Trishomoaromatic Cations

Trishomoaromatic Cations. Following Winstein s proposal1058 1061 of the formation of a trishomoaromatic cation in the solvolysis of s-bicyclo[3.1.0] hexyl tosylate, extensive effort was directed toward its generation under stable ion 

The ethano-bridged analog 611 was also prepared from the 8-chloroticyclo [3.2.1,02,4]octane precursor.1063 Ion 611 closely resembles the trishomoaromatic ion 610 in its spectral properties. 

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. 

DI values of cation 199 and its all-silicon analog SieH/ calculated by Esteves and coworkers are almost identical [DI = 0.47 among C(l)-C(3)-C(5), DI = 0.45 among Si(l)-Si(3)-Si(5)]. The electron delocalization of cation 199 and its silicon analog is, consequently, highly similar with less uniform distribution of the charge density in the latter. 

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. 

Two analogs of 199, the ions 202 and 203 with additional scaffolding, which favors stability, have been reported and both resemble the parent ion 

Olah et al. have calculated the structure of the hitherto unknown 

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Monohomoaromatic cations 279 Bishomoaromatic cations 286 Trishomoaromatic cations 290 Other cationic systems 292... 

Sekiguchi and coworkers have prepared and characterized the bishomocy-clopropenylium ion 242 and the trishomoaromatic cation cluster 243 Cation 242 shows the characteristic deshielded Si NMR resonances (S Si... 

The parent trishomocyclopropenyl cation [47], first proposed by Winstein et al. (1959), has been invoked as an intermediate in various solvolysis studies, observed under stable ion conditions by NMR spectroscopy, and studied theoretically (see Story and Clark, 1972 Paquette, 1978). Similarly, the related ions [48]—[51] have all been advanced as trishomoaromatics (see Story and Clark, 1972 Paquette, 1978). 

The trishomoaromatic 1,3-dehydro-5-adamantyl cation [52] has been invoked to explain the extremely high reactivity of the bromide [53] (Scott and Pincock, 1973). Recent calculations suggest that [52] does indeed enjoy homoaromatic stabilization to the extent of 25.4 kcal mol1 relative to the 1-adamantyl cation and 1,3-dehydroadamantane (Bremer et al., 1987). 

Olah et al. (1979) reinvestigated the preparation of [47] (and some alkyl and aryl substituted analogues) from the superacid treatment of alcohol [54] or chloride [55], In contrast with earlier studies, the alcohol [54] clearly gave the desired trishomocyclopropenyl cation [47] and not the previously observed cyclopentyl ion [56], Olah et al. (1979) attributed this difference to the highly purified (freshly distilled) SbF5 used in their experiments. These new data and the 13C NMR spectrum of [47] provide strong support for its classification as a trishomoaromatic. 

Coates cation of pentacyclo[A.7i.0.Q. 0 ]non-9-yUum cation The title ion [31] has unique structural and symmetry properties and shows multiple degeneracy. Through the bridge-flip mechanism (262) all nine carbons in this trishomoaromatic ion become equivalent. 

Bu3EGe]3+[Bp4] (E = Si, Ge), are not technically organometallic compounds, as no Ge-C bonds are present however, this remarkable discovery is included here because of the significance of this chemistry and its close relationship to the chemistry discussed in Section 5.3. These compounds can be further converted to a cationic trishomoaromatic cluster with a Geio core. ... 

A) is rather too great for trishomoaromatic stabilization by n-n bonding overlap to be a real physical possibility. Semi-empirical MINDO/3 calculations have been employed to optimize the structures of the triquinacen-l-yl and -10-yl cations, and reveal the former to be about 23 kcal mol more stable, with Eg = 9 kcal mol for its transformation into the C-10 cation. The open-shell version of MINDO/3 shows the same stability sequence for the analogous radicals, the C-1 radical being about... 

Monohomoaromatic Bishomoaromatic Trishomoaromatic Fig. 4.3 Cationic, anionic, and neutral homoaromatic molecules... 

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