Tropylium cation stability

Although this cleavage is probably driven by the stability of benzyl cation, evidence has been obtained suggesting that tropylium cation, formed by reanangement of benzyl cation, is actually the species responsible for the peak. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

The MP2/4-31G/STO-3G calculations show (83TL1863) the tropylium cation to be 112.7 kcal/mol more stable than the hypothetical bicyclo [4.1.0]hepta-2,4-dien-7-yl cation. This difference, attributed to the effects of aromatic stabilization, should be compared with the value of AE (172) - (124) = 44.8 kcal/mol (at the same computational level). Therefore, the energy of aromatic stabilization of borepin comes to about 40% of that of the tropylium cation. The 6-31G /6-31G calculated AE (172) -(124) = 37.6 kcal/mol, whereas nonaromatic cycloheptatriene is a mere... 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

In certain cases, the carbocations are so stable that their solid salts have been isolated. For example, triphenylmethyl perchlorate (2.1) exists as a red crystalline solid and tropylium bromide (2.2) has been isolated as a yellow solid. Tropylium bromide (2.2) is stabilized by aromatization as the tropylium cation is planar and has 6tt electrons like benzene. 

One of the most famous results of the simple MO approach is the prediction 24> of stability for the cyclopentadienyl anion and cyclohept-atrienylium (tropylium) cation, which has been fully confirmed by the experimental findings 161>. The Ji-electron configuration of these ions is fairly similar to that of benzene, and this explains the results obtained in the first approximation. 

HiickeFs rule also pertains to charged cyclic conjugated systems. The cyclo-propenyl (2 tt electrons), cyclopentadienyl anion (6 tt electrons), and cycloheptatrienyl (tropylium) cation (6 tt electrons) are examples of stabilized systems. We say much more about the relationship between MO configuration and aromaticity in Chapter 9. 

Cycloheptatrienyl bromide is an ionic compound because its cation is aromatic. The alkyl halide is not aromatic in the covalent form because it has an sp hybridized carbon, so it does not have an unintenupted ring of p orbital-bearing atoms. In the ionic form, however, the cycloheptatrienyl cation (also known as the tropylium cation) is aromatic because it is a planar cyclic ion, all the ring atoms are sp hybridized (which means that each ring atom has a p orbital), and it has three pairs of delocalized TT electrons. The stability associated with the aromatic cation causes the alkyl halide to exist in the ionic form. 

The pyrylium cation presents an intriguing dichotomy - it is both aromatic , and therefore, the beginning student would be tempted to understand, stable , yet it is very reactive - the tropylium cation and the cyclopentadienyl anion can also be described in this way. However, all is relative, and that pyrylium cations react rapidly with nucleophiles to produce adducts which are not aromatic, is merely an expression of their relative stability - if they were not aromatic it is doubtful whether such cations could exist at all. Pyrylium perchlorate is surprisingly stable - it does not decompose below 275 °C but, nonetheless, it will react with water, even at room temperature, producing a non-aromatic product. 

Cycloheptatriene reacts with vanadium hexacarbonyl to afford in 21% yield a dark green, diamagnetic 7r-tropylium-vanadium tricarbonyl [7t-C7H7V(CO)3] which is isoelectronic with [7r-C7H7Cr(CO)3]+ (132). Infrared and NMR spectra suggest, and an X-ray study (6) confirms, the presence of the tropylium cation symmetrically bound to the metal. The stability order (indicated qualitatively by sensitivity to air in solid state and in solution) in the isoelectronic series of aromatic metal tricarbonyls is 7r-C7H7V- (CO) < ir-C H.Cr (CO)s < Tr-CsHsMnHCO),... 

An aromatic compound with a carbon atom attached to the ring also participates in an a-cleavage. In this case the bond next to this benzylic carbon is cleaved if the ring is only monosubstituted, and the benzylic cation or its rearranged and resonance-stabilized tropylium cation (2) of miz 91 is the charged product ... 

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. 

The tropylium ion and the cyclopropenyl ions included in Table 5.1 are examples of cations stabilized by being part of a delocalized aromatic system. These ions are aromatic according to Hiickel s rule, with the cyclopropenium ion having two w electrons and the tropylium ion six. Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

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