1- phenyl-2-propanone, enolate

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

The acidifying effect of an adjacent phenyl group outweighs steric effects in the case of l-phenyl-2-propanone, and as a result the conjugated enolate is favored by both kinetic and thermodynamic conditions (Entry 5). 

The procedure reported here provides a convenient method for the a-hydroxylation of ketones which form enolates under the reaction conditions. The reaction has been applied successfully to a series of para-substituted acetophenones, 1-phenyl-1-propanone, 3-pentanone, cyclopentanone, cyclohexanone, cycloheptanone, cyclododecanone, 2-methyl cyclohexanone, 2-norbornanone and benzalacetone. In the case of a steroidal example it was shown that a carbon-carbon double bond and a secondary hydroxyl group are not oxidized. A primary amino function, as in the case of p-aminoacetophenone, is not affected.5 Similarly, a tertiary amino ketone such as tropinone undergoes the a-hydroxy at ion reaction.5... 

Coplanarity is optimal for the 2-indanone system42,43. Enolization of l-phenyl-2-propanone is inhibited by lack of enforced coplanarity43. It has been estimated that for l-phenyl-2-propanone pK% 16.2 and pKk — 6.442. 

Acyclic a-hydroxy ketones of high enantiomeric purity can be prepared by choice of the appropriate conditions and (camphorylsulfonyl)oxaziridine reagent, for example, treatment of deoxy-benzoin (159a) with NaHMDS followed by (+)-(114) at —78°C affords (+ )-(S)-benzoin (160) in 95% ee and 88% isolated yield (Scheme 29) <89JOC202i, 90JA6679). Similarly, (—)-(S)-2-hydroxy-1 -phenyl-1-propanone (161) is produced in 95% ee by hydroxylation of the lithium enolate of... 

Of course, under many conditions, the preformed enolate aldol reaction appears to be significantly exothermic. The additional driving force is presumably provided by the enthalpy of coordination of the ambident aldolate ion with a cation. The importance of cation solvation in providing a driving force for the aldol reaction has been elegantly demonstrated by Noyori and coworkers.In this important experiment, the tris(dimethylamino)sulfonium (TAS" ) enolate of l-phenyl-2-propanone was prepared as shown in equation (6). The naked enolate was obtained as a yellow crystalline material, free of trimethylsilyl fluoride, by concentration of the THF solution. 

Whereas keto and enol tautomers can be isolated only in the crystalline form and when in solution or melted pass rapidly one into the other, 2-hydroxy-1-phenyl-1-propanone CH3CH(OH)COC6H5 and l-hydroxy-l-phenyl-2-pro-panone CH3COCH(OH)C6H5 have not merely been isolated in pure form but also their different chemical properties have been demonstrated. Their interconversion is catalysed by alkali. 

A.ii. Diastereoselection in the Aldol Condensation. In addition to the alkylation reaction, enolates react with other carbonyl compounds to give aldol or Claisen products, as discussed in previous sections. An aldol condensation with the enolate of 1-phenyl-1-propanone and benzaldehyde generates two new stereocenters and gives two racemic diastereomers (four stereoisomers). These two diastereomers are the racemic anti diastereomer (340 and 343) and the racemic syn diastereomer (341 and 342). Diastereoselectivity in this reaction is dependent on the reaction conditions and the enolate and aldehyde partners, and this section will explore the origins of that diastereoselection. 

The procedure reported here provides a convenient method for the a-hydroxylation of ketones which form enolates under the reaction conditions. The reaction has been applied successfully to a series of para-substituted acetophenones, 1-phenyl-1-propanone, 3-pentanone, cyclopentanone, cyclohexanone, cycloheptanone, cyclododecannne, 2-methyl cyclohexanone, 2-... 

This approach is based on the kinetic of the protonation step indeed, the chiral enol-enolate hybrid should react faster than the uncomplexed enolate as well as faster than the achiral enolate complex, resulting from the complexa-tion of the achiral proton source with the Li enolate. To address these difficulties, Fehr and co-workers described an elegant and efficient catalytic enantioselective protonation using 20 mol% of (—)-//-E and 0.85 equiv of phenyl-2-propanone 23. This process affords the corresponding saturated (S)-a-damascone (5)-24) with 94% yield and excellent enantioselectivity (94% ee). It should be noted that this catalytic process might also be used for the enantioselective protonation of the key thioester (50-20, which could be obtained with 98% ee using 50 mol% of ( )-//-E. 

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