# How does the cyclopropyl group influence conjugation and aromaticity?

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):

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).

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

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

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

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.
• – Mithoron Aug 23 '17 at 12:29
• 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). – ron Aug 23 '17 at 18:11
• As to the "proper alignment", see here – ron Aug 23 '17 at 18:14
• Agree with ron. The analogy to compound 1 I expected was trimethylenecyclopropane. – orthocresol Jan 8 '18 at 16:21
• 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 – Zhe Jan 8 '18 at 17:02