Deprotonation is not the only way to use one simple organometallic reagent to generate another more useful one. Organolithiums can also remove halogen atoms from alkyl and aryl halides in a reaction known as halogen–metal exchange.

The bromine and the lithium simply swap places. As with many of these organometallic processes, the mechanism is not altogether clear, but can be represented as a nucleophilic attack on bromine by the butyllithium. But why does the reaction work? The product of our ‘mechanism’ is not PhLi and BuBr but a phenyl anion and a lithium cation. These could obvi-ously combine to give PhLi and BuBr. But is this a reasonable interpretation and why does the reaction go that way and not the other? The key, again, is pKa. We can think of the organolithiums as a complex between Li+ and a carbanion.

The lithium cation is the same in all cases: only the carbanion varies. So, the stability of the complex depends on the stability of the carbanion ligand. Benzene, (pKa about 43) is more acidic than butane (pKa about 50) so the phenyl complex is more stable than the butyl com plex and the reaction is a way to make PhLi from available BuLi. Vinyllithiums (the lithium must be bonded directly to the alkene) can also be made this way and a R2N– substituent is acceptable. Bromides or iodides react faster than chlorides.

Halogen–metal exchange tolls the knell of one appealing way to make carbon–carbon bonds. It may already have occurred to you that we might make a Grignard or organo lithium reagent and combine it with another alkyl halide to make a new carbon–carbon σ bond.

This reaction does not work because of transmetallation. The two alkyl bromides and their Grignard reagents will be in equilibrium with each other so that, even if the coupling were successful, three coupled products will be formed.

You will see later that transition metals are needed for this sort of reaction. The only successful reactions of this kind are couplings between metal derivatives of alkynes and alkyl halides. These do not exchange the metal as the alkynyl metal is much more stable than the alkyl metal. A good example is the synthesis of a substituted alkyne starting from acetylene (ethyne) itself. One alkylation uses NaNH2 as the base to make sodium acetylide and the other uses BuLi to make a lithium acetylide.

Transmetallation

 Organolithiums can be converted to other types of organometallic reagents by transmetallation—simply treating with the salt of a less electropositive metal. The more electropositive Mg or Li goes into solution as an ionic salt, while the less electropositive metal such as Zn takes over the alkyl group.

But why bother? Well, the high reactivity—and in particular the basicity—of Grignard reagents and organolithiums sometimes causes unwanted side reactions. Their combination with very strong electrophiles like acid chlorides usually results in a violent uncontrolled reaction. If a much less reactive organozinc compound is used instead, the reaction is more under control. These organozinc compounds can be made from either Grignard reagents or organolithium com pounds. E. Negishi, a pioneer of organozinc chemistry, got the Nobel Prize for Chemistry in 2010 with R. F. Heck and A. Suzuki for their work on organometallic compounds.

مواضيع ذات صلة

The product of nucleophilic addition to a carbonyl group is not always a stable compound

Oxidation of alcohols

?Secondary and tertiary alcohols: which organometallic, which aldehyde, which ketone

Making primary alcohols from organometallics and formaldehyde

Making carboxylic acids from organometallics and carbon dioxide

Using organometallics to make organic molecules

Making organometallics by deprotonating alkynes

Organometallics as bases

How to make organolithium reagents

Making organometallics, How to make Grignard reagents

Organometallic compounds contain a carbon–metal bond

Lewis acids and bases