Here is a thought-provoking recent study from Professor G. K. Surya Prakash, my PhD mentor. The work, done in collaboration with Professor Herbert Mayr, reports a surprising outlier in what might otherwise be an intuitively clear nucleophilicity trend. It turns out that fluorine-containing carbanions can be up to 35 times more nucleophilic than their hydrogen congeners. While the authors do not suggest a simple explanation for the observed effect, one might hazard to guess that a heteroatom in place of hydrogen would increase the pyramidalization of the proximal anionic carbon centre. This increase, induced by the electron-withdrawing heteroatom, could lead to augmented nucleophilicity. While this is admittedly sound, the chlorinated analog does not follow the trend, throwing a wrench into what we all crave – a simple explanation. Fluorine is never boring and this Angewandte report provides yet another illustration.
Interesting you bring this up today, as I’m currently using nitro(phenylsulfonyl)methane as an ester enolate equivalent for the first time in several years. Funny how things work.
Nice. Good luck with the experiment!
How about p-pi p-pi overlap of the fluorine with the anion, thus increasing the electron density on the trigonal carbon (and hence nucleophilicity)? Chlorine is sufficiently bigger that pyramidalization might occur to a significant extent (and the 2p pi- 3p-pi overlap would be poorer in the first place before accounting for change of geometry).
Yes, I agree with that. Most likely the reason is in the balance between electronegativity and what you just described. By the way, CF3+ species are also way more stable than CCl3+. This happens for the same reason.