Homoaromatic cations bishomoaromatic

For some homoaromatic carbocations the NICS values and chemical shifts have been calculated.105,106 IGLO-HF and GIAO-MP2 calculated 13C NMR chemical shifts for bishomoaromatic 7-norbornenyl 90 and 7-norbornadienyl cation 91 have also been reported.107... 

There is, no doubt, a decrease in the homoaromatic stabilization when conjugation is interrupted in two (bishomoaromatic) or more places (Paquette et al., 1977a Paquette, 1978). In spite of this, several bishomoaromatic cations have been well characterized. The simplest bishomoaromatic the 4-cyclopentyl cation [29] is, as yet, unknown (see Olah et al., 1972, and references cited therein). The cation [29] was not detected by NMR studies, nor was it invoked as an intermediate in the solvolysis of appropriately substituted cyclopentenes. 

The magnitude of homoaromatic stabilization is expected to decrease with increasing interruption by methylene groups of the otherwise tr-conjugated framework in neutral molecules. However, in an ionic species there is additional incentive for charge delocalization. Two of the most widely studied bishomoaromatic cations are the 7-norbornenyl (167) and 7-norbornadienyl (168) cations. ... 

The inability to form bishomoaromatic ions from cyclopentenyl derivatives is primarily due to steric reasons. The planar cyclopentene skeleton has to bend into an unfavorable chair conformation to achieve any significant overlap between the empty p-orbital and the k lobe of the olefinic bond. Such conformations, however, do exist in cations 167 and 168, resulting in homoaromatic stabilization. 

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

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