Delocalization bishomoaromatic

One of the most important features of the bicyclic bishomoaromatic germylium ion 6 is the considerably shielded NMR chemical shifts of the endocyclic atoms Cl and C2, relative to those of the neutral precursor 5. Despite the partial delocalization of the positive charge over Ge3, Cl and C2, significant high-field shifts... 

Overall, it is clear that 71 can rightly be considered to be a bishomoaromatic system. However, it was pointed out by Cremer, Ahlberg and colleagues that the relatively small stabilization energy associated with 71 means that substitution could have a major effect on the relative energies of 71-D and 71-0. The classification of 71 as bishomoaromatic does not mean that derivatives of 71 will also have a comparable electron delocalization pattern and structure. 

Cremer and colleagues concluded the 76 is non- or only very weakly, bishomoaromat-ic183. However, the transition state for the valence tautomeric interconversions of 75 and 76 was found to possess all the characteristics of homoaromatic electron delocalization. 

The bishomoaromatic neutral systems are of particular interest. Evidence for the importance of neutral homoaromatic delocalization appears to exist solely with certain substituted semibullvalenes. In terms of the latter systems the best candidates for experimental work appear to be 126 and 127. 

The extent of bishomoaromatic delocalization, as expected, is critically dependent upon structural geometry. Attempts to prepare the parent bishomoaromatic 4-cyclopentenyl cation 588 from 4-halocyclopentene 587 were unsuccessful... 

The novel four-center two-electron delocalized o-bishomoaromatic species 593, 594,599,601a, and 603 are representatives of a new class of 2jt-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations985 (Figure 3.25, 604). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 605 having a conventional p-type delocalized electron structure (Figure 3.25). 

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 quantum chemical calculations for the 7-norbomadienyl ion have confirmed its unsymmetrical bishomoaromatic structure. The first calculations for this ion were made as early as 1963 but this was done by using the method LCAO MO in the n-electron approximation at the same time the nondassical electron delocalization in the 7-norbomadienyl cation does not obviously have the 7c-character, but is intermediate between jt- and a-delocalizations. More reliable, therefore, are the Jesuits obtained in 1971-1972 by the CNDO/2 and the MINDO/2 method with all the valent electrons taken into account. According to these calculations, the positve charge is mainly concentrated on C, C, but not on C and C ... 

In-plane delocalized 4C/3e radical cations have been generated by one-electron oxidation of the corresponding dienes (Figure 3). The corresponding dications are cited as a-bishomoaromatic 4C/2e dications, and the lower oxidation potential of the diene shown (0.84 V) compared to the corrsponding mo-noene (1.46 V) is taken as evidence of stabilization of the radical cation by 4C/3e in-plane delocalization. "... 

Boron carbonyl (BCO) fragment is isolobal to a CH group. This relationship predicts BCO might mimic the aromaticity of their hydrocarbon counterparts. In 2003, Schleyer et al. applied this strategy to convert delocalized structure of SBV into bishomoaromatic minima by BCO replacement at appropriate positions (Fig. 4.11). 

The structure of 2,4,6,8-tetra BCO-substituted SBV 4-20 was optimized at B3P86/ 6-311-i-G as a C2v symmetrical delocalized stmcture. The bond length of B2-C3 was optimized as 1.494 A, which was close to the B-C bond length (1.503 A) in 1,3,5-tri BCO-substituted benzene. The nucleus-independent chemical shift (NICS) of 4-20 was calculated as -16.6 and confirmed the delocalization and its neutral bishomoaromaticity. The author ascribed the stabilization of the allylic moieties in delocalized stmcture to the greater radial extension of the orbitals of the more electropositive boron, which favors bridged stmctures. However, no synthesis effort toward 4-20 has been successful [17]. 

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