Geminal (2J) coupling

For coupling to be seen, the two hydrogen atoms in question must have different chemical shifts—identical protons do not couple. For 2J, or geminal, couplings the two hydrogen atoms are on the same carbon atom, so in order to discuss geminal coupling we must first consider what leads the two hydrogens of a CH₂ group to have different shifts. To introduce the topic, an example. It may seem to you that any six-membered ring might show different chemical shifts for axial and equatorial groups. But this doesn’t happen. Consider the result of this Robinson annelation reaction.

The two methyl groups at C4 give rise to a single signal in the 13C NMR at 27.46 ppm. Even though one of them is (pseudo)axial and one (pseudo)equatorial, the molecule exists in solu tion as a rapidly equilibrating mixture of two conformations. The axial green methyl in the left-hand conformer becomes equatorial in the right-hand conformer, and vice versa for the black methyl group. The equilibrium position must be 50:50 and fast exchange averages the chemical shifts of the two methyl groups. The same is true for the CH2 groups around the back of the ring, which each appear as a triplet. However, the enone is not the only product of this reaction. A methanol adduct is also formed by Michael addition of methanol to the conjugated enone. This product has two methyl signals at 26.1 and 34.7 ppm. If we examine the molecule by conformational analysis as we did for the fi rst product we see a similar situation.

Similar but not the same. This time, the two conformations are not identical. One has the OMe group equatorial and the other has it axial. Even the two methyl groups do not entirely change places in the two conformations. True, the green methyl is axial on the left and equatorial on the right, but it has a gauche (dihedral angle 60°) relationship with the OMe group in both conformations. The black Me group is gauche to OMe on the left but anti-periplanar to the OMe group on the right. Averaging the two different conformations, in each of which the black and green methyl groups are different (that is, they don’t just change places), does not lead to equalization of the two methyl groups. Perhaps a simpler way to discover this is to use a configurational, rather than a conformational, diagram. The green methyl group is on the same face of the molecule as the MeO group, while the black methyl group is on the other face. No amount of ring flipping can make them the same. They are diastereotopic, a term we shall defi ne shortly. And so are all three CH2 groups in the ring. The green Hs are on the same face of the molecule as the MeO group while the black Hs are on the other face. A proton NMR example confirms this, and here is one from an odd source. There are fungi that live on animal dung, called coprophilous fungi. They produce antifungal compounds, presumably to fight off competition! Anyway, in 1995 two new antifungal compounds were discovered in a fungus living on lemming dung. They were named coniochaetones A and B and their structures were deduced with the usual array of mass and NMR spectra. The proton spectra, run on a 600 MHz machine, are shown below, and they reveal considerable detail. Some of the spectrum is essentially the same for the two compounds, but other parts are quite different. Coniochaetone A has a very simple spectrum, very easily assigned. Coniochaetone B is rather more interesting. The spectrum is much more complicated, even though it has only one more C–H (the grey one) than coniochaetone A. The reason is that addition of that H atom creates a stereogenic centre and makes the top and bottom faces of the molecule different. Each H in both CH₂ groups becomes differentiated from its partner.

The green Hs are coupled to each other (J = 18 Hz) and to each of the black Hs with a different coupling constant. One of the green hydrogens also shows a long-range (4J = 1.4 Hz) W-coupling to the red H. The black Hs are too complex to analyse, even at 600 MHz, but the different couplings to the red Hs are shown by the signal at 5.43 ppm.

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مجلة تسليم تدعو الباحثين والأكاديميين للمشاركة في أعدادها القادمة

المجمع العلمي يواصل إقامة محاضراته الأسبوعية في مقام الإمام المهدي (عجل الله فرجه)

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المزيد

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دراسة علمية تكشف مفاجأة حول الأسباب الحقيقية للسمنة

دراسة: الأطباء يتجاهلون "سببا شائعا" لارتفاع ضغط الدم

نظامك الغذائي غني بالبروتين؟.. احذر هذه الآثار الجانبية

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المزيد

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اختبارات ناجحة لمحرك "VK-800" الروسي وتطويره لطائرة بايكال الخفيفة

رئيس إنفيديا: الذكاء الاصطناعي الصيني "محفز للتقدم العالمي"

المزيد

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

Heteroatoms in rings have axial and equatorial lone pairs

Four-membered rings

Ring size and NMR

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