Organolithium and Organocuprate Reagents

The three compared

Organolithium reagents

Made from an alkyl halide and lithium metal, or by exchange with another organolithium. The carbon–lithium bond is even more polarised than carbon–magnesium, so the carbon is more nucleophilic and more basic.

Where the extra reactivity helps

• Hindered ketones that a Grignard cannot add to are often attacked successfully.
• Carboxylic acids can be converted to ketones, since two equivalents give a dianion that survives until aqueous workup.
• Directed metallation — a strongly basic organolithium can remove an aromatic proton next to a coordinating group, allowing substitution at a position no electrophilic route reaches.

Where it hurts

The greater basicity means more competing deprotonation. With a ketone that has acidic alpha hydrogens, an organolithium may simply form the enolate rather than adding. Any acidic proton anywhere in the molecule destroys the reagent, so tolerance is poorer than for a Grignard.

Organocuprates

Made by treating two equivalents of an organolithium with a copper(I) salt. The resulting reagent is far softer, and that softness changes its behaviour completely.

Conjugate addition

With an enone, a cuprate adds at the beta carbon. The resulting enolate is then protonated on workup, giving the saturated ketone with the new group installed at the beta position — a transformation neither a Grignard nor an organolithium performs cleanly.

This is the standard method for that disconnection, and recognising when a target requires it is a common synthesis question.

Coupling with halides

Cuprates also couple with alkyl and vinyl halides to form new carbon–carbon bonds, with retention of alkene geometry in the vinyl case. That stereochemical retention makes it useful for building defined alkenes.

Better functional group tolerance

Being much less basic, cuprates tolerate esters and nitriles that would not survive an organolithium. This makes them the reagent of choice in molecules carrying other functionality.

Choosing between them

1. Need conjugate addition to an enone? Cuprate.
2. Need 1,2 addition to an enone, or to a hindered ketone? Organolithium.
3. Straightforward addition with other functionality present? Grignard, or cuprate if basicity is a concern.
4. Need to deprotonate rather than add? Organolithium, using its basicity deliberately.

Frequently asked questions

Why does a cuprate add conjugately?

Because it is a soft nucleophile and the beta carbon is the softer electrophilic site. Hard nucleophiles prefer the harder carbonyl carbon. This is a direct application of HSAB reasoning.

Why are organolithiums more reactive than Grignards?

Because the carbon–lithium bond is more polarised, making the carbon more carbanion-like and therefore both more nucleophilic and more basic.

Why do cuprates tolerate esters?

Because they are much less basic and less aggressively nucleophilic, so they do not attack the ester carbonyl under the conditions used.

Why are two equivalents of organolithium needed to make a cuprate?

Because the reagent contains two organic groups bound to copper. Only one is normally transferred to the substrate, which is a known inefficiency of the method.