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Regarding the IUPAC naming convention for compounds in which one ring is more highly $\pi$-conjugated than the other ring but contains fewer carbon atoms:

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I can name this compound as either:

(a) 1-[cyclohexenyl]-cyclopenta-1,3-diene, giving preference to the ring with the greater number of $\pi$ bonds; or

(b) 1-[cyclopenta-1,3-dienyl]-cyclohexenyl, giving preference to the ring with the greater number of carbon atoms.

What is the correct answer?

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    $\begingroup$ Before deciding the parent structure, there are two problems. Firstly, why square brackets? You only need square brackets as a second layer if you already have parentheses. If not, you should only use parentheses. Secondly, hyphens are not used to separate substituents from the main part of the name. You don’t say 2-methyl-butane, right? $\endgroup$ – orthocresol♦ Jul 21 at 9:20
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If you use "cyclopenta-1,3-diene" in the end of the name (your first attempt), then this would be the stem system. In case like the one presented in the question -- two rings connected via a single bond -- it is recommended to recognize the cyclohexene as the stem (like parent), and the cyclopentadienyl moiety as a substituent (like child). Because counting the number of ring atoms has a higher priority, than the number of double / triple bonds in the cycles, your second attempt is in this perspective the better one of the two.

To detail out the substituent, use normal parentheses. Square brackets would be suitable e.g., to describe annulated bicyclohexanes' bridges (e.g., decalin as bicyclo[4.4.0]decane); however, this motif is not present in your structure.

This yields the name of: 1-(cyclopenta-1,3-dien-1-yl)cyclohex-1-ene

Addition: The answer by Safdar Faisal refers to the 2013 edition of IUPAC's Blue Book (online content here). IUPAC published a brief guide about this revision (Hellwich et al. 2020) with the following flow chart relevant to the original question:

enter image description here

and specific polychromatic examples e.g., about structures with two cycles joined by a single bond:

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As SI to this publication, there equally is a four-page excerpt available, too.

Reference: Hellwich K.-H.; Hartshorn R. M.; Yerin A.; Damhus T.; Hutton A. T. Brief guide to the nomenclature of organic chemistry (IUPAC Technical Report). Pure Appl. Chem. 2020, 92, 527-539; doi 10.1515/pac-2019-0104 (open access).

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You have two choices for choosing the parent hydride here in this case.

1. Choosing the cyclohexene ring as the parent hydride

If we consider the cyclohexene ring as the parent hydride, we get the cyclopentadiene ring to be a substituent leading the name to become 1-(cyclopenta-1,3-dien-1-yl)cyclohex-1-ene

2. Choosing the cyclopentadiene as the parent hydride

If we choose the cyclopentadiene as the parent hydride, the cyclohexene ring becomes the substituent and we get the name of the compound to be 1-(cyclohex-1-en-1-yl)cyclopenta-1,3-diene

Now, what do we choose - the cyclopentadiene or the cyclohexene?

The preferred IUPAC naming

According to P-44.2.1 of the Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), we gain the rules to determine seniority in ring systems.

P-44.2.1 The senior ring or ring system:

(a) is a heterocycle;

(b) has at least one nitrogen atom;

(c) has at least one heteroatom (in the absence of nitrogen) that occurs earlier in the following sequence:

F > Cl > Br > I > O > S > Se > Te > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl;

(d) has the greater number of rings;

(e) has the greater number of skeletal atoms;

(f) has the greater number of heteroatoms of any kind;

(g) has the greater number of heteroatoms occurring earlier in the sequence:

F > Cl > Br > I > O > S > Se > Te > N > P > As > Sb > Bi > Si > Ge > Sn > Pb > B > Al > Ga > In > Tl.

Therefore, via P-44.2.1 (e), we have the cyclohexene ring as the the main parent hydride. Therefore, the name found in choice 1 is the PIN of the compound. Thus, the PIN of the compound becomes 1-(cyclopenta-1,3-dien-1-yl)cyclohex-1-ene

1-(cyclopenta-1,3-dien-1-yl)cyclohex-1-ene

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  • $\begingroup$ Because our university library did not (yet) engage in purchasing the 2013 edition of the Blue Book (neither paper, nor electronic). So it was a nice find (eventually) that there is a brief guide addressing at least some points (cf. vide infra). Do you know if the current Blue Book equally contains such flow charts and examples where color is used to highlight the structure motifs? $\endgroup$ – Buttonwood Jul 21 at 20:22
  • $\begingroup$ @Buttonwood IIRC the blue book is black and white only and it isn't actually text searchable with the pdf. So you have to do an OCR of your own if you would like to use that... $\endgroup$ – Martin - マーチン♦ Jul 21 at 20:47
  • $\begingroup$ @Martin-マーチン I thought a text of reference this relevant would retain the text layer. So the content mirror from Queen Mary University and Brief Guide mentioned below may be more accessible from the get-go. $\endgroup$ – Buttonwood Jul 21 at 21:28
  • $\begingroup$ @Martin-マーチン As it happens, an HTML version of the Blue Book was just published last month. Sadly, search engines have not gotten around to indexing it yet. $\endgroup$ – LegionMammal978 Jul 21 at 21:29
  • $\begingroup$ @Buttonwood I've found that OCR works reasonably well on the Blue Book PDF; the main annoyance is that names can pick up extraneous spaces around their punctuation, and 1 is often mistaken for l. The online HTML version is probably the better option now that it exists, though. $\endgroup$ – LegionMammal978 Jul 21 at 21:31

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