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Why do lithium enolates add to an electrophile in direct addition manner, while the sodium and potassium enolates tend to add in conjugated additions? The sodium and potassium enolates are well dissociated, making oxygen a hard base (which is why these enolates participate easily in O-acylations), but still, the alpha carbon atom is made softer, to react well in conjugated additions. This seems to me like a contradiction.

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I'll assume we're talking about aldol reactions, ie. the electrophile is a aldehyde, or less commonly, a ketone. Being a more strongly coordinating cation, lithium can actually coordinate to both the enolate oxygen atom AND the electrophile oxygen. This serves to "pre-organise" both reactants for a direct addition via a six-membered, cyclic transition state. The favoured transition states are often chair-like. This is known as the Zimmerman-Traxler model.

enter image description here

The model explains diastereselectivity when that is possible, ie. E enolates give predominantly anti products, while Z enolates give mainly syn products.

Lithium also stabilises the initial aldolate product, by chelation via both oxygens.

enter image description here

The more highly dissociated Na and K cations presumably don't greatly facilitate reaction via a cyclic transition state, nor form a stable chelate of the product. The reaction is also more likely to be reversible, and the products of conjugate addition more stable.

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Lithium has a very high affinity for oxygen (strong bonding interaction). For the larger (i.e. ion radius) more polarized alkali metals down the periodic table, most notably sodium and potassium, the bonding is more dissociated and hence the reaction of the enolate is more reversible. Usually the conjugated addition product is the thermodynamic product, while the aldol product is the kinetic product. More reversibility will increase reaction time and possibly yield. Therefore sodium and potassium bases are preferred over lithium bases when the conjugated product is needed.

The above was more or less paraphrased from Clayden et Al. Organic Chemistry textbook (p. 752, chapter 29).

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  • $\begingroup$ Could it be also because oxygen coordinates better to lithium, so the carbonyl group of Michael acceptor will be close for direct addition? $\endgroup$
    – EJC
    Feb 19 '15 at 15:59
  • $\begingroup$ I expanded the question a bit. $\endgroup$
    – EJC
    Feb 20 '15 at 14:53
  • $\begingroup$ @Marko I understand what you are saying. Potassium and sodium will indeed harden the nucleophile and this effect is used sometimes, but reversibility is much more important here, since most direct addition product will be converted back as it is only the kinetic product. $\endgroup$
    – Jori
    Feb 20 '15 at 23:15
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Because the lithium enolates are more strong than sodium or potassium enolates. The lithium enolates addition is under kinetic control, instead the other addition is under thermodinamic control.

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