Conjugate Nucleophilic Addition to a,3-Unsaturated Aldehydes and Ketones

A comparison of direct (1,2) and conjugate (1,4) nucleophiKc addition reactions. Direct addition 

The conjugate addition of a nucleophile to an a, -unsaturated ketone-or aldehyde is due to the same electronic factors that are responsible for direct addition. We ve seen that carbonyl groups are polarized so that the carbonyl carbon is positive, and we can even draw a dipolar resonance structure to underscore the point  

When we draw a similar resonance structure for an ,/3-unsaturated carbonyl compound, however, the positive charge is allylic and can be shared by the jS carbon. In other words, the /3 carbon of an ,j8-unsaturated carbonyl compound is an electrophilic site and can react with nucleophiles. A comparison of electrostatic potential maps of ethylene with an a,/3-unsaturated ketone shows that the double-bond carbon atoms of the unsaturated ketone are more positive (more green) than those of the isolated alkene ethylene. 

CHAPTER 19 Aldehydes and Ketones Nucleophilic Addition Reactions 

Primary and secondary amines add to a,j8-unsaturated aldehydes and ketones to yield jS-amino aldehydes and ketones. Reaction occurs rapidly under mild conditions, and yields are good. Note that the conjugate addition product is often obtained to the complete exclusion of the direct addition product. 

CHAPTER 14 ALDEHYDES AND KETONES NUCLEOPHILIC ADDITION REACTIONS 

Water can add reversibly to a,)8-unsaturated aldehydes and ketones to yield j8-hydroxy aldehydes and ketones, although the position of the equilibrium generally favors unsaturated reactant rather than saturated adduct. Related additions to a,j8-unsaturated carboxylic acids occur in numerous biological pathways, such as the citric acid cycle of food metabolism where cis-aconitate is converted into isocitrate by conjugate addition of water to a double bond. 

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