Download Properties of , -Unsaturated Aldehydes and Ketones

18-8
Properties of ,-Unsaturated Aldehydes and
Ketones
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.
,-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
18-9
Conjugate Additions to ,-Unsaturated
Aldehydes and Ketones
The entire conjugated system takes part in 1,4-additions.
Addition reactions involving only one of the two  bonds are called 1,2-additions.
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.
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.
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).
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
18-10 1,2- and 1,4-Additions of Organometallic
Reagents
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.
Grignard reagents with ,-unsaturated aldehydes and ketones may give 1,2addition, 1,4-addition or both, depending upon the particular substrates and
conditions.
Organocuprates are much more specific, undergoing primarily 1,4-addition
reactions.
The first isolable intermediate in a copper-mediated 1,4-addition reaction is an
enolate ion. This is trapped by the alkylating species.
18-11 Conjugate Additions of Enolate Ions: Michael Addition
and Robinson Annulation
Enolate ions undergo conjugate additions to ,-unsaturated aldehydes and
ketones in a reaction called the Michael addition.
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.