Synthesis of Anthracene: Mechanism

Recently, I happened to find a problem that involved the synthesis of anthracene from bromobenzene in the presence of tert-butyl lithium, using THF as the solvent, and at room temperature. I am wondering the mechanism for this transformation.


Taking into account that THF degrades in tert-butyl lithium this is the one that I thought of, 


It starts with aryne formation. Benzyne reacts with ethene in a [2+2] cycloaddition. This undergoes cycloreversion. The resulting product reacts with another mole of benzyne to create the dihydro-derivative. This reacts with another equivalent of tert-butyl lithium to form anthracene. 


I have a question regarding this. I do not think a cycloreversion could occur at room temperature (wiki gives a temperature of 180 degrees and states that the conrotary opening is unfavored). Is this the case or not? Am I missing another possible mechanism here? 


[![Drawing of structure](https://i.stack.imgur.com/ck0Pi.png)](https://i.stack.imgur.com/ck0Pi.png)




I found a research paper[[1]](https://doi.org/10.1039/P19750000964) which specifically talks about the reaction you mention. The paper also mentions the same mechanism as you do:



> 
> TETRAHYDROFURAN (THF) (1) is known 1 to react with strong bases like
> butyl-lithium, and the product (2) is known to decompose by a
> retro-cycloaddition to give ethylene (3), and the enolate ion (4) of
> acetaldehyde.  
> 
> [![retro-cycloaddition](https://i.stack.imgur.com/d7lvX.png)](https://i.stack.imgur.com/d7lvX.png)  
> 
> Enolate ions (6) are
> known 2 to react with benzyne (5) to give dihydrobenzocyclobutenes (7),
> and the dihydrobenzocyclobutenes are known to decompose by an
> electrocyclic opening to give o-xylylenes (9), which are  
> 
> [![electrocyclic opening to give o-xylylenes](https://i.stack.imgur.com/TS3f8.png)](https://i.stack.imgur.com/TS3f8.png)  
> 
> trapped by the
> benzyne to give dihydroanthracenols (11). By putting these known
> reactions together we have found a very simple ' one-pot ' synthesis
> of symmetrical 9,lO-unsubstituted anthracenes.
> 
> 
> 


The paper uses warm THF to conduct the reaction which shows that cycloreversion of the side ring actually takes place even at a temperature lower than 180°C.



> 
> When bromobenzene (13) is added to a solution of a four-fold excess of
> N-lithio-2,2,6,6-tetramethylpiperidine (15) in **warm** THF, the major
> neutral product is anthracene (16) (63% based on bromobenzene).  
> 
> [![bromobenzene to anthracene](https://i.stack.imgur.com/yzb9W.png)](https://i.stack.imgur.com/yzb9W.png)
> 
> 
> 


The paper also talks about what happens when the reaction temperature is changed:



> 
> *Variations in the Reaction Conditions.*-The reaction was carried out as described above, but with the following modifications. (i) When the
> butyl-lithium and the tetramethylpiperidine were mixed slowly at 0°
> and the reaction carried out at 20°, there was no anthracene formed,
> and Nphenyl-2,2,6,6-tetramethylpiperidine was the major product (75%
> by n.m.r.). (ii) When the butyl-lithium and the tetramethylpiperidine
> were mixed at reflux temperature and then cooled to 0° before adding
> the bromobenzene, anthracene was formed (40%, pure). We were not able
> to isolate dihydrobenzocyclobutanol. ... (iv) When butyllithium (5
> mmol) and tetramethylpiperidine (6 mmol) were mixed at -78° and then
> the mixture was brought to reflux temperature before adding the
> bromobenzene (1 mmol), the yield of anthracene was the same as in the
> original reaction.
> 
> 
> 




---


**Reference:**


([1](https://doi.org/10.1039/P19750000964)) Fleming, I.; Mah, T. A Simple Synthesis of Anthracenes. *J. Chem. Soc., Perkin Trans. 1* **1975**, No. 10, 964.


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