Ever since the term antiaromaticity was introduced by Breslow in the 1960s to describe the destabilization of cyclic conjugated systems containing 4n π-electrons, this concept was subject to intense debates. The cyclopentadienyl cation 1 in its singlet state S-1 is a prototypical antiaromatic molecule, whereas, according to Baird’s rule, the triplet state T-1 has to be considered being aromatic and not antiaromatic (Scheme 1). Recently, we developed a new approach for the synthesis of zwitterions by reacting carbenes with strong Lewis acids such as BF3. Using this method, the zwitterion S-2 containing a cyclopentadienyl cation fragment could be isolated and spectroscopically characterized in inert gas matrices at cryogenic temperatures (Scheme 2). Zwitterion S-2 shows the essential properties of a 4 π-electron antiaromatic molecule: (i) singlet ground state, (ii) strong bond alternation, (iii) large positive NICS(0) value and (iv) paratropic ring current. The IR spectrum is consistent with an allylic rather than a dienylic form of the cyclopentadienyl unit. In S-2 with a lower C2v symmetry, the structure is locked into the allylic form. Despite this distortion, the zwitterion retains a considerable amount of antiaromatic character. The chemistry of the antiaromatic zwitterion 2 and further derivatives will be discussed in the presentation.

Reaction of carbenes with Lewis acids: Isolation and spectroscopic characterization of antiaromatic molecules

256th National Meeting (2018)

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