Oxygen enolate

What kind of chemistry do enols have Because their double bonds are electron-rich, enols behave as nucleophiles and react with electrophiles in much the same way that aikenes do. But because of resonance electron dona lion of a lone-pair of electrons on the neighboring oxygen, enols are more electron-rich and correspondingly more reactive than aikenes. Notice in the following electrostatic potential map of ethenol (BbC CHOH) how there is a substantial amount of electron density (yellow-red) on the a carbon. 

Considering enolate 5, where X, Y, and Z stand for three connective points for cyclic enolate formation, the formation can occur between any two of such points, leading to three possible oxygen enolates endo-cyclic 6, 7, and exo-cyclic enolate 8. In such cases, the resident asymmetric center ( ) may be positioned on any connective atom in the substrate. 

Although ketones are essentially inert to molecular oxygen, enolates are susceptible to oxidation. The combination of oxygen and a strong base has found some utility in the introduction of an oxygen function at carbanionic sites.200 Hydroperoxides are the initial products of such oxidations, but when DMSO or some other substance capable of reducing the hydroperoxide is present, the corresponding alcohol is isolated. A procedure that has met with considerable success involves oxidation in the presence of a trialkyl phosphite.201 The intermediate hydroperoxide is efficiently reduced by the phospite ester. 

Deprotonation of complex 1 with butyllithium at — 78 °C generates the enolate species 2 (described in Section 1.1.1.3.4.1.1.), which reacts with electrophiles while in the anti conformation (acyl oxygen anti to carbon monoxide oxygen). Enolate 2 is inert to 1,2-epoxypropane (3a) at — 78 °C, but in the presence of a Lewis acid, rapid reaction ensues leading to preferred alkylation of the least hindered site of the epoxide13. Reaction of the enolate 2, derived from the racemic complex 1, with racemic monosubstituted epoxides results in preferential formation of one of two possible diastereomers this can be termed a double enantiomer-differentiating reaction. 

This enolate approach is also efficient for methylation as well as hydroxymethylation of ot-oxygenated enolates using methyl iodide or formaldehyde as the electrophile, as demonstrated in a series of papers by Klemer s group [163]. 

Oxygenation of cycloalkanones. FeCl, and a number of other Fe(lll) salts catalyze the O2 oxidation of cycloalkanones in an alcohol to lo-oxo esters. The yield is only moderate with unsubstituted ketones, but is considerably improved by an adjacent alkyl group. Trifluoromethyl or methoxy groups inhibit oxygenation. Enol acetates are easily oxidized, but lactones are merely hydrolyzed.-... 

Lead(IV) salts will a-oxygenate enol ethers as they do enols vide supra), although in this case the process involves bisoxygenation of the unsaturated Ullage and subsequent hydrolysis. For example, the combination of lead tetrabenzoate and triethylammonium fluoride at 0-25 C effects efficient a-benzoyl-oxylation, e.g. (68) to (69). 0, y-Unsaturated ketones are also successfully oxidized, e.g. (70) to (71). The correspon ng LTA a-acetoxylations are possible, but the benzoate salt remains the transition metal reagent of choice for these substrates.These reactions appear to be uniformly efficient and perluq)S deserve wider synthetic tq>plication. 

Bromine reacts at room temperature and in the absence of light with the acylmethylquinone (28.3) to give good-to-exceilent yields of the quinonoid heterocycle—a class of compounds which has been reviewed [2947, 3650], An oxygen enolate ion intramolecularly displaces a fluorine atom in pentafluoro-phenyl compounds with the formation of fused furan ring. 

These mechanisms should remind you of alkene bromlnation (p. 427)—except that here the attack on the bromine is assisted by an electron pair on oxygen. Enols are more nucleophilic than simple alkenes—the HOMO is raised by the interaction with the oxygen s lone pairs and looks not unlike the HOMO of the enolate anion we discussed on p. 453. The product, instead of being a bromonium ion (which would undergo further reactions), loses a proton (or the Lewis acid) to give a ketone. 

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