Cyclopropenyl cation, and

In 1997, Jemmis, Schleyer and coworkers studied the structure and energetics of lithi-ated cyclopropenyl cation and their acyclic isomers using ab initio MO (HF/6-31G ) and density functional theory (DFT, B3LYP/6-31G ) methods . Successive lithiation results in... 

Vibrational spectroscopy was used to study positively charged nonbenzenoid aromatic systems the substituted cyclopropenyl cations and the tropylium... 

Add electrons to your energy diagram to show the configuration of the cyclopropenyl cation and the cyclopropenyl anion. Which is aromatic and which is antiaromatic ... 

Tropylium cation and the two cyclopropenyl cations cited in Table 5.2 are of interest in that their ease of formation provides experimental support for the Hiickel 4/1+2 rule of aromatic stability. Both types contain planar monocyclic systems of 5p -hybridized atoms with two ir-electrons (n = 0) in the cyclopropenyl cations and six TT-electrons n = 1) in tropylium cation. It has been suggested that the curious fact of greater stability of the tri-/i-propyl substituted cyclopropenyl cation, as measured by pJ R+, compared to its triphenyl-substituted counterpart is a reflection of the greater stabilization of the covalent alcohol precursor of the latter because of its stilbene-like structure. 

In benzene, the maximum possible stabilization is achieved (and a total 7i-energy of 8 P), since all the binding orbitals are occupied with a total of 6 rc-elec-trons. It can also be deduced from HiickeTs rule, that the cyclopropenyl cation and the cyclopentadienyl anion exhibit aromatic character this is also true for azulene, which has been found in coal tar. 

Figure 6.25. A depiction of the cyclopropenyl cation and the cyclopropenide anion. The former, with 4 -I- 2 electrons ( = 0) is particularly stabilized and is considered aromatic while the latter, with 4n electron (n = 1) is considered antiaromatic.

A. The aromatic ions cyclopropenyl cation and cyclopentadienyl anion. B. Cyclooctatetraene is nonplanar, but the dianion is planar and aromatic. C. 10 TT electron systems naphthalene, all-cis [lOJannulene, and a possibly planar [lOJannulene with severe transannular interactions. D. A bridged [lOjannulene. E. Dimethyldihydrophenanthrene. 

The G2 calculated reaction enthalpy derived by eq 4 for the cyclopropenyl radical is only 8.9 kcal/mol compared to values of 59.1 and —4.3 kcal/mol for the comparable reactions of the cyclopropenyl cation and anion (eq 5), and the much lower stabilization of the radical compared to the cation was interpreted as indicating that the cyclopropenyl radical is not aromatic. The large differences between the three... 

Figure 12.7 shows the energy levels for the stable, aromatic cyclopropenyl cation, and Figure 12.8 shows the energy levels for the unstable, antiaromaric cyclobutadiene molecule. 

Preparation of Aromatic Compounds via Dehydrogenation. Dihydroaromatic compounds are easily converted into the corresponding aromatic compound by treatment with triphenylcarbenium tetrafluoroborate followed by base. Certain a,a-disubstituted dihydroaromatics are converted to the 1,4-dialkylaromatic compounds with rearrangement (eq 1). Nonbenzenoid aromatic systems, e.g. benzazulene or dibenzosesquifulvalene, are readily prepared from their dihydro counterparts. Aromatic cations are also easily prepared by hydride abstraction, for example, tropylium ion (e.g. in the synthesis of heptalene (eq 2)), cyclopropenyl cation, and others, including heterocycUc systems. Some benzylic cations, especially ferrocenyl cations, can also be formed by either hydride abstraction or trityl addition. 

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