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In another question, I mentioned that O-protonation of an enolate followed by tautomerization may lead to increased amounts of the thermodynamic product over the kinetic product (when those are different species).

The idea that the oxygen is likely protonated first has been discussed previously.

The way, I think, to get the most kinetic product is to use a bulky acid that would do C-protonation rather than O-protonation (assuming sterics explain the difference in kinetic/thermo product). The proton would have to be a softer electrophile, i.e. less positively charged than a proton from a stronger acid.

To continue with the example I gave in the previous question, I think it's likely that C-protonation would look like this, where the major product would be the kinetic product:

enter image description here

So my question is: are there acids that are known to specifically protonate the carbon of an enolate rather than the oxygen? I was thinking maybe BHT would work. Or maybe the thiol analogue of BHT - yes, it would be more acidic than BHT, but it would also be a softer nucleofuge than the BHT anion. This reasoning works with alkyl halides, i.e. softer leaving group = more C-attack, but I'm not sure if the same goes here.

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  • $\begingroup$ I guess $\sigma ^{*} _{\ce{S-H}}$ would be lower in energy than $\sigma ^{*} _{\ce{O-H}}$, so even though BHT would a weaker acid than the equivalent thiol, the thiol would still have better HOMO-LUMO overlap with carbon and lower $\delta +$ on the hydrogen. $\endgroup$ – SendersReagent Mar 24 '16 at 16:13
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    $\begingroup$ I think it's reasonable to extend the logic of O- vs. C-alkylation to protonation. I don't see any good way of proving the results of the experiment though. $\endgroup$ – jerepierre Mar 24 '16 at 18:15
  • $\begingroup$ @jerepierre What do you mean you don't see any good way of proving the results? You mean that even if the results came out as I expected, it wouldn't be definitive that it happened the way I described it? $\endgroup$ – SendersReagent Mar 24 '16 at 19:44
  • $\begingroup$ what technique would you use to prove that the oxygen or carbon is protonated first and the observed product is not the result of equilibration? $\endgroup$ – jerepierre Mar 24 '16 at 19:59
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    $\begingroup$ Trap it as the silyl ketene acetal and then protonate that, eg. doi.org/10.1016/S0040-4039(00)84541-4 $\endgroup$ – Organic Chemistry Explained Nov 25 '18 at 3:22

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