notes-5-hmr-6-longrange-coupling
H
H
4
J = 2 to 3 Hz
H
H
Allylic 4
J = -3 to +3 Hz
H
H
Propargylic 4
J = 2 to 4 Hz Allenic 4
J = 6 to 7 Hz H
H
H
H
W-Coupling 4
J = -1 to +3 Hz
"U-Coupling"J = -1.1 Hz
"W-Coupling"J = +2 Hz
H
J = 2-3 Hz
H
H 4
J = 1.2 Hz
H
H 4
J = 7 Hz
4
J = 18 Hz H
H
4
J = 10 Hz
5
J = 0 to 8 Hz H
H
Homoallylic
Homopropargylic
5
J = 2 to 4 Hz Homoallenic
5
J = 3 to 6 Hz
H
H
H
H
O
H H
H
H
4J cis = +4.84
J trans = -2.8
The "U" couplings are very small (<0.1 Hz)
4
J ee = 0-2 Hz
H H
5.6 Long-Range (4J and higher) Proton-Proton Couplings
Proton-proton couplings over more than three bonds are usually too small to detect easily (< 1 Hz). However,there are a number of important situations where such couplings are present, and can provide useful structural information. Coupling across π-systems are the most frequently encountered 4J couplings: the meta-coupling in aromatic compounds, and the 4-bond allylic, propargylic and allenic couplings. 4-Bond couplings across saturated carbons (sp 3) or heteroatoms are rarer, and are usually seen only when there is a favorable geometric alignment along the H-C-C-C-H chain ("W-Coupling"). Longer range couplings (5J and higher) are also observed, particularly in acetylenes and allenes (Chem. Rev. 1977, 77, 599).
W-Coupling in Saturated Systems . Normally long-range couplings across saturated carbons (or O and N) are negligibly small (<1 Hz). However, if there is proper orbital alignment between C-H bonds and the intervening C-C bonds then detectable 4-bond and higher couplings can be observed. The most favorable alignment is the W
arrangement of the connecting bonds ("W-coupling"), in which the H-C-C and C-C-H fragments are close to coplanar in an anti-arrangement. Thus coupling between 1,3-equatorial protons in cyclohexanes is frequently seen.
However, couplings across U-shaped HCCCH fragments can also sometimes be detected. Long-range couplings can become quite large in rigid strained bicyclic ring systems and/or when there are multiple coupling pathways available.
? Copyright Hans J. Reich 2010All Rights Reserved University of Wisconsin
Do not expect to see such large long-range couplings
in unstrained (5,6,7-membered) rings.
Cyclobutanes generally show substantial cross-ring 4J couplings, with 4J cis , which has the proper orientation for a W-coupling, greater than 4J trans . In fact, J cis > 0 and J trans < 0 in almost all cases (A. Gamba, R. Mondelli
Tetrahedron Lett. 1971, 2133), so this coupling can be used to assign stereochemistry in cyclobutanes. The figure below illustrates the effect of the long range couplings in a cyclobutanone (a simple AB quartet would be expected if there were no long-range couplings - top simulation). An AA'BB' simulation gives the parameters shown on the figure. The pattern is not completely centrosymmetric because there is a small long-range coupling from the side-chain CH 2 (δ 1.75) to one of the cyclobutane protons.
7654
3210
ppm
640
620
600580
560
Hz
O
OMe
Decoupling at δ 1.75 symmetrizes the AA'BB" pattern (there is a small
rong-range coupling between the pentyl CH 2 group and one of the ring ptotons).
νAB = 45.20J AA' = 6.50J BB' = 5.70J AB = -18.70J AB' = -2.00
3.2 3.1
3.0 2.9 2.8
ppm
νAB = 45.20J AA' = 6.50J BB' = 5.70J AB = -18.70J AB' = -2.00
J AA' = 0J BB' = 0
J AB = -18.70J AB' = 0J AA' = 0J BB' = 0
J AB = -18.70J AB' = -2.00
Spectrum
Simulation
200 MHz 1H NMR Spectrum Source: Art Cammers/Vedejs
Simulation
Simulation
1.75
C H
H
H
H
H
C H
H
H
H
trans-Decalin
cis-Decalin
w1/2 = 0.90 Hz
(TMS: 0.38 Hz)
w1/2 = 0.39 Hz
(TMS: 0.33 Hz)
Stereochemistry of cis/trans Decalins. A useful application of long range couplings for the assignment of
ring-fusion stereochemistry in decalin ring system bearing an angular methyl group has been developed (Williamson, K. L.; Howell, T.; Spencer, T. A.
J. Am. Chem. Soc.1966, 88, 325). In the trans-decalins, there are usually several ideal W-pathways for long range coupling between the methyl group and axial protons. In
cis-decalins, there are fewer or no such pathways. Thus in a pair of cis/trans isomers, the methyl group in the trans isomer will usually be broader (or will actually show splitting), whereas the cis isomer will have a sharper (unsplit) methyl group.
