# Is there a way to force C-protonation of an enolate?

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 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:

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.

• 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. Mar 24 '16 at 16:13
• 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. Mar 24 '16 at 18:15
• @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? Mar 24 '16 at 19:44
• Trap it as the silyl ketene acetal and then protonate that, eg. doi.org/10.1016/S0040-4039(00)84541-4 Nov 25 '18 at 3:22
• You might use tert-butyl bromide as your "acid" source via an E2 elimination. Apr 22 '20 at 23:02