Is cyclopropane really the σ-aromatic paradigm?

Abstract

Dewar proposed the a-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1980. While a detailed, but indirect energetic evaluation in 1986 raised doubts-"There is no need to involve 'a-aromaticity',"-other analyses, also indirect, resulted in wide-ranging estimates of the a-aromatic stabilization energy. Moreover, the aromatic character of "in-plane", "double", and cyclically delocalized a-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A 2007, 111, 8163-8169) challenged the existence of an induced a-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the a-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the a-aromatic stabilization energy of cyclopropane is, at most, 3.5 kcalmol-1 relative to propane, and is close to zero when nbutane is used as reference. Trisilacyclopropane also has very little a-aromatic stabilization, compared to Si3H8 (6.3 kcalmol-1) and Si4H10 (4.2 kcal mol -1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.