Ring size and NMR

we considered the effect of changes in the dihedral angle on coupling constants. But the dihedral angle is not the only angle worth measuring: we should also consider how the two C–H bonds are spread out in space. The dihedral angle is what we see when we look down the spine of the book in our earlier analogy—now we want to look at the pages in the normal way, at right angles to the spine, as if we were going to read the book. We can show what we mean by fixing the dihedral angle at 0° (the C–H bonds are in the same plane) and looking at the variation of J with the ring size of some simple cyclic alkenes.

The wider apart the hydrogens are spread, the smaller the coupling constant. Remember, the dihedral angle stays the same (0°)—we are just varying the angle in the plane. A dramatic illustration of this comes with the product of dehydrogenation of the natural product guaiol with elemental sulfur. From the brown, smelly reaction mixture, guaiazulene, a deep blue oil, can be distilled.

Some assignments are clear. The 6H doublet and the 1H septuplet are the isopropyl group, and the two 3H singlets belong to the two methyl groups—we can’t really say which belongs to which. The 1H singlet must be the green hydrogen as it has no neighbours and that leaves us with two coupled pairs of protons. One pair has J = 4 Hz and the other J = 11 Hz. From the values above, we expect to find larger coupling where the H–C–C–H angle is smaller, so we can say that the 4 Hz coupling is between the pair on the five-membered ring and the 11 Hz coupling is between the pair on the seven-membered ring. When protons on a double bond in a ring have neighbours on saturated carbon, the coupling constants are all small and for the same reason—the angles in the plane of the ring are approaching 90° even though the dihedral angles are 45–60° in the examples on the left. A bizarre result of this is that the 3J coupling between the red and black hydrogens is often about

the same as the allylic (4J) coupling between the red and the green hydrogens. An example follows in a moment. The ‘spreading out’ effect also affects vicinal (3J) couplings in simple saturated rings. No other ring size has so well defined a conformation as that of the six-membered ring, but we can still note useful trends as we move from 6 to 5 to 4 to 3. Briefly, in five-membered rings, cis and trans couplings are about the same. In four- and three-membered rings, cis couplings are larger than trans. But in all cases the absolute values of J go down as the ring gets smaller and the C–H bonds are ‘spread out’ more. Indeed, you can say that all coupling constants are smaller in small rings, as we shall see. But we need to examine a few examples in a bit more detail.

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مواضيع ذات صلة

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

How to tell if protons are homotopic, enantiotopic, or diastereotopic

The size of the geminal coupling constant

Geminal (2J) coupling

Heteroatoms in rings have axial and equatorial lone pairs

Four-membered rings

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Thermodynamic control over ring size

Making heterocycles: ring-closing reactions

Related effects in other types of compounds