In most organic chemistry textbooks, double bond cis/trans isomerism is exemplified by alkenes. It is also observed in cycloalkenes such as cyclooctene that can exist as either cis (a) or trans (b) isomer:
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| (a) | (b) |
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To the trans isomer, there is a twist — and the pun is fully intended. Have a look at the structures (c) and (d) (or at their 3-D models here, Fig. 2 and Fig. 3, respectively).
-cyclooctene.jpg) |
-cyclooctene.jpg) |
| (c) | (d) |
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Isn’t it obvious that (c) and (d) are a pair of enantiomers? Like it is the case with axially chiral compounds, the molecules (c) and (d) do not have any chiral centre. What’s more, they also lack a chiral axis! What we have here is an example of planar chirality, which the Gold Book not-quite-defines as
Term used by some authorities to refer to stereoisomerism resulting from the arrangement of out-of-plane groups with respect to a plane (chirality plane).
“Some authorities”? It’s as if IUPAC doesn’t want to have anything to do with those anonymous authorities. Luckily, the Blue Book assumes a bit of responsibility modifying the above definition thus [1, P-92.1.2.1.3]:
Planar chirality is a term used to refer to stereoisomerism resulting from the arrangement of out of plane groups with respect to a plane (stereogenic plane).
To specify the configuration of the molecules with planar chirality, the stereodescriptors ‘Rp’ and ‘Sp’* or ‘M’ and ‘P’ are employed, with the latter pair being preferred by IUPAC. Although the Blue Book uses trans-cyclooctene to illustrate planar chirality, it does not explain very well how these descriptors are assigned to the enantiomers [1, P-93.5.1.4.1]. Let’s see if I can do it better.
I find it much easier to think of a part of cyclooctene as a helix. Focus on the chain –HC=CH–CH2–CH2–, that is, carbon atoms C-1 through C-4. The carbons C-1 and C-2 are trigonal planar (TP-3), which means that the hydrogens attached to C-1 and C-2, as well as C-3 (and also C-8) are all in the same plane. This is our chirality plane. In the structure (c), if we look at this plane from the side of the ring, we’ll see the path from C-1 to C-2 to C-3 going clockwise (↻) before plunging down to C-4. Just like a right-handed helix would do. So this structure is the ‘P’ (“plus”) isomer. Conversely, in (d) the path C-1 → C-2 → C-3 goes anticlockwise (↺), so it’s the ‘M’ (“minus”) isomer.
Still sounds too complicated? I have an easier method. Take a piece of paper (A5 will do). Hold it with two hands in front of you. Now turn either hand clockwise (↻). What you have in your hands now looks like the structure (c), that is, the ‘P’ isomer, right? Anticlockwise (↺) — ‘M’ isomer (d).
The preferred IUPAC names for (c) and (d) will be (1E,1P)-cyclooct-1-ene and (1E,1M)-cyclooct-1-ene, respectively. Do we really need locant ‘1’ if there is only one double bond and, accordingly, only one chirality plane? No we don’t: (E,P)-cyclooctene and (E,M)-cyclooctene are completely unambiguous.
Paracyclophanes are another class of structures exhibiting planar chirality. Observe the enantiomers (e) and (f):
-1.2-iodo-1,4(1,4)-dibenzenacyclohexaphane.jpg) |
-1.2-iodo-1,4(1,4)-dibenzenacyclohexaphane.jpg) |
| (e) | (f) |
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We can name them systematically using phane nomenclature. Let’s refresh how it’s done. But first, I have to clutter the diagrams some more with locants:
-1.2-iodo-1,4(1,4)-dibenzenacyclohexaphane_numbered.jpg) |
-1.2-iodo-1,4(1,4)-dibenzenacyclohexaphane_numbered.jpg) |
| (e) | (f) |
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The name is based on a “simplified skeletal name”, which is cyclohexane, modified to contain ‘phane’, i.e. cyclohexaphane. Adding the locants for superatoms 1 and 4 gives us 1,4-dicyclohexaphane. Replacing the superatoms 1 and 4 with benzene rings, we get 1,4-dibenzenacyclohexaphane. Specifying that the benzene rings are attached at positions 1 and 4, we add these locants in parentheses following the superatom locants: 1,4(1,4)-dibenzenacyclohexaphane. Substututing the first benzene ring at the second position (i.e. 12) by iodine results in 12-iodo-1,4(1,4)-dibenzenacyclohexaphane†.
