Electron delocalization ketones

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

This electron delocalization stabilizes a conjugated system Under conditions cho sen to bring about their mterconversion the equilibrium between a p 7 unsaturated ketone and an a p unsaturated analog favors the conjugated isomer... 

In the case of 18, the various C—C bond distances in the basal seven-membered ring are consistent with there being a significant degree of cyclic electron delocalization and the ion being classified as homoaromatic. The C(1),C(7) bond order of 18 was estimated to be 0.56 on the basis of the measured internuclear distance91. It is interesting to note that the bond distances found for 18 are substantially different from those reported for the proto-nated cyclopropyl ketones discussed above. 

The liquid ketone exists 85% in the enol form and is moderately acidic. The Ka in water is = 10 9. The enol form is stabilized significantly by both electron delocalization and hydrogen bonding. The amount of enol present at equilibrium depends on the solvent, and is smallest in hydrogen-bonding solvents and largest in nonpolar solvents such as carbon tetrachloride. 

Resonance is more important in a,j8-unsaturated ketones. Conjugation of the carbonyl group with the carbon-carbon double bond increases opportunities for electron delocalization. 

The oxyallyl cation with its two electrons delocalized over the allylic system would add to furan in a [2 + 4] cycloaddition to give a new cation stabilized by the oxyanion or, in more familiar guise, a ketone. The reaction was supposed to go like this. 

This heteroannular dienone system causes rings A and B to assume a half-chair conformation. Carbon atoms 1, 3, 4, 5, 6, 8, 9, 10, 11 are all in one plane while carbon atom 2 projects above and carbon atom 7 below the plane. The rest of the molecule is below the plane. Addition of the second unsaturation to the A -3-ketone enone system provides an additional electron-rich unsaturated residue, i.e., it enhances the electron delocalization. The result will be an enlarged contribution of ionic resonance structures such as... 

The facile reduction of the -COOH group by BHj THF or BH3 SMej has been employed for chemoselective reductions of the carboxyl group in the presence of ester or lactone functionalities using a stoichiometric quantity of the borane. The carbonyl group in triacylboranes resembles the reactivity of an aldehyde or a ketone more than of an ester (ester resonance) due to electron delocalization from the acyl oxygen into the p orbital of boron. 

In the fluorenyl radical [130], the shielding of the 9-carbon is obviously not so effective as in HO—PTM- (p. 364), so allowing immediate hydrogen abstraction from HI. The isomeric ketone [132] is not reduced at all, this being explained by the loss of n-electron delocalization between the quino-methane moiety and the fluorene system that would occur in the reduction of cation [133] caused by twisting of the p-hydroxytetrachlorophenyl group around its bond to C(9) of the fluorene system (steric inhibition of resonance) (177) (p. 384). 

Isolated carbon-carbon double bonds are not normally reduced by dissolving metal reducing agents. Reduction is possible when the double bond is conjugated, because the intermediate anion can be stabilized by electron delocalization. The best reagent is a solution of an alkali metal in liquid ammonia, with or without addition of an alcohol - the so-called Birch reduction conditions. Under these conditions conjugated alkenes, a,p-unsaturated ketones and even aromatic rings can be reduced to dihydro derivatives. 

Two factors—one an electron-withdrawing inductive effect, the other electron delocalization—combine to make an H—C—C=0 unit of an aldehyde, ketone, or ester relatively acidic compared with most other C—H bonds. The inductive effect of the carbonyl group increases the positive character of the a hydrogen, and resonance stabilizes the conjugate base. 

The decreased acidity of ester a protons compared with those of aldehydes and ketones reflects the decreased electron-withdrawing ability of an ester carbonyl. Electron delocalization of the type ... 

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