fluorodefluorosulfonation: a new arene fluorination?
SNAr works with highly electronegative groups: NO2, N2+, SO3-, F are more likely to be exchanged than Cl, Br, I.
On highly electron-deficient arenes (3-nitroisonicotinate) the nitro group can be substituted for fluoride with CsF in DMSO:
https://www.mdpi.com/1420-3049/11/2/130/htm
and we are all aware of the alkaline hydrolysis of sulfonates to phenols at around 300 C. Both of these reactions require strong nucleophiles at high
temperatures, but they work.
So my suggestion is that the fluorosulfonyl group, -SO2F, could probably be substituted by the fluoride anion F-. The pKa of the conjugate acids is as
follows:
NO2- <> HNO2 pKa ~= 3.4 (fluoro-denitration)
SO3(2-) <> HSO3- pKa ~= 7 (hydroxy-desulfonation)
SO2F- <> HSO2F pKa ~= 0 (fluoro-defluorosulfonation, conjugate acid is unstable)
pKa is a poor measure of electronegativity, but overall this looks promising.
Perhaps more interestingly, fluorosulfite SO2F- should decompose at high temperatures (required anyway) to SO2 + F-, regenerating fluoride. Since the
fluoride ion is regenerated, that means that the reaction could be catalytic in fluoride. That should allow you to use e.g. NBu4F
economically. I'm not certain of this, though: sources tell me that KSO2F decomposes between 100 and 160 C, but the decomposition temperature may be
higher with no K+ ion present.
The FSO2- group is "easily" attached by treating any arene with FSO3H, which will readily sulfonate all but the most deactivated arenes: 2 FSO3H + ArH
>> ArSO2F + H2SO4 + HF
The net reaction is an electrophilic monofluorination of arenes with no electrophilic fluorine or metal catalysts.
These reactions are roughly at the edge of what an amateur might be able to try -- the worst part by far is the preparation of fluorosulfuric acid,
although Wikipedia claims this can be done by the reaction of CaF2 and oleum (I'm not holding my breath!). The fluorosulfonation is "simple"
but requires handling FSO3H. The key step probably looks something like the reaction of a sulfonyl fluoride with CsF or TBAF in refluxing propylene
carbonate (242 C).
tl;dr: I was looking at sulfonate hydrolysis and wondered "what if hydroxy was fluoro?" and then I wrote this post about it
[Edited on 04-20-1969 by clearly_not_atara]
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