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

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a Eclipsing strain energy (kcal/mol).

b Angle strain energy (kcal/mol).

c σ-Aromatic energy (kcal/mol).

9.0 74.4 55.1 28.3 28.3

12.0 15.4 _ 27.4 27.4

_ 107.0 55.1 51.9 54.5

3.0 29.7 _ 32.7 30.6

conventional “Strain” Energy Calculated Observed

Table 1. Contributions to the CSE of Small Ring Compounds (kal/mol)

Compound ESEa ASEb σ-AEc

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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).

J. Phys. Chem. 1996, 100, 6992-6997

Aromaticity of Substituted Cyclopropenes: A Theoretical Study

Table 2. Calculated Magnetic Susceptibilities, Components of Magnetic Susceptibility Tensor, Magnetic Susceptibility, and 1H Chemical Shifts, respectively in (ppm cgs) and (in ppm).

χ χzz χyy χxx Δχ δ

-31. 7 -44. 16 -27. 9 -22. 9 -18. 8 7. 9

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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.

-6.2 -17.0 8.3(+6.5)

7.7(-5.6)

8.4(-5.7)7.3

(-4.0)

7.9(+8,0)

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.

Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

Org. Lett., Vol. 7, No. 8, 2005

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)].

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NICS δ1H σⅡ (π)

-42.8 0.0 0.0-29.1 0.7 0.1

7.0 0.62.6 2.0 0.00.6 5.9 0.34.6 5.5 0.2

6.1 0.4-4.1 6.5 -0.7

6.8 -1.2-8.8 7.3 -3.1

1,3-cyclohexadiene-H(2)cyclopentadiene-H(1)cyclopentadiene-H(2)

benzene

cyclopropane-H

cyclopropene-H(1)cyclobuane-H

cyclopropene-H(3)

cyclohexene-H(1)1,3-cyclohexadiene-H(1)

Table 3. Magnetic Properties of Monocyclic Hydrocarbons, The Protons of Neither the σ-Aromatic Cyclopropane nor the σ-Antiaromatic Cyclobutane Are Shifted Downfield

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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 AromaticMetallacyclopropenes? Organometallics 2010, 29, 76–81.2. NIST Chemistry WebBook

(http://webbook.nist.gov/chemistry/).