I want to enolize a ketone, for instance acetone with a $\mathrm{p}K_\mathrm{a}$ value of about 20 (in $\ce{H2O}$).

I'd like to know if there exists a rule of thumb to predict how the $\mathrm{p}K_\mathrm{a}$ of the ketone will decrease if we add one of the following Lewis acids:

a) $\ce{(c-Hex)2BCl}$ (chlorine anion can be eliminated)

b) $\ce{Bu2BOTf}$ (triflate anion is a good leaving group)

c) $\ce{NaI}$ (sodium is bigger than lithium, and I expect the bond to oxygen is less strong meaning the decrease of the $\mathrm{p}K_\mathrm{a}$ will be less than in d))

d) $\ce{LiI}$ (iodine is bigger than lithium, therefore good hard-soft-mismatch; and oxygen is hard as well)

e) $\ce{HCl}$ (As @SYK suggested in his answer of my recent question [1], the proton is the strongest Lewis acid as possible and will lower the $\mathrm{p}K_\mathrm{a}$ as much as possible, that even a very weak base, may be of $\mathrm{p}K_\mathrm{a}~9$ as $\ce{NH4Cl}$ will (or won't it?) form the cis-enolate)

I'm especially interested in the lowering values for a), b) and c). My questions:

  • Is my qualitative lowering of $\mathrm{p}K_\mathrm{a}$ values a < b < c < d < e correct?
  • Do you have somes guesses for decreasing values for a, b and c ?
  • $\begingroup$ You think a,b will be worse than c,d? I don't think so. $\endgroup$ – Mithoron Jul 18 '15 at 22:00
  • $\begingroup$ I think $\ce{Na+}$ is harder and hence has a higher positive charge acting on the oxygen. Why do you prefer the other ones? $\endgroup$ – laminin Jul 18 '15 at 22:23
  • $\begingroup$ It coordinates weakly, for borium keton would act as third ligand - bound strongly. $\endgroup$ – Mithoron Jul 18 '15 at 23:01
  • $\begingroup$ @Mithoron Why ? $\endgroup$ – laminin Jul 20 '15 at 13:37

I used to work in a group that specialised on the total synthesis of polyketides. For obvious reasons, syntheses of polyketides rely on aldol additions a lot. Yet, I never heard of any simple rule of thumb for the $\mathrm{p}K_\mathrm{a}$ value differences so I suspect there isn’t one.

Both boron reagents enolise ketones practically by themselves, $\ce{NEt3}$ is typically added to capture any excess protons. On the other hand, I have never heard of lithium or sodium enolising a ketone by themselves.

Finally, $\ce{HCl}$ (or any strong proton source for that matter) will protonate the ketone but not to the enolate. Instead, the protonated ketone with $\ce{C=\overset{+}{O}-H}$ will be obtained.


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