Fernández Israel, Dyker C Adam, DeHope Alan, Donnadieu Bruno, Frenking Gernot, Bertrand Guy
Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.
J Am Chem Soc. 2009 Aug 26;131(33):11875-81. doi: 10.1021/ja903396e.
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-1,2-diium salt 3g, which has also been crystallographically characterized.
小环丙二烯通常是高张力且高反应活性的物种，长期以来仅被视为瞬态中间体。最近五元杂环丙二烯1f的分离引发了关于其稳定性影响因素的质疑和争论。由于1f是由吡唑鎓离子2f去质子化得到的，有人认为1f的稳定性源于其芳香性。在此我们报告计算证据，包括HOMA和NICS芳香性指数，表明由3,5 - 双（π - 供体）取代的吡唑鎓盐衍生的丙二烯具有弱芳香性至非芳香性，甚至它们的吡唑鎓离子前体的芳香性也显著降低。涉及π - 供体取代基的环外离域以芳香性为代价发生，并且随着供体强度的增加而增强。这些结论的实验支持来自于通过晶体学确定的3,5 - 双（二甲氨基）吡唑鎓离子2g的结构，其显示出高度锥形化的环内氮中心但环外氮中心呈平面化，以及来自于容易的C4质子化得到稳定的吡唑 - 1,2 - 二鎓盐3g，其也已通过晶体学表征。
