Ring assemblies

How shall we call the structure (a)?

(a)

biphenyl (trivial) 1,1′-biphenyl (ring assembly, PIN) phenylbenzene (substitutive)

We can name it substitutively, i.e. substituting one hydrogen atom in the parent hydride benzene with phenyl group: phenylbenzene. This name, however, does not reflect the obvious symmetry of the molecule.

Similar story with (b) whose substitutive name, cyclopentylidenecyclopentane, is barely pronounceable.

(b)

1,1′-bi(cyclopentylidene) (ring assembly) cyclopentylidenecyclopentane (substitutive)

The structures (a) and (b) are examples of ring assemblies, that is, systems where two ring components with no atoms in common are directly connected by a single [as in (a)] or a double [as in (b)] covalent bond^*.

Ring assemblies consisting of identical ring components can be named by combining the name of the component with a multiplier. For example, (a) contains two phenyl groups and known as 1,1′-biphenyl; (b) contains two cyclopentylidene groups and named 1,1′-bi(cyclopentylidene). Now ‘1,1′’ bit appears to be redundant since there is only one way of connecting two phenyl (or two cyclopentylidene) groups by a direct bond. However, we do need locants in most of ring assemblies to avoid ambiguity. Compare the structures (c) and (d):

(c)	(d)

2,2′-bipyridine (ring assembly, PIN) 2,2′-bipyridyl (ring assembly) 4,4′-bipyridine (ring assembly, PIN) 4,4′-bipyridyl (ring assembly)

Do these names remind you multiplicative names? This is because they are built almost exactly like multiplicative names, except there are no linkers. Oh, and instead of Greek-derived multipliers ‘di’, ‘tri’, ‘tetra’, etc., the Latin-derived multipliers are used:

2	bi
3	ter
4	quater
5	quinque
6	sexi
7	septi
8	octi
9	novi
10	deci
11	undeci
12	dodeci

You may have noticed that ring assemblies named in this fashion can contain either unchanged parent hydride name, as in 2,2′-bipyridine, or a substituent group name, as in 2,2′-bipyridyl. The former method is preferred (i.e. used to create PINs) in most ring assembies; the later is used for assembiles of benzene rings, as in (a), and for rings linked by a double bond, as in (b) [1]. On the other hand, CAS always names the ring assemblies linked by a double bond substitutively [2].

As soon as there are three or more ring components in an assembly, the names start to lose elegance.

(e)	(f)	(g)

2,3′:4′,2′′-terthiophene (ring assembly) 12,23:24,32-terthiophene (ring assembly, PIN) benzerythrene (trivial) 1,1′:4′,1′′:4′′,1′′′-quaterphenyl (ring assembly) 11,21:24,31:34,41-quaterphenyl (ring assembly, PIN) 4,4′-diphenyl-1,1′-biphenyl (ring assembly + substitutive) 4,4′-bi-1,1′-biphenyl (ring assembly + substitutive) 2,2′:6′,2′′:6′′,2′′′:6′′′,2′′′′:6′′′′,2′′′′′-sexipyridine (ring assembly) 12,22:26,32:36,42:46,52:56,62-sexipyridine (ring assembly, PIN)

The locants in 2,3′:4′,2′′-terthiophene (e) simply indicate that the atom 2 in the first thiophene ring is linked to the atom 3 in the second thiophene ring, and the atom 4 in the second thiophene ring is linked to the atom 2 in the third thiophene ring. The name of (f), 1,1′:4′,1′′:4′′,1′′′-quaterphenyl, follows the same logic. One may wonder if you can give it a shorter systematic name. For example, if we substitute each phenyl ring of one 1,1′-biphenyl by a phenyl group at position 4, we get 4,4′-diphenyl-1,1′-biphenyl. Alternatively, we can join two 1,1′-biphenyl molecules through the position 4 to get 4,4′-bi-1,1′-biphenyl. This is the shortest systematic name for (f) I can think of, yet I never seen it in any publication (until I put it in this post, that is). The problem is, IUPAC does not allow the ring assembly names, such as ‘1,1′-biphenyl’, as parent hydrides.

The systematic name of (g), 2,2′:6′,2′′:6′′,2′′′:6′′′,2′′′′:6′′′′,2′′′′′-sexipyridine, is taking the primed locant system to the extreme. I mean, the Romans knew better than to use five ‘I’s for ‘five’. So in the New Blue Book IUPAC came up with a different system which is used to generate PINs for ring assemblies [1]. The rings are numbered sequentially (1, 2, 3, etc.) and the attachment points are indicated by superscript locants. Thus (e) is named 1²,2³:2⁴,3²-terthiophene, (f) 1¹,2¹:2⁴,3¹:3⁴,4¹-quaterphenyl, and (g) 1²,2²:2⁶,3²:3⁶,4²:4⁶,5²:5⁶,6²-sexipyridine.

The same basic approach can be used to name ring assemblies of identical polycyclic components:

(h)	(i)

1H,1′H-3,3′-biindole-5,5′,6,6′-tetrol (ring assembly + substitutive) 5,5′:15′,5′′-terporphyrin (ring assembly) 15,25:215,35-terporphyrin (ring assembly, PIN)

Assemblies of non-identical ring components are named using simple substitutive nomenclature.

(j)	(k)	(l)

3-phenylfuran (substitutive) 2,5-diphenylfuran (substitutive) apholate (trivial) 2,2,4,4,6,6-hexakis(aziridin-1-yl)-1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine (H-W + substitutive)

*	I see no reason why the IUPAC definition can’t be extended to include a triple bond linking cyclic components.

References

1. Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
2. Bünzli-Trepp, U. Systematic Nomenclature of Organic, Organometallic and Coordination Chemistry. EPFL Press, 2007, pp. 141—144.