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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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About the molecule that you remember cited in your textbook, there's a study in ref.1. According to these authors:

enter image description here

The delocalization of the C1-C2 and Cl-C3 electrons of spiro[2.4]hepta-4,6-diene (1a) and partial rehybridization at C1, C2, and C3 result in a contributing structure which is perhaps best represented as a $\pi$ complex (lb) of cyclopentadienylidene and ethylene.

From NMR studies they concluded that:

The comparison of the predicted downfield shift of approximately 6.6 ppm for the complex and the observed shift of approximately 1.0 ppm leads to the the conclusion that the $\pi$ complex may contribute as much as 15 % to the total electronic structure of 1.

The most recent reference I have found is from 1997(ref.2). They studied these structures: enter image description here

In the series of the investigated compounds 1-5, spi- ro[2.4]hepta-4,6-diene (1) shows the strongest influence of cyclopropyl conjugation on molecular structure in the group of doubly unsaturated cyclopropyl hydrocarbons

The conjugation properties of cyclopropane are subject of debate and other references don't support this. You can find a blog about this on the royal society of chemistry website(http://my.rsc.org/blogs/84/1009).

I have found another paper in ref.3 and according to this:

In one chemical attempt to test the transmission of conjugation by cyclopropane rings, Cannon, et al.were unable to alkylate or acylate the methyl group in diethyl 2-methylcyclopropane-l, 1-dicarboxylate and concluded that transmission of conjugative effects in the transition state for carbanion formation are small compared with analogous, unsaturated compounds.

Also:

Trachtenberg and Odian concluded from their review of the various conflicting reports that a cyclopropane ring is incapable of transmitting conjugation in the ground state but may be able to do so in some excited states. It is likely that, during chemical reactions, such transmission occurs only in a transition state, i.e., after bond breaking of the ring has commenced.

From what I have read, even assuming a conjugate effect, this would be much less evident than a C-C double bond.

About your questions:
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 π-character and can, therefore, act as an alkene?
Partially, the effect is not comparable to a true double bond. This hypothesized conjugation effect was found to be stronger in piro[2.4]hepta‐4,6‐diene(1) compared to the other molecules studied in ref.2(molecules 2 to 6 in the second picture).

1a. Are electronic structures 1 and 1a; 2 and 2a; 3 and 3a similar, and to what extent?
Considering that the supposed conjugation contribution of the cyclopropane ring is much less than a C-C double bond, I think they differ substantially. Unfortunately, I can't find any spectra or other additional data supporting this.

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 system?
The Huckel rule is not valid in these complex cases. The Hückel's rule is based on calculations using the Hückel method and therefore has a limited validity. Here you need advanced MO calculations and as you have seen from all the references mentioned in such systems there's a lot of debate.

References:
1) Aromaticity via cyclopropyl conjugation. Electronic structure of spiro[2.4]hepta-4,6-diene, J. Am. Chem. Soc.197092154736-4738 https://pubs.acs.org/doi/abs/10.1021/ja00718a047

2) Structural Aspects of Cyclopropyl Conjugation: Experimental Studies and Ab Initio Calculations. https://doi.org/10.1002/jlac.199719971007

3)Transmission of conjugation by the cyclopropane ring, Stewart JM, Pagenkopf GK, The Journal of Organic Chemistry, 1969. https://pubs.acs.org/doi/pdf/10.1021/jo00838a003?rand=vimssm6d

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  • $\begingroup$ The paper from 1969 is an interesting find, however, it seems like authors give an overview of their observations and rather speculative results even for their own ether systems. I'm afraid neither question 1 nor question 2 is really answered here (not definitively, at least). I haven't really dive into the reference mentioned in the RSC Blog, but it also seems to be tangentially relevant here. Please correct me if I'm wrong, or feel free to expand your answer so that it's less broad and targets the systems mentioned in the question. $\endgroup$ – andselisk Aug 18 at 21:09
  • $\begingroup$ I have edited it. Unfortunately, I couldn't find more recent studies. your molecule 2 was mentioned in ref.2 $\endgroup$ – blu potatos Aug 18 at 22:36
  • $\begingroup$ Before going to your specific analogies, it's important to understand if the conjugation properties of the cyclopropane are a real thing, just the memory of a textbook that cite an example is not enough. About your analogies there isn't any study available. Finding an complete and exhaustive answer from the literature data available so far is impossible. I think it was a nice question that makes people reflecting about this topic, but the main question should be: Is really cyclopropane conjugated with neighboring C=C bonds? $\endgroup$ – blu potatos Aug 19 at 9:25

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