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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

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  • $\begingroup$ Bromobenzene is fairly stable towards lithium halogen exchange, without generation of a benyzyne. $\endgroup$ – NotEvans. Jul 6 '17 at 6:06
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    $\begingroup$ @AS_1000 It might represent some work (e.g., barrelene), however a small graphic illustration of your suggested mechanism would ease to follow your argumentation. As a word of caution, BuLi is known to react with etheral solvents (hence tBuLi is sold in alkanes, and never in THF) to a degree that in conjunction with TMEDA you may use it synthetically (in case of THF, yielding ethene and the Li enolate of acetaldehyde). $\endgroup$ – Buttonwood Jul 6 '17 at 10:42
  • $\begingroup$ Thanks for that, I proposed another mechanism using THF, I also added drawings, but I still have a question regarding it because it involves a cycloreversion that I don't think would be possible at room temperature. $\endgroup$ – AS_1000 Jul 6 '17 at 12:16

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