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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, 2017 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, 2017 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, 2017 at 12:16

1 Answer 1

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I found a research paper[1] 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
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
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

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) Fleming, I.; Mah, T. A Simple Synthesis of Anthracenes. J. Chem. Soc., Perkin Trans. 1 1975, No. 10, 964.

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