Do eta-4 benzene ligands participate in Diels Alder or other cycloadditions?

While refreshing myself on denticity to answer another question, I started to read about [hapticity](http://en.wikipedia.org/wiki/Hapticity), which is the coordination of a ligand through multiple continuous atoms. A famous case is ferrocene (bis[$\eta^5$-cyclopentadienyl]iron, $\ce{Cp2Fe}$), where all five carbon atoms in each cyclopentadienyl ligand (Cp) are coordinated to the iron center. 


Benzene can also be a polyhaptic ligand, forming $\eta^4$ or $\eta^6$ complexes depending on the oxidation state of the metal. In particular, bis(benzene)ruthenium complexes can undergo redox shuttling, where one benzene ligand switches between $\eta^4$ and $\eta^6$.


![enter image description here](https://i.stack.imgur.com/pPnWU.png)


In the $\eta^4$ complex on the right, the top benzene ligand is now bent out of planarity, implying a break of aromaticity. This compound looks like a good dienophile for a [Diels-Alder reaction](http://en.wikipedia.org/wiki/Diels_Alder). The [article linked](http://dx.doi.org/10.1002/anie.197105561) in the Wikipedia page on this switching phenomenon is published in Angewandte Chemie, International Edition, which is behind a paywall for me. 



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> Does coordination of benzene to a metal in the $\eta^4$ mode activate it toward cycloadditions? Because of paywalls, I would appreciate summaries of articles in addition to links to them. My attempts to find articles using Google Scholar led me to [articles about the construction of benzenes by metal mediated cycloadditions](https://www.google.com/search?q=metal%20mediated%20cycloaddition%20benzene&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#q=metal%20mediated%20cycloaddition%20of%20benzene&rls=org.mozilla:en-US:official), and not to reactions of benzene itself.
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### Coordinated benzene as a dienophile


It would be logical to suggest that an η4-benzene ligand could behave as a dienophile. The reason why benzene is characteristically unreactive, both as a dienophile and as a diene, is because of its aromaticity; if aromaticity is lost, one would expect it to become more reactive.


However, to the best of my knowledge (and SciFinder-fu), such reactivity does not seem to have been achieved. :(


In the rest of this answer I'll try to summarise some of the research that is (somewhat) related to what you were asking about.




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### Benzene as a dienophile - in general


Benzene itself (and some substituted benzenes) can act as a dienophile under some conditions:


* with 1,2,4,5-tetrazines which act as electron-poor dienes (i.e. inverse demand Diels–Alder).1 Presumably, the reaction is favoured due to the loss of $\ce{N2}$ gas.
[![Benzene as a dienophile with electron-poor dienes](https://i.stack.imgur.com/KWsSR.png)](https://i.stack.imgur.com/KWsSR.png)
* Under high pressure.2
[![Benzene as a dienophile under high pressure](https://i.stack.imgur.com/QKvhp.png)](https://i.stack.imgur.com/QKvhp.png)
* Intramolecularly, when constrained to be in close proximity with a reactive diene.3
[![Benzene as a dienophile in an IMDA](https://i.stack.imgur.com/uPTz8.png)](https://i.stack.imgur.com/uPTz8.png)
* With incredibly reactive dienophiles (cyclobutadienes).4
[![Benzene reacting with cyclobutadienes](https://i.stack.imgur.com/aupKc.png)](https://i.stack.imgur.com/aupKc.png)


(This list is probably not exhaustive.)




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### Coordinated benzene, but as a diene


In ref 4, Inagaki *et al.* also summarised the ways in which benzene has been activated towards Diels–Alder reactions:



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> Currently, there are only a limited number of options to activate benzene sufficiently, including **coordination of a transition metal complex to benzene**, naphthalene with small resonance energy and the use of benzene derivatives with a sterically induced strained geometry [...] (emphasis mine)
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The only reference given for the bolded sentence is the same article that Klaus has linked in his answer.5 It describes the reaction of η2-benzene as a *diene*, with a hypothesis very similar to what you have written in the question:



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> In the proper coordination environment, both osmium and rhenium can form stable complexes with arenes in which only two of the six carbons are coordinated. Once complexed in a dihapto fashion, the uncoordinated portion of the arene more closely resembles a conjugated diene. We therefore hypothesized that the complexation should increase the propensity of arenes to undergo cycloaddition reactions.
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In this reaction scheme taken from the paper, the carbons numbered 1 through 4 are acting as the diene component in a Diels–Alder reaction with *N*-methylmaleimide:


[![Reaction scheme](https://i.stack.imgur.com/Glwe5.png)](https://i.stack.imgur.com/Glwe5.png)




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### References


1. Seitz, G.; Hoferichter, R.; Mohr, R. Benzol und linear anellierte Arene als Dienophile in der Diels-Alder-Reaktion mit inversem Elektronenbedarf. *Angew. Chem.* **1987,** *99* (4), 345–346. [DOI: 10.1002/ange.19870990413](https://doi.org/10.1002/ange.19870990413).
2. Jarre, W.; Bieniek, D.; Korte, F. Benzene as Dienophile in the Diels-Alder Reaction. *Angew. Chem. Int. Ed.* **1975,** *14* (3), 181–182. [DOI: 10.1002/anie.197501812](https://doi.org/10.1002/anie.197501812).
3. Saito, K.; Omura, Y.; Maekawa, E.; Gassman, P. G. The Addition of 1,3-Diphenylisobenzofuran to Cyclooctatetraene and to Dimethyl Cyclooctatetraene-1,2-dicarboxylate. The Benzene Ring as a Dienophile in an Intramolecular Diels–Alder Reaction. *Bull. Chem. Soc. Jpn.* **1990,** *63* (2), 395–400. [DOI: 10.1246/bcsj.63.395](https://doi.org/10.1246/bcsj.63.395).
4. Inagaki, Y.; Nakamoto, M.; Sekiguchi, A. A Diels–Alder super diene breaking benzene into C2H2 and C4H4 units. *Nat. Commun.* **2014,** *5.* [DOI: 10.1038/ncomms4018](https://doi.org/10.1038/ncomms4018).
5. Chordia, M. D.; Smith, P. L.; Meiere, S. H.; Sabat, M.; Harman, W. D. A Facile Diels−Alder Reaction with Benzene: Synthesis of the Bicyclo[2.2.2]octene Skeleton Promoted by Rhenium. *J. Am. Chem. Soc.* **2001,** *123* (43), 10756–10757. [DOI: 10.1021/ja011689q](https://doi.org/10.1021/ja011689q).


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I'm definitely not an expert in organometallic chemistry, and I don't know which aspect ($\eta^{\mathbf{4}}$, any other hapticity, or benzene as a dienophile or diene) is more important for you.


Anyway, here's a [*JACS* paper](http://dx.doi.org/10.1021/ja011689q), which describes the the Diels-Alder reaction of N-methyl maleimide with an $\eta^2$-rhenium complex of benzene as the **diene**.


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