron count from $18$ to $16$, causing the complex to dissociate and liberate free cyclobutadiene.

The tricarbonyliron complex of cyclobutadiene is sufficiently stable to undergo reactions

typical of aromatic hydrocarbons. What is the product of the reaction shown?