Benzocyclobutadiene resonance forms

[olegursu]

2 resonant forms of benzocyclobutadiene are assigned different hash keys.

benzocyclobutadiene is unstable, there are stable substituted derivatives.

[caodac]

Hi Oleg, we currently do not have an general approach to handle resonance beyond the standard aromaticity model. Do you have any recommendation here?

[olegursu]

The example above can't/shouldn't be handled by aromaticity models because it is not aromatic 8 pi electrons and breaks Huckel's rule, the non-substituted compound is unstable the substituted derivatives are most likely in form A according to experimental data. Considering how likely is to find these structures in a chemical library we can either ignore or add custom rule to transform form B to A if encountered.

[caodac]

Sure, we can make an exception in this particular case but is that sufficient? What about the general case of conjugate system in larger rings (e.g., clyclooctane)?

[olegursu]

I think we should make a distinction between aromatic resonance structures which can be identified using simple rules like Hukel's (or standard aromaticity model rules) and valence isomerism/tautomerism which aren't aromatic resonance structures like cyclooctatetraene or benzocyclobutadiene, or even more interesting flux structures like bullvalene, see http://onlinelibrary.wiley.com/store/10.1002/ijch.199900017/asset/199900017_ftp.pdf?v=1&t=hsi5rqrv&s=654aa96e799d8d90723fce232ecdf695f05b624e. Valence isomerism/tautomerism is beyond simple rules level of theory, we most likely need quantum ab initio methods to even consider handling these, if course if we find a particular example of valence isomerism/tautomerism which is frequent enough we can probably write rules to handle it.