W-Coupling Across Heteroatoms. In conformationally well-defined systems significant 4J couplings can be seen to OH and other XH protons. In the example below, the long-range W-coupling between the OH proton and the axial proton at C6 was used to assign configuration to the major isomer formed in the reaction. In the minor isomer the OH proton was not detectably coupled. The well-defined 3J ax-ax and 3J ax-eq at C2 in both isomers shows that the ring-flip isomer shown predominates (Bueno, A. B.; Carreno, M. C.; Ruano, J. L. G Tetrahedron Lett. 1995, 36, 3737).
d, J = 1.9
broad s
H
H
O
O
O
Tol
H O
:
AlMe
ZnBr3
96 : 4
+
6
2
O
O
i Pr H
CH3
O
S
O
4J = 1.14
O
O
i Pr H
O
O
S
CH3
In a related system, the observation of an unusually large 4J across the sulfone sulfur was interpreted in terms of the conformation shown, in which the methyl group is over the ring, rather than alternative conformations in which the sulfone oxygen is over the ring (Kaloustian, M. K.; Dennis, N.; Mager, S.; Evans, S. A.; Alcudia, F. Eliel, E. L. J. Am. Chem. Soc.1976, 98, 956-965).
Allylic Coupling . 4-Bond coupling of vinyl to allylic hydrogens is usually easily observable. We can think of the coupling as having two components, the usual W-coupling transmitted through the σ-system, which is maximized for the trans proton when the allylic C-H bond is in the plane of the vinyl C-H group ( Θ = 0 °, J > 0), and a
π-component, which is maximized when the allylic C-H bond is perpendicular to the double bond (Θ = 90 °, J < 0)(Garbisch, J. Am. Chem. Soc. 1964, 86, 5561). The positive σ-contribution added to the larger negative
π-contribution normally results in a numerically slightly smaller (negative) coupling to the trans vinyl proton, but the effect is small, and not reliable enough for the unambiguous determination of double bond stereochemistry (note the marked entry below in which J trans > J cis ) (Barfield, M.; Chakrabarti, B. Chem. Rev. 1969, 69, 757).
H H
?
?
H ?H ?
H
H
H
?
H
?H
Θ = 0°
Θ = 180°Θ = 90°4J σ =4
J π =
0.00.00.0-2.6
+1.30.04J = 1.3 cos 2Θ - 2.6 sin 2Θ (Θ = 0 - 90°)4
J = 2.6 sin 2Θ (Θ = 90 - 180°)
H
CH 3
X X = CH 3-1.25H
H
CH 3
X
-1.25X = Cl -1.3-0.7X = Ph
-1.5
-0.8
H
CH 2
H X = CH 3-1.7H
H
CH 2
H
-1.2X = Cl -1.41-0.93X = Ph -1.47-1.47X
X
X = CH=CH 2
-1.30
-1.50
X = OH -1.55-1.25
J t
J c
J t
J c
Allylic Coupling (4J )
Benzylic Coupling. Coupling between benzyl protons and ortho hydrogens on aromatic rings are often not resolved, but almost always cause significant broadening of both the aromatic and benzyl protons. The coupling is related to π-bond order, so it is usually smaller than allylic coupling.
4
J
4J
= -0.45 Hz
4
J = -1.06 Hz
4
J = -1.75 Hz
J = -1.5 Hz
Benzylic Coupling (4J )
Homoallylic Coupling. Couplings across 5 bonds are unusual, but can be seen under favorable circumstances. Optimum coupling is seen when both C-H bonds are aligned with the π-orbital of an intervening double bond (perpendicular to the plane of the double bond). Especially large long-range couplings are seen for 1,4-cyclohexadienes and related structures where there are two paths for the coupling.
?
H ?
?
?
H
?
?
?
H
?
?
?
H
?
?
Optimal alignments for 5J
H
H
5J = 2-5 Hz
C H
5J = 2.7 Hz
4J = 1.6 Hz H
H
H
H
5J
cis
= 9.63 Hz
5J
trans = 8.04 Hz
N
H
H
H
H
5J
cis
= 3.0 Hz
5J
trans
= 7.2 Hz
H
Ph3C
H
Ph
5J
cis
= 11 Hz
H
Ph3C
Ph
H
5J
trans
= 7.5 Hz
Some extreme examples of large homoallylic coupling:
Homoallylic
H
H 4J = 3 Hz
C H3C H2-R
5J = 2.5 Hz
C H3C H2-OH
9J = 0.4 Hz
H
t Bu
C
H
4J = 6.5 Hz
H2CH2CH3
5J = 3.3 Hz
C H3
C H3
H
6J = 0.4 Hz
Acetylenes and Allenes
Long range Couplings in Acetylenes and Allenes. No special structural features are required to observe 4-and 5-bond couplings across acetylenes and allenes - such couplings are usually present. Even couplings across 5, 6, and even more bonds are detected across polyacetylene or cumulene chains