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The term enol is an abbreviation of alkenol, a portmanteau deriving from "-ene"/"alkene" and the "-ol". Many kinds of enols are known. [1] Keto–enol tautomerism refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol.
Under such conditions, they exist in low concentrations, but they still undergo reactions with electrophiles. Many factors affect the behavior of enolates, especially the solvent, additives (e.g. diamines), and the countercation (Li + vs Na +, etc.). For unsymmetrical ketones, methods exist to control the regiochemistry of the deprotonation.
This is the 'enol mechanism'. Carbonyl compounds, being carbon acids, can also be deprotonated to form enolates, which are much more nucleophilic than enols or enol ethers and can attack electrophiles directly. The usual electrophile is an aldehyde, since ketones are much less reactive. This is the 'enolate mechanism'.
If the enol has an adjacent stereocenter, then the two stereocenters flanking the carbonyl in the product are naturally syn: [23] The underlying mechanistic reason depends on the enol isomer. For an E enolate, the stereoinduction is necessary to avoid 1,3-allylic strain, while a Z enolate instead seeks to avoid 1,3-diaxial interactions: [24]
The general structure of a silyl enol ether. In organosilicon chemistry, silyl enol ethers are a class of organic compounds that share the common functional group R 3 Si−O−CR=CR 2, composed of an enolate (R 3 C−O−R) bonded to a silane (SiR 4) through its oxygen end and an ethene group (R 2 C=CR 2) as its carbon end.
The electrophilic amination of enolates yields α-amino carbonyl compounds. When chiral oxazolidinones are used in conjunction with azo compounds, enantioselectivity is observed (see above). BINAP can also be used for this purpose in the amination of silyl enol ethers.
In the protonation of an enolate ion, the kinetic product is the enol and the thermodynamic product is a ketone or aldehyde. Carbonyl compounds and their enols interchange rapidly by proton transfers catalyzed by acids or bases, even in trace amounts, in this case mediated by the enolate or the proton source.
Baldwin's rules also apply to aldol cyclizations involving enolates. [ 9 ] [ 10 ] Two new descriptors need to be defined: enolendo and enolexo , which refer to whether both carbons of the enolate C-C fragment are incorporated into the ring formed or not, respectively.