Alkane deprotonation by organolithiums

Question

Butyllithium isomers are all stored in alkane solvents. Theoretically, equilibrium should (given enough time) lead to formation of mostly n-hexyllithium in the case of n-BuLi stored in (mostly) n-hexane, I believe. I understand that the kinetics of deprotonating an alkyl proton are extraordinarily slow, apparently even in the case of the much stronger t-BuLi in pentane (which isn't merely thermodynamically neutral; it should be thermodynamically favorable for the lithium to exchange with the proton of a less substituted carbon).

I was wondering, though, does anyone know of any instance where a significant portion (or any observable portion, really) of something like t-BuLi has been noted to have reacted with alkane solvents?

All searches I've attempted have given me completely unrelated results, and even attempting to sift through them, I have come away with no mention of the idea, even.

Also, would the addition of some kind of lithium-binding reagent such as TMEDA or 15-crown-5 increase or decrease the rate here (assuming no Brønsted reaction between BuLi and the ligand)?

I ask because BuLi will typically deprotonate a heteroatom before doing nucleophilic addition, but will do nucleophilic addition (to a carbonyl) before deprotonating most "softer" carbon acids, suggesting alkyllithiums are pretty hard bases. I'm assuming making a more "naked" carbanion would make a harder base (more charge-driven than "HOMO-LUMO overlap"-driven), and I'm wondering if that could make it slower to react with an alkyl proton (though, an unstabilized carbanion is a fairly hard nucleofuge, so I could see the reaction going faster, too).

Answer 1

I would bet activation energy for this sort of reaction is quite high, and since primary and tertiary carbons aren't hugely different in stability the sort of H for Li exchange reaction would be very slow. For nBuLi, the reaction would practically not happen, and for sBuLi and tBuLi intramolecular rearrangement (proton shift) is more likely than reaction with solvent, resulting in nBuLi and iBuLi respectively.

Agreed that elimination and precipitation of LiH will be faster than alkyl exchanges in these solution, though I've never checked an old bottle of tBuLi for any precipitate so this is entirely speculative.

Investigating this would be quite challenging, as loss of base power in practice usually happens from slow exposure to O2 and H2O,which would resolve Li2O and LiOH respectively, also basic precipitates.