Homoaromaticity is a special case of aromaticity in which π-conjugation is interrupted by a single sp3 hybridized carbon atom (it is sometimes referred to as a suspended π-bond with no underlying σ-foundation). But consider the carbene shown below. This example comes from a recently published article[1] which was highlighted on Steve Bachrach’s blog. Here aromaticity has resulted from a slightly different phenomenon, whereby a 4π-electron planar (and hence nominally anti-aromatic) molecule is elevated to aromatic peerage by promoting the two carbene σ-electrons to have π-status.
References
- B. Chen, A.Y. Rogachev, D.A. Hrovat, R. Hoffmann, and W.T. Borden, "How to Make the σ<sup>0</sup>π<sup>2</sup> Singlet the Ground State of Carbenes", Journal of the American Chemical Society, vol. 135, pp. 13954-13964, 2013. https://doi.org/10.1021/ja407116e