Bishomoaromatic System

As noted earlier, in exceptionally weakly nucleophilic media the NMR method is used to observe directly many nonclassical ions — intermediates postulated in explaining unusual rates, products and stereochemistry of the above solvolysis reactions. This enables research under stable-ion conditions may result in dis-coverii new, earlier unknown kinds of carbocation rearrangements illustrated by the 7-norbomenyl and 7-norbomadienyl cations. 

Judging by the chemical shifts the cations 19 and 236 are similar in stracture and are likely to have a nonclassical symmetrical stracture. Deno and Brown expressed an alternative viewpoint suggesting the rapid equilibrium of the tricycHc cations 279  

The symmetrical ion structure was supported by the PMR data of the 2-methyl-7-norbornenyl cation 426 

The assumed equilibrium 223 223a must be very much shifted towards the tertiary ion 223. In the spectra of a-cyclopropylcarbinyl ions the signals of a- and g-cyclopropane protons are in a comparatively high field (r 7-8) The signal of the proton (t 3.53) of ion 426 in by 3 ppm in a lower field than the signals of (t 6.58) and of the a-protons of cyclopropylcarbinyl ions (also by 3 ppm) on the other hand, the positions of the signals for ions 19 and 426 differ only by 0.6-0.7 ppm. These results are incompatible with the assumed equilibrium 223 223a they indicate that the electron distributions in ions 19 and 426 are very similar, in accordance with their bishomocyclopropenyl structure. 

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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. 

Third, no examples of bishomoaromatic systems with a tram orientation of the rings are known. 

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

Monohomoaromatic neutral species 296 Bishomoaromatic neutral systems 299 Trishomoaromatic neutral systems 308 Higher homoaromatic neutral systems 311 Homoaromaticity in the bridged annulenes 312 Other neutral homoaromatic systems 313... 

Presumably, in these systems the energy cost of ring puckering is greater than the stabilization due to bishomoaromaticity (Bartlett and Rice, 1963). However, if the ring is already forced into a puckered conformation (e.g. by bridging between the 3 and 5 positions) then homoaromatic stabilization becomes effective. Solvolytic studies on the bridged systems [30] and [31]... 

In 1970 Ahlberg, Harris and Winstein reported the preparation of 71 and 72, the first examples of bishomotropenylium ions167. These 1,4-bishomoaromatic cations were prepared by ionization of the corresponding barbaralyl systems as shown in Scheme 30. The formation of 71 and 72 proceeds by way of an initial barbaralyl cation, the structure and nature of which has been the subject of a considerable amount of work168-170. The initially formed unsubstituted barbaralyl cation rearranges to 71 at -125 °C. 

Both 84 and 85 have been prepared as stable ions in super-acid media and their NMR properties studied198-201. The conclusions reached from the extensive amount of work done with the parent and a variety of substituted systems is that both ions can be considered to be bishomoaromatic. In the case of 85 it should be noted that the C(7) bridge was found to lean towards and interact with one of the double bonds. Cation 85 was found to undergo an inversion process in which there is an interchange of the participating double bond (Scheme 35). Winstein and coworkers were able to place a lower limit of 19.6 kcalmol 1 on... 

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 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 C —Br bond is ionized without the participation of the C —C bond to form the classical ion 318. Thus, if the arising cation centre at in the bicyclo-[3,2,l]octane system can be stabilized by the classical p,7t-interaction, the non-classical participation does not manifest itself (cf. besides, the homobenzylic participation would result in the antiaromatic (4jt-electrons) destabilized cation 319 (cf. But in the ion 315 the arising secondary cation centre can be effectively stabilized only by the nonclassicai a- or 7t-participation formating ion 316 of the bishomoaromatic type. 

All the homoallylic ions so far discussed have been bishomoaromatic. A repre ta-tive of the system two-electron is the pentamethylcyclobutenyl cation 453 an allylic ion with unusually high 1,3-interaction. Its UV absorption maximum (245 nm) is intermediate between the allylic and cyclopropenyl cations. The calculations show the value of the 1,3-resonance integral for ion 453 to be Pjj = 0.33 Po... 

The bicyclo[3.2.1]octa-3,6-dien-2-yl anion [27] was proposed as the prototype of bishomoaromatic anions. The isotopic perturbation method has been applied to this system to obtain experimental evidence on its nature (Christl et al., 1983). 

The dication 29 could be considered a sandwich bishomoaromatic dication or a 4 jr-electron longicyclic aromatic system or as the alternative homoaromatic species 30. To resolve this problem, Olah and Prakash et al. undertook a theoretical study of these compounds and also, for comparison, monocation 31. Using HF/6-31G and B3LYP/6-31G methods, they obtained optimized structures for 29—31. There is no evidence for longicyclic stabilization in 29, and both... 

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