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σ-Aromaticity about cyclopropene

σ-Aromaticity about cyclopropene. Dewar firstly deduced in 1979, that cyclopropene should have σ-Aromaticity with the aromaticity energy to cyclopropane (55.1kcal/mol) as in Table 1.

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σ-Aromaticity about cyclopropene

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  1. σ-Aromaticity about cyclopropene Dewar firstly deduced in 1979, that cyclopropene should have σ-Aromaticity with the aromaticity energy to cyclopropane (55.1kcal/mol) as in Table 1. σ-Conjugation and σ-Aromaticity M. J. Dewar, Bull. Soc. Chim. Belg. 1979, 88, 957-967

  2. aEclipsing strain energy (kcal/mol). b Angle strain energy (kcal/mol). c σ-Aromatic energy (kcal/mol).

  3. Then, in 1996, Peeter Burk and his group calculated the 1HNMR of cyclopropene, obtainted the chemical shift 7.9 (δ, in ppm), which is higher than 7, indicating aromatic character. He explained this phenomenon as a result of in-plane delocalization of electrons in σ-bonds (σ-aromaticity). Table 2. Calculated Magnetic Susceptibilities, Components of Magnetic Susceptibility Tensor, Magnetic Susceptibility, and 1H Chemical Shifts, respectively in (ppm cgs) and (in ppm). Aromaticity of Substituted Cyclopropenes: A Theoretical Study J. Phys. Chem. 1996, 100, 6992-6997

  4. However, in 2005, Schleyer denied the downfield proton chemical shift for a suitable aromaticity criteria. In the paper, he calculated various δ values of different compounds compared them with NICS values, they are not always consistent with each other. Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators Org. Lett., Vol. 7, No. 8, 2005 Figure1. Computed isotropic NMR δ1H values referenced to the benzene proton chemical shift (taken to be δ=7.3). The magnitude of the out-of-plane component of the shielding tensor, σⅡ(π)is given in brackets. NICS(0) values are in ring centers.

  5. He also concluded that cyclopropane and cyclopropene (Table 3.) cannot be characterized as aromatic, either by the 1HMR of the sp3 hybridized CH groups (δ=0-0.7 ppm) or by the π-electron contributions to the out-of-plane component of the shielding tensor [ σⅡ(π)]. Table 3. Magnetic Properties of Monocyclic Hydrocarbons, The Protons of Neither the σ-Aromatic Cyclopropane nor the σ-Antiaromatic Cyclobutane Are Shifted Downfield

  6. In 2010, Jemmis calculated the isomerization reaction 1, isomerization energy endothermic by 12.6 kcal/mol 1(the corresponding energy from experimental enthalpy of formation is 10.3 kcal/mol)2，indicating nonaromaticity.. ΔH =+12.6 kcal/mol 1.Are Metallocene-Acetylene (M=Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes? Organometallics 2010, 29, 76–81. 2. NIST Chemistry WebBook (http://webbook.nist.gov/chemistry/).

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