Is this sydnone aromatic?

Question

Is the following molecule aromatic?

It does seem like it will be aromatic because it is similar to furan. I am confused because of the positive charge on the $\ce{N}$.

Answer 1

As correctly pointed out in the comments, the compound you present belongs to the class of sydnones. According to Hückel's rule, it should be aromatic, as the molecule is planar and contains $4n+2$ π-electrons.

However, calculations performed in 1998^[1] that mainly looked at chemical shifts and inferred aromaticity from the magnitude of observed diamagnetic shielding, show that sydnones in fact are not fully delocalized, i.e. they are not aromatic. To quote from the linked publication:

Mesoionic compounds are planar five-membered heterocyclic betaines with at least one side chain whose α-atom is also in the ring plane and with dipole moments of the order of 5 D. Electrons are delocalized over two regions separated by what are essentially single bonds. One region, which includes the α-atom of the side chain, is associated with the HOMO and negative π-charge whereas the other is associated with the LUMO and positive π-charge.

The publication is also riddled with images that depict syndones like this:

It ends with another quote from Gilchrist's book "Heterocyclic Chemistry", 2^nd edition, 1992, p. 329:

It does not therefore seem useful or appropriate to classify sydnones and other mesoionic compounds as aromatic, despite the existence of a cyclic array of p-orbitals.

Reference

1. Simas, A. M.; Miller, J.; de Athayade Filho, P. F. Are mesoionic compounds aromatic? Can. J. Chem. 1998, 76 (6), 869–872. DOI: 10.1139/v98-065.

Answer 2

One of Paul Schleyer's students, Matthew Duvall, has studied the aromaticity (or lack thereof) of mesoionic compounds, such as sydnones and münchnones, using various techniques.^[1] In his thesis, Duvall directly refutes the article linked in tschoppi's answer (emphasis mine):

In contrast to the major arguments of Simas et al. (the endocyclic C–O bond with little or no π character and the fact that mesoionic compounds had well-separated regions of negative and positive charges separated by what were essentially single bonds), sydnones and münchnones are aromatic and are comparable to the aromaticity of benzene, furan, and pyrrole by magnetic (NICS(0)_πzz) and energetic (ASE) criteria.

$$\begin{array}{cccc} \hline \text{Molecule} & \text{NICS}(0)_{\pi zz}\text{ / ppm} & \text{ASE / }\pu{kcal mol-1} & \text{Point group} \\ \hline \text{Benzene} & -36.3 & 31.0 & D_\mathrm{6h} \\ \text{Pyrrole} & -33.5 & 16.7 & C_\mathrm{2v} \\ \text{Furan} & -28.9 & 12.5 & C_\mathrm{2v} \\ \text{Sydnone} & -23.7 & 27.0 & C_\mathrm{s} \\ \text{Münchnone} & -20.6 & 28.2 & C_\mathrm{s} \\ \hline \end{array}$$

Negative nucleus-independent chemical shift (NICS)^[2] values correspond to magnetic shielding in the middle of the ring, which you would expect to see in an aromatic compound with ring currents (this is strikingly illustrated in [18]annulene). On the other hand, aromatic stabilisation energies (ASEs)^[3] refer to the difference in energy between the compound and a non-aromatic isomer, although the exact details are somewhat more involved. For example, these were the reactions used to determine the ASEs of the benzene and sydnone rings. The value for benzene must be corrected in order to account for the different number of individual butadiene units in syn or anti conformations.

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References

1. Duvall, M. B. The Aromaticity of Mesoionic Compounds: Sydnones and Münchnones. M.S. Thesis, University of Georgia, Athens, GA, 2013 (website, PDF).

2. Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Nucleus-Independent Chemical Shifts (NICS) as an Aromaticity Criterion. Chem. Rev. 2005, 105 (10), 3842–3888. DOI: 10.1021/cr030088%2B.

3. Schleyer, P. v. R.; Pühlhofer, F. Recommendations for the Evaluation of Aromatic Stabilization Energies. Org. Lett. 2002, 4 (17), 2873–2876. DOI: 10.1021/ol0261332.