α,β-UNSATURATED ALDEHYDES AND KETONES
α,β-Unsaturated aldehydes and ketones are aldehydes and ketones whichare conjugated with a double bond.
The carbonyl oxygen of an α,β-unsaturated aldehyde or ketone is a nucle-ophilic center. The carbonyl carbon and the β-carbon are electrophilic centers. Nucleophilic addition can take place at either the carbonyl carbon (1,2-addition), or the β-carbon (1,4- or conjugate addition).
1,2-Addition to α,β-unsaturated aldehydes and ketones takes place with Grignard reagents and organolithium reagents.
1,4-Addition to α,β-unsaturated aldehydes and ketones takes place with organocuprate reagents, amines and the cyanide ion.
α,β-unsaturated ketones are reduced to allylic alcohols with lithiumaluminum hydride.
α,β-Unsaturatedaldehydes and ketones are aldehydes and ketones which are conjugated with a double bond. The α-position is defined as the carbon atom next to the carbonyl group, while the β-position is the carbon atom two bonds removed (Fig. 1).
The carbonyl group of α,β-unsaturated aldehydes and ketones consists of a nucleophilic oxygen and an electrophilic carbon. However, α,β-unsaturated aldehydes and ketones also have another electrophilic carbon – the β-carbon. This is due to the influence of the electronegative oxygen which can result in the resonance shown (Fig. 2).
Since two electrophilic centers are present, there are two places where a nucleophile can react. In both situations, an addition reaction takes place. If the nucleophile reacts with the carbonyl carbon, this is a normal nucleophilic addition to an aldehyde or ketone and is called a 1,2-nucleophilicaddition. If the nucleophile adds to theβ-carbon, this is known as a 1,4-nucleophilic addition or a conjugate addition.
The mechanism of 1,2-nucleophilic addition is the same mechanism already described. It is found that Grignard reagents and organolithium reagents will react with α,β-unsaturated aldehydes and ketones in this way and do not attack the β-position (Fig. 3).
The mechanism for 1,4-addition involves two stages (Fig. 4). In the first stage, the nucleophile uses a lone pair of electrons to form a bond to the β-carbon. At the same time, the C=C π bond breaks and both electrons are used to form a new π bond to the carbonyl carbon. This in turn forces the carbonyl π bond to break with both of the electrons involved moving onto the oxygen as a third lone pair of electrons. The resulting intermediate is an enolate ion. Aqueous acid is now added to the reaction mixture. The carbonyl π bond is reformed, which forces open the C=C π bond. These electrons are now used to form a σ bond to a proton at the α carbon.
Conjugate addition reactions can be carried out with amines, or a cyanide ion. Alkyl groups can also be added to the β-position by using organocuprate reagents. A large variety of organocuprate reagents can be prepared allow-ing the addition of primary, secondary and tertiary alkyl groups, aryl groups, and alkenyl groups.
The reduction of α,β-unsaturated ketones to allylic alcohols is best carried out using lithium aluminum hydride under carefully controlled conditions (Fig. 6). With sodium borohydride, some reduction of the alkene also takes place.
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