Bishomoaromatic Cations

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 C NMR spectrum of the cation 167 shows substantial shielding of both the C(7) cationic and vinylic carbon chemical shifts at 34.0 and 125.9, respectively. A similar shielding phenomenon is observed for the ion 168. 

At -77°C the signals from protons bonded to carbons C(7), C(l), C(6), C(5), and C(4) showed broadening but the other protons, at C(2) and C(3), did not. The mechanism for the rearrangement of the protons was elucidated using several deuterium-labeled precursors. 

On the basis of several studies, including the application of the tool of increasing electron demand,ion 167 can be best described as a nonclassical symmetrical bridged 27i-bishomocyclopropenium cation as opposed to a rapidly equilibrating pair of cyclopropylmethyl cations, for example 171. The observed unusually large coupling constants at the C(7) position of 167 

DI values calculated by Esteves and coworkers for bonds involved in 3c-2ebondingof7-norbornenylcationl67[C(2)-C(7) = 0.52,C(3)-C(7) = 0.52, C(2)-C(3) = 1.34] and a bond order of 1.61 of the C(2)-C(3) bond are further pieces of evidence of strong interaction of the three carbon atoms and delocalization of the bond. DI values for the all-silicon analog SijH./ [Si(2)— Si(7) = 0.55, Si(3)-Si(7) = 0.54, Si(2)-Si(3) = 0.94] and the Laplacian of charge densities indicate more effective interaction but less uniform electronic charge distribution in this species. 

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 

To further explore the limiting structural and energetic prerequisites for the unique o-bishomoconjugation the structurally related seco-l,16-dodecahedradiene 596 and 1,16-dodecahedradiene 597 were also studied. For 596, only the corresponding bisallylic dication could be observed,1051 1052 whereas for 597 the dication could not be observed at all.1053 The o-bishomoaromatic species 599, however, could be generated via electrochemical oxidation of alkene 598.1054 

Isopagodanes 600 and 602 were also studied under the usual oxidative conditions (SbF5-S02ClF, 130°C, prolonged stirring) and both gave diamagnetic solutions as a result of two-electron oxidation.1055 The number of absorptions of the H and 13C 

Precedents of this type of bonding pattern are the assumed bicyclo[2.2.2]octane-1,4-diyl dication 606,393 which was found to be the monocation monodonor-acceptor 

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

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. 

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. 

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 are numerous reports of the direct NMR observation of the bishomocyclopropenyl cation [32 n = 2] under stable ion conditions. The first reports of the H NMR spectrum of [32 n = 2] appeared simultaneously (Brookhart et al., 1966 Richey and Lustgarten, 1966), and subsequently various accounts of the 13C NMR spectrum appeared, culminating in an extensive study by Olah and Liang (1975). The 13C data were taken as clear evidence for the bishomoaromatic nature of [32 n = 2] and to preclude the equilibrating classical ions [34] and [35] (Olah and Liang, 1975). 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

A5 C(C1) = —29.8, (A5 C(C1) = —54.5) are observed and a relatively large /(C2H) coupling constant of 165.9 Hz is detected. This counter-intuitive low-frequency shift of the C NMR resonance of Cl and C2 as well as the large scalar CH coupling constant was rationalized for similar bishomoaromatic carbon cations like the 7-norbornenyl cation, 79, by the hypercoordinated nature of the vinylic C atoms and was put forward as spectroscopic evidence for bishomoaromaticity. " ... 

The structure of the sandwiched bishomoaromatic dication was confirmed by theoretical calculations at the HF/6-31G level. The stabilization of the carbocation was shown to be mainly due to the bishomoaromatic interactions. The long-postulated longicyclic interactions do not contribute to the stability of the cation, as shown by the energies of the following isodesmic reactions... 

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... 

Eq. (3.145)].222,1043 They gave instead the cyclopentenyl cation. The lack of formation of bishomoaromatic ions from cyclopentenyl derivatives is mainly due to steric reasons. The planar cyclopentene skeleton has to bend into the chair conformation to achieve any significant overlap between the empty p orbital and the 7i-p lobe of the olefinic bond, which is sterically unfavorable. However, such conformation already exists in ions 581 and 582. 

The novel four-center, two-electron delocalized a-bishomoaromatic species 182,183,188,190a, and 192 are representatives of a new class of 27i-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 cations (Fig. 5.11,193). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 194 having a conventional p-type delocalized electron structure (Fig. 5.11). 

An n.m.r. study 243> of the bicyclo[4.3.0]nonatrienyl cation now provides the most clear-cut evidence for the existence of bishomoaromatic species. The 1,4-bishomotropylium ion 243>, 90, shows n.m.r. absorptions which are in general quite reminiscent of those observed for the monohomo-tropylium ion, 73, and the tropylium ion, 59. 

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. 

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 ... 

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... 

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