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I was wondering whether allenes can form rings; geometrically speaking, this seems like it would be energetically unfavorable due to their 180 degree linear geometry.

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    $\begingroup$ Related: A monocyclic 6 carbon ring with 6 double bonds $\endgroup$
    – user7951
    Mar 7 '16 at 20:44
  • $\begingroup$ Strictly speaking, cumulenes with even numbered of C=C bonds (odd numbers of carbons) are tetrahedral, so the bond angle strain not automatically a show-stopper. That being said, these kinds of systems are not common. $\endgroup$
    – Lighthart
    Mar 7 '16 at 21:57
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    $\begingroup$ Cyclonona-1,2-diene is the smallest allene in a ring that has been isolated (it's stable at room temperature, but dimerizes at higher temperatures). Trapping experiments have suggested the possible intermediacy of cycloocta- , cyclohepta- and cyclohexa-1,2-diene. $\endgroup$
    – ron
    Mar 7 '16 at 22:22
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Yes they do. Such compounds are even found in nature, e.g. vernonallenolides.

James R. Hanson, Terpenoids and Steroids: A Review of Chemical Literature, volume 12, 1983

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  • $\begingroup$ I think OP means "Do allenes react with themselves to form rings," not "Do allenes exist within rings?" I like the post, I just don't think it answers the question's intended meaning. Actually, I may well be wrong. Maybe OP could elaborate. $\endgroup$ Mar 9 '16 at 23:01
  • $\begingroup$ Well, I guess my main question was whether rings of allenes would be stable enough to exist at all, which was answered by the examples above...However, your question is an interesting one too that I would love to know the answer to. $\endgroup$
    – John Smith
    Mar 10 '16 at 23:30
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As small as six-membered rings with allenes can exist, albeit fleetingly. Cyclohexa-1,2-diene has been known since a 1966 report by Georg Wittig (Angew. Chem. Int. Ed. 1966, 5 (9), 846), in which it is prepared by direct elimination of HBr from a vinyl bromide using t-BuOK:

Cyclohexa-1,2-diene reactions

For a more modern take on the matter, see e.g. Nat. Chem. 2018, 10 (9), 953–960 where Neil Garg and coworkers report the generation of azacyclic six-membered cycloallenes. The allene is generated by treatment of a 1,2-silyl triflate with CsF (this strategy will be familiar from benzyne chemistry). The cycloallene intermediates are trapped in various cycloaddition reactions.

Azacyclic allenes

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