N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
175
5.2 Alkenes
175
Coupling Over More than Three Bonds in Alkenes (Long-Range
Coupling, J in Hz)
Allylic Coupling
cisoid:
4 J ab -3.0 to +2.0
transoid: 4 J ac -3.5 to +2.5
In acyclic systems, the coupling constants range from -0.8 to -1.8 Hz and, usually,
|J| cisoid is larger than |J| transoid . The magnitude of the coupling constants depends
on the conformation. Largest absolute values are observed if the C–H bond of the
substituents overlaps with the π electrons of the double bond (φ = 0 or 180 o ):
φ
4 J ab
4 J ac
0 o
-3.0
-3.5
90 o
+1.8
+2.2
180 o
-3.0
-3.5
270 o
0.0
0.8
Homoallylic Coupling
cisoid:
| 5 J ab | 0–3
transoid: | 5 J ac | 0–3
The values of homoallylic coupling constants between methyl groups and of allylic
ones are comparable:
5 J H 3 C–C=C–CH 3
≈ 4 J H–C=C–CH 3
In acyclic systems, |J cisoid | < |J transoid | usually holds. Large homoallylic coupling
constants are occasionally observed in cyclic systems with fixed conformation
between the protons:
5 J ab 5–11
4 J ab 0–7
X
H
H
R
R
H
H
H
C
H
H
H
φ
b
b
a
a
a
b
a
b
c
c
X: CH, N
R: any substituent
X: O, NH
R: any substituent
X
X
H
R
H
R