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While answering the question Why isn't cyclobutane planar?, I recollected that cyclopropane and its derivatives behave like double-bond compounds, e.g. a cyclopropyl group can potentially be conjugated with neighboring $\ce{C=C}$ bonds and affect aromaticity, like in spiro[2.4]hepta‐4,6‐diene which has pronounced aromatic character (I remember this example from an early edition of March's Organic Chemistry):

A funny looking kite that's supposed to be an aromatic compound. Chemists . . .

Cyclopropyl group is generally known to be a poor $\pi$-electron acceptor, but a very good $\pi$-electron donor [1]. So, I'm wondering whether the cyclopropyl conjugation in general may result in altered aromatic properties, and to what extent; also, on how Hückel's $4n + 2$ rule is applicable here.

I've picked up existing cyclopropane derivatives which I think mimic electronic configuration of well-known aromatic and non-aromatic compounds, assuming each cyclopropyl group acts as a double bond:

  1. [3]Rotane $\ce{C9H12}$ [2] (1) vs. benzene (1a);
  2. Dispiro(2.2.2.2)deca-4,9-diene $\ce{C10H12}$ [3] (2) vs. cyclooctatetraene (2a);
  3. 9,10-bis(Spirocyclopropyl)-9,10-dihydroanthracene $\ce{C18H16}$ [4] (3) vs. dibenzo(a,e)cyclo-octatetraene (3a).

2D aromatic structures which could be my next avatar.

Crystal structure of (1), obviously planar triangle (1a is planar as well):

3D nature scenery

Crystal structure of (2), planar $\ce{C6}$ ring (2a is not planar):

The next generation of can openers

Crystal structure of (3), all three $\ce{C6}$ rings are planar and are within a single plane (3a is not planar):

Can opener with aromatic handles

Provided with these three examples of planar systems for illustration (you can also pick up the systems of your interest), I would like to state a couple of questions:

  1. Is there always a conjugation between a cyclopropyl group and an adjacent double bond? In other words, do cyclopropane's "bent" bonds always have extensive $\pi$-character and can therefore act as an alkene?
    1a. Are electronic structures 1 and 1a; 2 and 2a; 3 and 3a similar, and to what extent?
  2. Can we use Hückel's $4n + 2$ and $4n$ rules for cyclopropyl-containing monocyclic rings? If not, how to estimate aromaticity/anti-aromaticity in such systems?

Bibliography

  1. de Meijere, A. Angew. Chem. Int. Ed. Engl. 1979, 18 (11), 809–826. DOI 10.1002/anie.197908093.
  2. Boese, R.; Miebach, T.; De Meijere, A. J. Am. Chem. Soc. 1991, 113 (5), 1743–1748. DOI 10.1021/ja00005a043.
  3. Haumann, T.; Boese, R.; Kozhushkov, S. I.; Rauch, K.; de Meijere, A. Liebigs Ann./Recl., 1997 (10), 2047–2053. DOI 10.1002/jlac.199719971007.
  4. Rosenfeld, S.; White Tingle, C.; Jasinski, J. P.; Whittum, J. E.; Woudenberg, R. C.; Van Epp, J. J. Am. Chem. Soc. 1994, 116 (26), 12049–12050. DOI 10.1021/ja00105a054.
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    $\begingroup$ related chemistry.stackexchange.com/questions/33602/… $\endgroup$ – Mithoron Aug 23 '17 at 12:29
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    $\begingroup$ Cyclopropane often acts like a double bond in terms of stabilizing adjacent positive charge, when the 3-membered ring is properly aligned. I've never see it discussed in the sense you suggest (removing one cyclopropane $\ce{CH2}$ group and then enlarging the other ring to include the remaining $\ce{CH2}$ group). The appropriate analogy would be to replace the cyclopropane with an exo-methylene type of double bond (e.g. removing one of the cyclopropane $\ce{CH2}$ groups and creating a double bond between the remaining $\ce{CH2}$ group and the former spiro-carbon). $\endgroup$ – ron Aug 23 '17 at 18:11
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    $\begingroup$ As to the "proper alignment", see here $\endgroup$ – ron Aug 23 '17 at 18:14
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    $\begingroup$ Agree with ron. The analogy to compound 1 I expected was trimethylenecyclopropane. $\endgroup$ – orthocresol Jan 8 '18 at 16:21
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    $\begingroup$ If you go with Walsh cyclopropane, then there is a $p$-orbital right there. e.g., bluffton.edu/homepages/facstaff/bergerd/chem/walsh/derive.html $\endgroup$ – Zhe Jan 8 '18 at 17:02

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