Exocyclic Delocalization at the Expense of Aromaticity in 3,5-bis(π-Donor) Substituted Pyrazolium Ions and Corresponding Cyclic Bent Allenes

Small ring allenes are usually highly strained and highly reactive species, and for a long time considered only as transient intermediates. The recent isolation of a five membered heterocyclic allene 1f has raised questions and debate regarding the factors responsible for its stability. Since 1f has been derived by deprotonation of a pyrazolium ion 2f, it has been suggested that the stability of 1f comes from its aromatic character. Here we report computational evidence, including HOMA and NICS aromaticity indices, that allenes derived from 3,5-bis(pi-donor) substituted pyrazolium salts are weakly aromatic to nonaromatic, and that even their pyrazolium ion precursors have dramatically reduced aromaticity. Exocyclic delocalization, involving the pi-donor substituents, occurs at the expense of aromaticity and increases with the strength of the donor. Experimental support for these conclusions is found in the crystallographically determined structure of 3,5-bis(dimethlamino)pyrazolium ion 2g, which exhibits highly pyramidalized endocyclic nitrogen centers but planarized exocyclic ones, and from the facile C4 protonation to give a stable pyrazole-1,2diium salt 3g, which has also been crystallographically characterized.