Cyclopropenyl cations stability

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

Simple HUckel calculations predict the resonance energy of the benzocyclopropenyl cation (273) (3.65 P) to be higher than that of the tropylium ion (274) (2.99 P) and that of the cyclopropenyl cation (275) (2.00 P). Recent ab initio calculations at the 3.2 IG level attribute the same stability to 273 and 275 with respect to their hydrocarbon precursors The enthalpy change in going from 1 to... 

The stability of these cations is enhanced by the amino substituent, which is also known to increase considerably the stability of the cyclopropenyl cation [118]. The 1.69 and 1.74 A PC bond length of 13 (calculated) and 12 (measured), respectively [118], are clearly longer than the 1.63 A value of the neutral 27/-phos-phirene [119], whereas the CN bond length is clearly in the CN double-bond range. Thus, for description of the bonding in 12 and 13 the b-type mesomeric structure was preferred [118],... 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

The special effect of cyclopropyl is noteworthy since the tricyclopropyl-cyclopropenyl cation, the most stable of all the hydrocarbon cyclopropenyl cations, is nearly as stable as cations stabilized by complexation with organometallic reagents. The triferrocenyl and tri(3-guaiazulenyl)cyclopropenyl ions can be cited as examples since these species have P r+ values greater than 10. Other metal complexed cyclopropenyl species have also been reported " " ... 

Cyclopropenone (27) and its derivatives are expected to share some of the aromatic stability of the cyclopropenyl cations, to which they are related by structure 27a. It is not... 

Stability of methylenecyclopropene (stable below — 75°C) stands in contrast to cyclobutadiene, an isomer, which appears to dimerize in a diffusion controlled process The triapentafulvenes or calicenes " constitute another type of methylenecyclopropene which gives some indication of dipolar character and thus a degree of aromaticity. These compounds link the positively charged cyclopropenyl cation to a cyclopentadienyl... 

Another consequence of the donor/acceptor properties of the lithium atom is the dramatic stabilization by 37 kcal/mol (155 kJ/mol) of the cu-planar dilithiomethane 3b (cis) compared with the rra s-planar structure 3b (trans). In contrast to 3b (trans) the lithium atoms in 3b (cis) c n interact electronically to form together with the p orbital at the carbon atom a (4n + 2)-Huckel system (n = 0) isoelectronically to the cyclopropenyl cation 6 ... 

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares with 29.1 kcal/mol for benzene by the same approach. " The gas phase heterolytic bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene and 268 kcal/mol for the 1-propyl cation from propane. It is clear that the cyclopropenyl cation is highly stabilized. 

Although there are several locations in this molecule that are reasonable sites for protonation, one in particular, the Lewis basic carbonyl oxygen, is unusual Upon protonation, a resonance-stabilized cation is formed, and one of the resonance forms can be recognized as a substituted cyclopropenyl cation, an aromatic species (Problem 59). In fact, with the additional phenyl substitution, this species is remarkably stable. 

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. 

Because charged ring systems can satisfy the Hiickel criteria for aromaticity, they are highly stabilized compared to other nonaromatic cations or anions. Examples include the cyclopropenyl cation, cycloheptatrienyl cation, and cyclopenta-dienyl anion. 

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.

In some cases, a resonance structure is required to see an aromatic system. The increased stability associated with an aromatic system is found for the structure, although the compounds do not appear aromatic unless the resonance structure is considered. Azulene, which can be drawn as a cyclopentadienyl anion fused to a cycloheptatriene cation, and cyclopropenone, which can be written as possessing a cyclopropenyl cation, are two examples (see margin). 

One can compare the reactivity of cyclopropenyl cation to allyl cation as a means of estimating the stability imparted by aromaticity. In the following equilibrium, it... 

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