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In Carey, Sundberg Advanced Organic Chemistry it is written:

The relative slowness of the abstraction of protons from carbon acids by organolithium reagents is probably also due to the compact character of the carbon-lithium clusters. Since the electrons associated with the carbanion are tightly associated with the cluster of lithium cations, some activation energy is required to break the bond before the carbanion can act as a base. This kinetic sluggishness of organometallic compounds as bases permits important reactions in which the organometallic species acts as a nucleophile in preference to functioning as a strong base.

Won't the same reasons apply for nucleophilic displacement? Compactness of the aggregates should affect nucleophilicity, too. So why will BuLi add to a carbonyl instead of deprotonating the alpha carbon atom?

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  • $\begingroup$ tBuLi and LDA? Both organolithiums that act solely as bases (or at least never as nucleophiles) $\endgroup$ – NotEvans. Jul 23 '16 at 15:14
  • $\begingroup$ They act so because of they are bulky. I am more interestsed in BuLi and similar organolithiums. $\endgroup$ – RBW Jul 23 '16 at 15:43
  • $\begingroup$ 'Why are organolithiums better as nucleophiles than bases' Maybe update your title and question to be more specific. The statement isn't really a general one other than n-Buli $\endgroup$ – NotEvans. Jul 23 '16 at 15:45
  • $\begingroup$ Edited. Hope that it is more clear now. $\endgroup$ – RBW Jul 23 '16 at 16:48
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This is a trick question, which explains why the first sentence from the book takes so many precautions. It is very difficult to provide a general explanation because there are many factors influencing the result.

  1. Solvent: the solvent is extremely important in such reactions. It plays an important role in the order of oligomerization of the organolithium reagent; monomers, dimers, tetramers or polymers. Of course, the reactivity is really different.
  2. Co-solvent: a co-solvent like HMPT or TMEDA can drastically change the course of the reaction by changing the order of oligomerization.
  3. Temperature: it is well known that n-butyllithium is a very strong base at low temperatures (<-60°C) but will become a good nucleophile at higher temperatures. It can even open a THF ring.
  4. Thermodynamic or kinetic control: organolithium compounds are usually used under kinetic control, but...
  5. HSAB: how the oligomerization affects the hardness/softness of the base and its nucleophilicity?
  6. 3D shape of the organolithium compound: it greatly influences the shape and order of oligomers.
  7. ...

So now you feel completely depressed, I can try to answer your question. I think that a key factor is actually that in a cluster (dimer, tetramer and so on), charges are more diluted which enhances the nucleophilic behavior. Also, the geometry of the cluster and of its 3D-complex with the substrate could explain why a reaction is favored over the other.

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  • $\begingroup$ Great answer! :D I concluded that oligomeriation softens the organolithium. $\endgroup$ – RBW Jul 23 '16 at 18:53

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