Trishomocyclopropenium cation

In many respects the homotropenylium ion can be considered to be the archetype or benzene of homoaromatic systems. It is not only one of the earliest examples of a homoaromatic system to be described, but well more than forty substituted derivatives of the homotropenylium cation have now been reported69. These substituted ions have been examined by a broad range of experimental techniques and theoretical methods. It is interesting to note that unlike the trishomocyclopropenium cation, the initial homoaromatic system to be studied, characterization of the homotropenylium ion did not rely on a... 

The key feature of the NMR spectra of 91 is its simplicity. Thus the 13C NMR spectrum consists of only two resonances at 4.9 and 17.6 ppm, indicating either a symmetrical trishomocyclopropenium cation, 93, or rapid equilibration between three equivalent structures (Scheme 37). The positions of the 13C NMR resonances of the cation strongly suggested the formulation of its structure as the trishomocyclopropenium ion, 93210. This conclusion was reinforced by the preparation of the deuterated cation and examination of the isotopic perturbation of its 13C chemical shifts208 211, and measurement of the 13C H coupling constants209. 

Attempts to prepare methyl- and phenyl-substituted trishomocyclopropenium cations by the ionization of various czx-bicyclop.l.OJhexyl alcohols and halides have been unsuccessful and have led instead to rearranged cyclo-hexenyl (200) and cyclopentenyl (201) cations [Eqs. (5.30) and (5.31)]. Jorgensen s theoretical calculations on the CyHif potential energy surface have shown that the delocalized methyltrishomocyclopropenium cation is less stable than the corresponding classical tertiary ions. 

The trishomocyclopropenium cation 1 was one of the first species proposed and convincingly demon-... 

Prakash, Olah, and Rasul found that the trisho-mocyclopropenium cations 40 are 0.2 kcal mol more stable than the corresponding hypercoordinate square-pyramidal cation 41. From their calculated C NMR chemical shifts, they determined that these ions exist as a rapidly equilibrating mixture (40 41 = 2 1) in superacid solution. In other related hypercoordinate square-pyramidal cations, the potential trishomocyclopropenium cations were not stable species. 

The trishomocyclopropenium species 91 has been an important cation in terms of the development of the concept of homoaromaticity. It also provides an example of a different mode of cyclopropyl homoconjugation to that encountered in the systems discussed thus far. In 91, the cyclopropane is formally interacting with the remote positive charge in an edge-on manner. All of the previous examples discussed have involved a cyclopropyl carbinyl-type interaction with the conjugating group being joined to the cyclopropane. 

The trishomocyclopropenium ion (CeH/, 199) was first proposed by Winstein and coworkers as an intermediate in the solvolysis of czs-bicyclo[3.1.0] hexyl tosylate and extensive efforts were directed toward its generation under stable ion conditions. The persistent cation 199 was first prepared by Masamune et al. by the ionization of czx-bicyclo[3.1.0]hexane in superacid media and it has since been generated from the corresponding alcohol [Eq. (5.29)]. The NMR spectra of structure 199 are consistent with an ion of Csv symmetry. The three equivalent C-H groups are found at high field in the C NMR spectrum (8 C 4.9,7c h = 195.4Hz) in accordance with their hypercoordinate environments. 

The symmetrical trishomocyclopropenium ions (56) have been generated from the analogous 9-alcohols and trapped by methoxide ion to afford the 9-methyl ethers n.m.r. data appears to exclude classical structures. Reaction of (27 X = CH2), (57), and (58a) with FSO3H-SO2CIF at - 135 °C affords in each case the bishomo-cycloheptatrienyl cation (60), whereas the isomeric CioHjo hydrocarbon (59) reacted cleanly at — 80 °C to yield the hitherto unknown, yet stable ion (61). No antibicyclo-aromatic characteristics could be detected for (61) from the chemical and physical... 

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