There are several useful esters of this kind, of which the four in the margin are the most important. They cannot act as the enol partner, and the first three are more electro philic than most esters, so they should acylate an ester enolate faster than the ester being enolized can. These four are arranged in order of reactivity towards nuclophiles, the most electrophilic at the top and the least electrophilic at the bottom. Oxalates are very reactive because each car bonyl group makes the other more electrophilic. The molecular LUMO is the sum of the two π* orbitals and is lower in energy than either.

Formate esters look a bit like aldehydes but their ester character dominates. The hydrogen atom just makes them very electrophilic as they lack the σ conjugation (and steric hindrance) of simple esters. Carbonates are particularly useful as they introduce a CO₂R group on to an enolate. It is perhaps not immediately obvious why they are more electrophilic than simple esters. Normal esters are (slightly) less electrophilic than ketones because the deactivating lone pair donation by the oxygen atom is more important than the inductive effect of the electronegative oxygen atom.

The result is a small difference between two large effects. In carbonate esters there are two oxygen atoms on the same carbonyl group. Both can exert their full inductive effect but the lone pairs have to share the same π* orbital. The balance is changed—the summed inductive effects win out—and carbonates are more electrophilic than ordinary esters.

Finally, esters of aromatic acids cannot enolize but are less reactive than ordinary esters because of conjugation from the aromatic ring. These compounds may still be useful, as we shall see.

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مواضيع ذات صلة

Sulfur-stabilized anions

Sulfur is a very versatile element

Symmetry in intramolecular crossed Claisen condensations

Intramolecular crossed Claisen ester condensations

Crossed Claisen ester condensations between two different esters

Directed C-acylation of esters

Claisen condensations between ketones and esters

The Claisen ester condensation and other self-condensations

Problems with acylation at carbon

Cross-condensations

The aldol reaction

Nitroalkanes are superb nucleophiles for conjugate addition