Enolate

of an enolate anion. In organic chemistry, enolates are the organic anions derived from the deprotonation of carbonyl ( ) compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.

of an enolate anion.]] In organic chemistry, enolates are the organic anions derived from the deprotonation of carbonyl ( ) compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds. ==Bonding and structure== Enolate anions are electronically related to allyl anions. The anionic charge is delocalized over the oxygen and the two carbon sites. Thus they have the character of both an alkoxide and a carbanion. Although enolate salts are often drawn as simple ion pairs, in fact they adopt complicated structures often featuring aggregates. ==Preparation== Carbonyl compounds with an α hydrogen atom deprotonate to give enolates: Base mediates the process, but Lewis acidity plays a key role in stabilizing the product. Often the (weak) Lewis acid is simply the alkali counterion to an Arrhenius base (1.); unsurprisingly, reactivity with such salts varies from lithium to cesium. Alternatively, enolates can be generated from a molecular Lewis acid and a weak Brønsted base (a frustrated Lewis pair; 2.): Most substrates have multiple α hydrogen atoms, and in principle could give multiple enolate isomers. For example, with methylcyclohexanone: However, reaction conditions can control both the resulting enolate's regio- and stereochemistry. This provides one of the best understood synthetic strategies to introduce chemical complexity in total syntheses. Alternatively, an enone can serve as a protecting group, masking a specific enol. Reaction with a hydride or dissolving-metal reduction then forms an enol, as in this total synthesis of progesterone: ==Reactions== Powerful nucleophiles, enolates react with a variety of electrophiles at oxygen and carbon. Controlling which atom enolates react at has drawn much attention. Reaction at carbon is thermodynamically favored. Kinetically, the negative charge in enolates is concentrated on the oxygen. However, the oxygen center is also highly solvated, which can lead to alkylation at carbon. Reaction at oxygen traps the enolate as a (silyl) enol ether or ester. Such species can be carried through other transformations relatively inertly, but then are released in the presence of Lewis acids (the Mukaiyama aldol reaction): Other important electrophiles are aldehydes/ketones (the aldol reaction) and Michael acceptors.

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