Is (cycloprop-2-en-1-ylidene)propanedinitrile ((cycloprop-2-en-1-ylidene)malononitrile) aromatic or not? If yes? How?
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At least formally, the target compound is the product of a Knoevenagel reaction of cyclopropenone with malononitrile.
For cyclopropenone, a zwitterionic mesomeric structure is conceivable. This species, a cyclopropenyl cation with a negative charge on the oxygen atom attached, is aromatic. Think that this is the little brother of tropone.
For the compound in question, charge separation is possible as well. The resulting species would again be an aromatic cyclopropenyl cation with a dicyanomethyl substituent over which the negative charge can be delocalized.
answered Nov 13, 2016 at 17:11
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$\begingroup$ Are you basing that on experience or can you back that up with some more evidence? How would a zwitterionic tautomer differ from the non-zwitterionic form in structure, and are you implying that the resonance interpretation of cyclopropenone is wrong. The NMR shifts of these protons are more deshielded than just aromatic protons (Org. Synth. 1977, 57, 41; DOI: 10.15227/orgsyn.057.0041). $\endgroup$– Martin - マーチン ♦Commented Nov 14, 2016 at 10:36
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$\begingroup$ @Martin-マーチン I do have published experimental experience on spin and charge stabilization through dicyanomethyl substituents. Apparently, the old NMR findings were confusing, but there seems to be quite some evidence from later studies by Arthur Greenberg and coworkers at NJIT, cf. Chem. Eng. News, 1983, 61 (38), 33 (DOI), J. Am. Chem. Soc., 1983, 105 (23), 6855-6858 (DOI, Chem. Rev., 1989, 89, 1225-1246 (DOI). $\endgroup$ Commented Nov 14, 2016 at 11:07
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$\begingroup$ I suggest adding those to your answer.\\ The c&en article is like a little newspaper clipping somewhat summarising the JACS `83 paper. They conclude that resonance stabilisation is present in this compound, but from my point of view they fail to prove that this is due to aromaticity. As stated there, the NMR experiment cunducted until that point showed no aromatic ring current. I personally find heats of formation are not enough (at least not conclusive enough) evidence of aromaticity for this heatedly discussed topic to be settled. $\endgroup$– Martin - マーチン ♦Commented Nov 14, 2016 at 12:46