Why is BuLi a better nucleophile than base?

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?




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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