Loptr
International Hazard
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Reactions with Aldehyde/Ketone Bisulfite-Adducts
As most of us know, the reaction between an aldehyde/ketone and sodium bisulfite produces a bisulfite-adduct, otherwise known as an
alpha-hydroxysulfonate. This sulfonate has a hydroxyl group on the alpha-carbon, next to the electron withdrawing sulfonate group.
The hydroxyl is a primary alcohol and should be able to leave through an E2 reaction with an acid. The E1 reaction would try to form a somewhat
unstable carbocation, and if it was able to, unless there is the potential for a more stable adjacent carbocation, it wouldn't live long enough in the
reaction. Otherwise, a rearrangement would take place and the carbocation will migrate to the more stable location. However, since that alpha-carbon
is relatively acidic due to the adjacent sulfonate group, I am thinking the double bond will still be between those groups from deprotonation of the
acidic hydrogen, given all things are the same.
Is there any precedence for this reaction as a way to take the alpha-hydroxalkylsulfonate (bisulfite-adduct) and eliminate the alpha-hydroxy group to
an unsaturated sulfonate?
EDIT: I understand this is the E1 mechanism. I have since changed my opinion on the reaction pathway.
R-CH(-OH)-SO2(OH) + H3PO4 -> R-C+H-SO2(OH) + H2O +
H2PO-4
R-C+H-SO2(OH) + H2PO-4 -> R=CH-SO2(OH)
I am trying to figure out what other reactions with bisulfite-adducts are possible.
The bisulfite-adduct is poorly soluble in solvents that would be required to keep it from hydrolyzing, so water, alcohols, etc., would be a problem.
One idea that I had would be to form the tetrabutylammonium salt of the alpha-hydroxyalkylsulfonate, and that possibly it could be used to get it into
a suitable solvent.
I haven't done any experiments on this yet, and was looking for some guidance, or for someone to tell me that this would be unlikely to be a success.
Any thoughts this reaction, the solvent, and acid?
One of my hopes would be to use toluene, and possibly phosphoric acid, since it can be extracted into toluene. The azeotropic drying doesn't hurt
either, however, I know the solvent effects on this reaction play an important role.
EDIT: It's been a while since I have dove into the reaction mechanisms, and I think I have combined E1/E2 into my logic. I believe
that the elimination would take place via E2, which means there wouldn't be a carbocation due to stability issues of a primary carbocation. The base
would abstract a hydrogen from the adjacent carbon, leading to the alkene.
[Edited on 12-1-2024 by Loptr]
"Question everything generally thought to be obvious." - Dieter Rams
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clearly_not_atara
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This would amount to a double deprotonation, so you would need a very strong base. At the same time, you would need to avoid the competing
nucleophilic displacement of sulfur. Transformations of the alpha-hydroxy have been reported before:
https://www.sciencedirect.com/science/article/abs/pii/S00404...
E2 elimination of alcohols is almost never favorable, though, due to competing formation of the alkoxide. And the carbocation formation is also rather
unlikely due to positive charge on sulfur in the resonance isomers. Sometimes this can be encouraged by metal catalysts, e.g. alumina.
There is another possibility. Apparently the bisulfite adducts can be converted in some cases to the amino-derivatives, which would be the bisulfite
adducts of imines. There is a review:
https://pubs.acs.org/doi/pdf/10.1021/jo01094a029
Apparently the quaternization of these alpha-aminosulfonates is possible:
https://www.sciencedirect.com/science/article/abs/pii/S00404...
However the application of Hofmann-type elimination on them is not documented, probably because nobody considers it practically competitive with other
methods. The unsaturated sulfonic acids are apparently very strong dienophiles and dipolarophiles, and in particular might react also with bisulfite
again to form the vicinal bis-sulfonates. Cf. the PhD thesis of CJ Wygant, "UNSATURATED SULFONIC ACIDS: DIELS-ALDER AND BROMINATION REACTIONS".
It is also known that the whole thing can be perchlorinated with excess PCl5 to the alpha-chlorosulfonyl chloride, but nobody wants to do this.
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