Diels-Alder Reaction (pericyclic reaction)

Diels-Alder Reaction
(pericyclic reaction)
H
H
C
C
CH 2
CHCH
CH 2
CH 2
O
100 o
O
CH
1,3-butadiene
the diene
propenal
the dienophile
3-cyclohexene-1-carboxaldehyde
(100 %)

Additional unsaturation enhances the reactivity of
the dienophile’s C=C by electron-withdrawal
O
+
CN
heat
O
CN
(39 %)
O
O
+ O
heat
O
O
(100 %)
O

Stereochemistry
OHC
OHC
H
H
heat
H
H
CHO
CHO
a cis-dienophile
a cis-product

cis or syn-addition
bridge
endo (favored)
OHC
H
H
H
heat
OHC
H
H
H
trans to bridge
exo
H
H
CHO
H
heat
H
H
H
CHO
cis to bridge

O
+ CH 2 =CHCOCH 3
cyclopentadiene
methyl acrylate
O
C
H
OCH 3
+
H
C
O
OCH 3
endo isomer
(75 %)
exo isomer
( 25 %)
1) both isomers are of almost equal stability
2) kinetically controlled preference for
formation of endo product
Alder rule - accumulation of maximum unsaturation

O
furan
O
NPh
NPh
+ +
H
O
O
H
O
exo
O
O
endo
3
H
O
O
H
NPh
N-phenylmaleimide
7-Oxabicyclo[2.2.1]hept-5-ene-
2,3-dicarboxy-N-phenylimide
1) Synthesis
Separate endo & exo isomers
using column chromatography
TLC to monitor separation
Obtain 1 H NMR spectra of endo & exo isomers
2) Follow reaction by obtaining 1 H NMR spectra
of reaction mixture over a period of 4 weeks
at either room temperature or 0 o C

7
Hb
3
2
1
4
6
5
Hx
Hx
norbornene
bicycle[2.2.l]heptene)
Hb
Hn
Hn
stereochemistry can be deduced experimentally
the coupling constants between the bridgehead
protons on C-1 and C-4 (H b )
and the exo (H x ) protons or endo (H n ) protons
on C-5 and C-6

H
O
7-oxabicyclo[2.2.l]heptene ring system
H
H
H
O
O
NPh
H
O
H
O
H
H
O
NPh
Karplus equation: J = 8.5cos 2 θ -0.5cosθ -0.028
θ = the dihedral angle

J (Hz)
10
9
8
7
6
5
4
3
2
1
0
0 30 60 90 120 150 180
Dihedral angle (θ)