[emyonee]

Hello,

Please I am trying to synthesize 4-bromobenzene-1,2-diol and I came across an article that synthesized the product and gave the procedure as "high potential selectivity in the electrophilic aromatic bromination of catechol (15) was accomplished by carefully controlling the reaction temperature. The reaction of catechol (15) with N-bromosuccinimide and fluoroboric acid in acetonitrile was performed at −30 °C, the reaction mixture was allowed to warm up to room temperature, and then stirred at this temperature overnight to afford 4-bromobenzene-1,2-diol (16a) in 100% yield." (extracted from the article below).
If anyone can tell me the role of the fluoroboric acid in the synthesis and what alternatives I can use in its place. Thank you.

Li, H.J., Wu, Y.C., Dai, J.H., Song, Y., Cheng, R. and Qiao, Y., 2014. Regioselective electrophilic aromatic bromination: theoretical analysis and experimental verification. Molecules, 19(3), pp.3401-3416.

[orthoformate]

Hey, I read that article.

Possibly stabilization of the arenium cation transition state? this would decrease the energy barrier, acting as a catalyst.

This is just my best guess,

https://en.wikipedia.org/wiki/Fluoroboric_acid#Organic_chemistry

[clarkstill]

Hey, I read that article.

Possibly stabilization of the arenium cation transition state? this would decrease the energy barrier, acting as a catalyst.

I'm not sure what mode of stabilization you are suggesting? How can a strong acid stabilize a cation? I would suggest that the acid protonates NBS, making it a more electrophilic source of Br, as postulated in ref 44 (http://pubs.acs.org/doi/pdf/10.1021/jo9622993).

[emyonee]

Hey, I read that article.

Possibly stabilization of the arenium cation transition state? this would decrease the energy barrier, acting as a catalyst.

I'm not sure what mode of stabilization you are suggesting? How can a strong acid stabilize a cation? I would suggest that the acid protonates NBS, making it a more electrophilic source of Br, as postulated in ref 44 (http://pubs.acs.org/doi/pdf/10.1021/jo9622993).

If this is true what other strong acids can be suggested to also give the desired product? Thank you

[orthoformate]

Hey, I read that article.

Possibly stabilization of the arenium cation transition state? this would decrease the energy barrier, acting as a catalyst.

I'm not sure what mode of stabilization you are suggesting? How can a strong acid stabilize a cation? I would suggest that the acid protonates NBS, making it a more electrophilic source of Br, as postulated in ref 44 (http://pubs.acs.org/doi/pdf/10.1021/jo9622993).

If this is true what other strong acids can be suggested to also give the desired product? Thank you

I was thinking that the BF₄ ion coordinates to the arenium cation, but after reading a little more about Electrophilic Aromatic Halogenation, I think Clarkstill is correct. The acidity of the Fluoroboric acid drives the alkylation.

Personally, I wouldn't change this persons 4-bromobenzene-1,2-diol prep. what is your concern with Fluorboric acid? Dangers of Fluorine?

[emyonee]

I was thinking that the BF₄ ion coordinates to the arenium cation, but after reading a little more about Electrophilic Aromatic Halogenation, I think Clarkstill is correct. The acidity of the Fluoroboric acid drives the alkylation.

Personally, I wouldn't change this persons 4-bromobenzene-1,2-diol prep. what is your concern with Fluorboric acid? Dangers of Fluorine?
[/quote]

I was thinking of an alternative because the article mentioned others strong acids and I wanted to know if there are certain criteria in choosing this particular strong acid.

[orgopete]

I really have no idea, but I wondered if the mechanism was somewhat different. Potentially I think one can write a mechanism in which HBr catalyzes the formation of Br2 and succinimide from NBS. My thinking is that Br2 would be pretty reactive and non-selective. If BF4(-) were simply a source of fluoride, then it could convert Br2 into BrF. This should be less reactive, more selective, reagent.