Aromaticity of Substituted Cyclopropenes: A Theoretical Study Peeter Burk,* ,²,‡ Jose-Luis M. Abboud, ‡ and Ilmar A. Koppel ² Institute of Chemical Physics, Tartu UniVersity, Jakobi 2, EE2400, Tartu, Estonia, and Instituto de Quimica Fisica “Rocasolano”, C.S.I.C., c/Serrano, 119, E-28006, Madrid, Spain ReceiVed: NoVember 7, 1995; In Final Form: February 5, 1996 X The aromaticity of several heterosubstituted cyclopropenes and corresponding protonated forms was investigated using ab initio calculations at the G2-MP2 level of theory, topological charge density, and NBO analysis. It was shown that according to geometric, energetic, charge density, and magnetic criteria these systems are remarkably aromatic, especially the protonated forms. It was shown that the aromatic delocalization of double bond in the cyclopropene ring can be well modeled using a three-center two-electron bond in the framework of NBO analysis proposed by Reed and Weinhold. Introduction Aromaticity is one of the most important concepts for the understanding of organic reactivity. 1 However, this concept has proved difficult to define and to express quantitatively. 2,3 In recent years many reports have appeared, where new criteria based on geometry, 4 electron structure, 3,6 magnetic properties, 6 IR spectra, 7 and different energetic parameters, 8 for establishing the aromaticity of different compounds have been proposed and also the usefulness of these different criteria has been dis- cussed. 2,3 The whole concept of aromaticity has recently been reviewed in a series of interesting theoretical studies. Shaik et al. have demonstrated 9 that the electron delocalization in several six- and three-membered rings is a by-product phenomenon. This concept is in accord with the work of Malar and Jug 10 on antiaromaticity and was later confirmed by Jug et al. 11 To rationalize the properties of some small rings, the concept of σ-aromaticity was proposed. 12 First suggestions about the possible aromaticity of methyl- enecyclopropene and cyclopropenone date back to Manatt and Roberts’ 13 calculations of their delocalization energies and Breslow et al.’s 14 synthesis of diphenylcyclopropenone. The stability of these systems can be thought to arise from the participation of resonance structures a, b, and c, Scheme 1, which formally contain a three-membered ring of sp 2 carbons with two π electrons. In the last 35 years extensive theoretical and experimental studies of methylenecyclopropene and cyclopropenone have been reported, but no definite conclusion has been reached concerning their aromaticity. Cyclopropenone is usually found to be aromatic or moderately aromatic on the grounds of its thermal stability, 14-19 low pK b value, 17 large dipole moment (4.39 D), 20 low CdO stretch (1640 cm -1 ), 15 large charge buildup on the O, as seen by 17 O NMR spectroscopy, 21 and the molecular geometry (elongated CdC and CdO bonds, contracted C-C bonds 20,22 ). Calculations 18,22-26 have suggested large resonance energies, charge concentration on oxygen, and geometries similar to experimental ones. On the other hand, very small magnetic susceptibility anisotropy 20 suggests little aromaticity. When the electron distribution was used as the criterion for aromaticity, the results were contradic- tory: some authors 24,27 suggest that cyclopropenone is not aromatic, while others 25,28 conclude that it possesses at least some aromatic character. Tobey 1 has argued in favor of a somewhat intermediate model. Less work has been reported concerning the aromaticity of methylenecyclopropene. The geometry, determined by micro- wave spectroscopy, 29 indicates short C-C bond lengths and a very large dipole moment (1.90 D). Calculations 21,22,26,29,30 suggest relatively small resonance energy. Nevertheless, meth- ylenecyclopropene has been deemed by various authors as aromatic, 30,31 nonaromatic, 26,29,32 or even antiaromatic. 33 Staley and co-workers have recently reported ab initio and microwave spectroscopy studies of methylenecyclopropene 29 and cyclopropenone, 22 addressing the problem of the aromaticity. They suggested three criteria for estimation of the degree of aromaticity: the amount of π-electron density at C 2 , the length of the C-C single bond compared to that in the corresponding cyclopropane derivative, and the resonance energy. All these criteria correspond to the “classical” aromaticity as determined by Katritzky et al. 2 and require reference to some arbitrary model. For π-electron density and the C-C bond lengths, the comparison is made to cyclopropenium cation as aromatic reference and to the corresponding cyclopropane derivative as nonaromatic reference. Staley et al. 22,29 and others 18,26 have em- ployed the isodesmic reaction (1) from Scheme 2 to approxi- mate the resonance stabilization of cyclopropenone and meth- ylenecyclopropene, respectively. There have been some criti- cisms regarding the use of this reaction, 34 as it does not conserve the groups as defined by Benson 35 and therefore contains also the energy of rehybridization. Staley et al. 29 found methylene- cyclopropene to possess 15% aromaticity using their first two aromaticity criteria (22%, when using calculated structures) and ² Tartu University. ‡ Instituto de Quimica Fisica “Rocasolano”. X Abstract published in AdVance ACS Abstracts, April 1, 1996. SCHEME 1 SCHEME 2 6992 J. Phys. Chem. 1996, 100, 6992-6997 0022-3654/96/20100-6992$12.00/0 © 1996 American Chemical Society