organic chemistry - By how many pKa units will the pKa value of a ketone be minimized by addition of a Lewis acid

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 ?