Exocyclic delocalization at the expense of aromaticity in 3,5-bis(π-donor) substituted pyrazolium ions and corresponding cyclic bent allenes

Israel Fernández, C. Adam Dyker, Alan DeHope, Bruno Donnadieu, Gernot Frenking, Guy Bertrand

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106 引用 (Scopus)

摘要

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(π-donor) substituted pyrazolium salts are weakly aromatic to nonaromatic, and that even their pyrazolium ion precursors have dramatically reduced aromaticity. Exocyclic delocalization, involving the π-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-1,2-diium salt 3g, which has also been crystallographically characterized.

源语言英语
页(从-至)11875-11881
页数7
期刊Journal of the American Chemical Society
131
33
DOI
出版状态已出版 - 26 8月 2009
已对外发布

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