This is all fine, I hear, what’s about chirality? According to the Blue Book, the configuration is indicated by the stereodescriptors ‘Rp’ and ‘Sp’ assigned to the carbon C-11 [1, P-92.1.2.1.3]. The explanation involves a tetrahedron whose base is formed by the three atoms attached to C-11. I appreciate organic chemists’ love of tetrahedra, however here it does not help much. Elsewhere, a different and slightly more transparent exposition is given for the descriptors ‘P’ and ‘M’ [1, P-92.1.2.2.2]. Yet another run-down on stereogenic planes invokes the “pilot atom”, never defined and never mentioned again [1, P-93.5.5.1]. Can’t we assign the helicity descriptors ‘P’ and ‘M’, once again, using a helix? Let me have a go.
Our chirality plane is the one of the benzene ring 1. All its carbon atoms, as well as the atoms attached to them — that is, hydrogens, iodine, and carbon C-2 — are in the same plane. We start from C-12 (and not from C-16) because it is the highest priority carbon atom according to the Cahn–Ingold–Prelog (CIP) rules. In the structure (e), the path from C-12 to C-11 to C-2 is going clockwise (↻) before heading down to C-3. Just like a right-handed helix. So (e) is the ‘P’ isomer and the complete name will be (11P)-12-iodo-1,4(1,4)-dibenzenacyclohexaphane. On the other hand (literally), in (f) the path C-12 → C-11 → C-2 goes anticlockwise (↺), so it’s the ‘M’ isomer. Easy? Easy.
Looking at the PINs, one can feel somehow uncomfortable that, in spite of not having a chiral centre, the ‘P’ and ‘M’ descriptors have to be attached to a particular atom: C-1 in (c) and (d), C-11 in (e) and (f). I don’t think there’s any pressing need to specify the locant in cyclooctene. This is not the case with paracyclophanes where each ring can have its own planar chirality, as shown in [1, P-93.5.5.1]. If I had my way, I would replace ‘(11P)’ and ‘(11M)’ with ‘(1P)’ and ‘(1M)’, respectively, for it is more logical to attach the descriptor to the whole ring 1 than just to the atom C-11.
Given that IUPAC prefers the descriptors ‘P’ and ‘M’ to ‘Ra’ and ‘Sa’ as well as to ‘Rp’ and ‘Sp’, one can feel that there is reluctance to bring the notions of axial and planar chirality into the systematic names. Constable [3] points out that the structures like biaryls can be described either as axially or planar chiral. For instance, the same ‘P’ biaryl isomer will be assigned either ‘Sa’ or ‘Rp’ descriptor.
| * | Like with ‘Ra’ and ‘Sa’, it is not clear how the descriptors ‘Rp’ and ‘Sp’ are meant to be pronounced. “R planar” and “S planar” sound more Spanish than English because the adjective “planar” follows the descriptor. On the other hand, in Spanish chemical literature you’ll find the descriptors ‘pR’ and ‘pS’ [2, p. 127]. |
| † | If you think this name is long or clumsy, check out its von Baeyer alternative: 5-iodotricyclo[8.2.2.24,7]hexadeca-1(12),4,6,10,13,15-hexaene. Of course, the atom numbering here is totally different to that in (e)/(f).
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References
- Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
- Quiroga Feijóo, M.L. Estereoquímica: conceptos y aplicaciones en química orgánica. Editorial Síntesis, Madrid, 2007.
- Constable, E.C. (2021) Through a glass darkly — Some thoughts on symmetry and chemistry. Symmetry 13, 1891.
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