Transcript alpha-beta unsaturated carbonyls

α,β-Unsaturated Carbonyl Compounds
C C C O
 
conjugated system
CH2=CHCH=O
acrolein
CH3
H2C C COOH
methacrylic acid
C C CH=O
H H
cinnamaldehyde
CH2=CHCOOH
H2C CHC N
acrylic acid
acrylonitrile
O
CH3CH=CHCH=O
crotonaldehyde
mesityl oxide
O
C C C
H H
benzalacetophenone
chalcone
H
H
C
C
O
C OH
C OH
O
maleic acid
H
C
C
O
C OH
HO C
H
O
fumaric acid
O
O
O
maleic anhydride
α,β-Unsaturated Carbonyl Compounds, reactions:
1. electrophilic addition
deactivated
2. nucleophilic addition
activated
3. Michael Addition
4. Diels-Alder Reaction
1. electrophilic addition
The carbonyl group is an electron withdrawing group
when conjugated with a double bond. This decreases the
electron density and deactivates the double bond to
electrophilic addition.
C C C O + HZ
C C C O
Z H
HCl
CH2CH2CHO
Cl
H2C C C O
H H
O
CH3OH
H2SO4
H2C C COOH
H
H2O,H+
O
OCH3
H2
H2C C COOH
OH
electrophilic addition mechanism:
C C C O + H
H
C C C OH
C C C OH
more stable
C C C O
H
C C C O
H
electrophilic addition mechanism:
C C C O + H
C C C OH
C C C OH
Z
C C C OH
Z
C C C OH
Z
unstable
enol
C C C O
Z H
2. nucleophilic addition
The carbonyl group is an electron withdrawing group
when conjugated with a double bond. This decreases the
electron density and activates the double bond to
nucleophilic addition.
C C C O + Z
H2O
C C C O
Z H
C C COOH
H H
O
NH2OH
CH3NH2
CHCH2COOH
NHOH
O
NHCH3
NaCN,aq.
H3C C C COOEt
H H
H H2
H3C C C COOEt
CN
nucleophilic addition mechanism:
C C C O + Z
C C C O
Z
Z
C C C O
Z
C C C O
Z
more stable
nucleophilic addition mechanism:
C C C O
C C C O
Z
C C C O
Z
Z
H
C C C O
Z H
keto-enol tautomers
C C C OH
Z
3. Michael Addition.
Carbanions as the nucleophiles in nucleophilic addition
to α,β-usaturated carbonyls. (The enolate anion must be
in reasonably high concentration for the Michael
Addition to take place. Such enolates can be obtained
from removal of alpha-hydrogens that are next to two
electron withdrawing groups.)
O
C C C
H H
benzalacetophenone
+
COOEt
CH2
COOEt
piperidine
diethylamalonate
O
H H2
C C C
CH
EtOOC
COOEt
CH3
H2C C COOEt
+
O
H
C C C CH3
H
COOEt
CH2
C N
+
COOEt
CH2
C CH3
O
OEt
base
CH3
H2C C COOEt
H
CH
N C
COOEt
O
H H2
C C C CH3
CH
H3C C
COOEt
O
4. Diels-Alder reaction: diene + dienophile  cyclohexene
G
G
+
mechanism: (concerted)
G
G
O
CH=O
+
1,3-butadiene
acrolein
O
+
1,3-butadiene
O
O
maleic anhydride
O
O
O
O
O
+
1,3-butadiene
O
O
p-benzoquinone
O
O
O
O
+
O
O
O
O
O
O
exo
O
endo
endo is preferred over the exo product
any unsaturated groups in the dienophile
tend to lie near the developing double bond
in the diene rather than farther away (exo).
α,β-Unsaturated Carbonyl Compounds, reactions:
1. electrophilic addition
deactivated
2. nucleophilic addition
C C C O
C C C O
Z H
+ HZ
activated
3. Michael Addition
O
C
4. Diels-Alder Reaction
+
O
C