properties of , -unsaturated aldehydes and ketones 18-8 conjugated unsaturated aldehydes and...
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, -Unsaturated aldehydes and ketones undergo the reactions typical of their component functional groups. The conjugated carbonyl group of , -unsaturated aldehydes and ketones can undergo reactions involving the entire functional system by: Acid-catalyzed mechanisms Radical mechanisms Nucleophilic addition mechanismsTRANSCRIPT
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Properties of ,-Unsaturated Aldehydes and Ketones
18-8
Conjugated unsaturated aldehydes and ketones are more stable than their unconjugated isomers.
Enones, or ,-unsaturated carbonyl groups, are stabilized by resonance.
As a result, acids or bases catalyze a rearrangement of ,-unsaturated carbonyl compounds to their conjugated --isomers.
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,-Unsaturated aldehydes and ketones undergo the reactions typical of their component functional groups.
The conjugated carbonyl group of ,-unsaturated aldehydes and ketones can undergo reactions involving the entire functional system by:
Acid-catalyzed mechanisms
Radical mechanisms
Nucleophilic addition mechanisms
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Conjugate Additions to ,-Unsaturated Aldehydes and Ketones
18-9
The entire conjugated system takes part in 1,4-additions.
Addition reactions involving only one of the two bonds are called 1,2-additions.
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Several reagents add to the conjugated system in a 1,4-manner. This is called conjugate addition.
The nucleophilic part of the reagent attaches to the -carbon and the electrophilic part (proton) attaches to the carbonyl oxygen.
When A is H, the initial product is an enol, which then tautomerizes to its keto form.
The end result then appears to be a 1,2-addition.
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Oxygen and nitrogen nucleophiles undergo conjugate additions.Conjugate additions of water, alcohols, amines and similar nucleophiles undergo 1,4 additions:
These reactions are generally faster and result in higher yields when a base is used as the catalyst.
These processes are readily reversed at elevated temperatures.
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1,4-products (carbonyl compounds) usually form rather than 1,2-products (hydrates, hemiacetals and hemiaminals) because they are more stable.
Exceptions include amine derivatives for which 1,2-addition results in an insoluble product (hydroxylamine, semicarbazide or the hydrazines).
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Hydrogen cyanide also undergoes conjugate addition.A conjugated aldehyde or ketone may react with cyanide in the presence of acid.
The reaction proceeds through a 1,4-addition pathway. • Protonation of the oxygen• Nucleophilic attack• Enol-keto tautomerization
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1,2- and 1,4-Additions of Organometallic Reagents
18-10
Organometallic reagents may attack the ,-unsaturated carbonyl function in either 1,2- or 1,4-fashion.
Organolithium reagents react almost exclusively by attacking the carbonyl carbon.
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Grignard reagents with ,-unsaturated aldehydes and ketones may give 1,2-addition, 1,4-addition or both, depending upon the particular substrates and conditions.
Organocuprates are much more specific, undergoing primarily 1,4-addition reactions.
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The first isolable intermediate in a copper-mediated 1,4-addition reaction is an enolate ion. This is trapped by the alkylating species.
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Conjugate Additions of Enolate Ions: Michael Addition and Robinson Annulation
18-11
Enolate ions undergo conjugate additions to ,-unsaturated aldehydes and ketones in a reaction called the Michael addition.
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With some Michael acceptors, the products of the initial addition are capable of a second intramolecular aldol condensation reaction, resulting in ring formation.
This sequence of Michael addition followed by intramolecular aldol condensation is called a Robinson annulation.