So far we have considered only ‘self-condensations’—dimerization reactions of a single car bonyl compound. These form only a tiny fraction of known aldol reactions. Those that occur between two different carbonyl compounds, one acting as a nucleophile in its enol or enolate form, and the other as an electrophile, are called cross-condensations. They are more interesting than self-condensations, but working out what happens needs more thought. We shall start with an example that works well. The ketone PhCOMe reacts with 4-nitrobenzaldehyde in aqueous ethanol under NaOH catalysis to give a quantitative yield of an enone.

The fi rst step must be the formation of an enolate anion using NaOH as a base. Although both carbonyl compounds are unsymmetrical, there is only one site for enolization as there is only one set of α protons, on the methyl group of the ketone. The aldehyde has no α protons at all.

To get the observed product, the enolate obviously attacks the aldehyde to give an aldol, which then dehydrates by the E1cB mechanism.

Now, in this step there was a choice. The enolate could have attacked another molecule of unenolized ketone. It didn’t, because ketones are less reactive than aldehydes. In this case the aldehyde has an electron-withdrawing nitro substituent too, making it even more reactive. The enolate selects the better electrophile—that is, the aldehyde. In other cases the balance may shift towards self-condensation. You might think that a crossed aldol reaction between acetaldehyde and benzophenone (diphenylketone Ph₂C=O) should work well.

After all, only the aldehyde can enolize and the enolate could attack the ketone.

But it won’t work. The ketone is very hindered and very conjugated. It is less electrophilic than a normal ketone and normal ketones are less electrophilic than aldehydes. Given a choice between attacking this ketone and attacking another (but unenolized) molecule of acetaldehyde, the enolate will choose the aldehyde every time. The reaction at the start of the chapter occurs, while the ketone is just a spectator.

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المزيد

مواضيع ذات صلة

The Claisen ester condensation and other self-condensations

Problems with acylation at carbon

The aldol reaction

Nitroalkanes are superb nucleophiles for conjugate addition

A variety of electrophilic alkenes will accept enol(ate) nucleophiles

Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product

Enamines are convenient stable enol equivalents for conjugate addition

Alkali metal (especially Na, K) enolates can undergo conjugate addition

1,3-Dicarbonyl compounds undergo conjugate addition

Using Michael acceptors as electrophiles

Conjugate addition to enones gives enolates regiospecifically

Enones provide a solution to regioselectivity problems