Bishomoaromaticity

A valence bond isomer of pentakis-(trifluoromethyl)-l, 3-diazepine (44) was prepared from (43) (81TL1113) (44) can be transformed thermally or photochemically to a 2,4-diazabicyclo(3.2.0)hepta-2,6 diene (45), which was subsequently photolysed to an imidazole in an anionic process. Compound (45) is highly acidic arising out of the bishomoaromaticity of the anion and forms a salt with Et3N (81TL1369). 

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... 

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... 

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. 

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]... 

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). 

Several interesting dications have been prepared and characterized as homoaromatic. Olah et al. (1983) prepared the bishomoaromatic/allylic dication [61], Prakash et al. (1987) studied the related bishomoaromatic... 

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. 

C. Deprotonation of Bicyclo[3.2.1]alkadiene, Some Other Cycloalkadienes and Cyclooctatetraene Bishomoaromaticity and Transannular Cyclization... 

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... 

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... 

The structures of the intriguing dications 156 and 158 were also computed by DFT calculations. The C NMR chemical shifts were also calculated using both GIAO and IGLO methods. Both dications 156 and 158 can also be characterized as 4c/2e a-bishomoaromatic rectangular cyclobutane dications as well as frozen Woodward Hoffmann transition state analogs. 

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. 

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. 

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... 

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

As is shown in Scheme 40, the activation enthalpy of the degenerate valence isomerization in 120 is only 5.2 kcalmol"1257. Recent ab initio calculations by Szabo and Cremer give values of 4.0 (MP2/DZ+P+ZPE) and 6.5 kcalmol"1 (MP4/DZ+P+ZPE) for this barrier258. Calculated geometrical, electron density and magnetic properties of the transition state clearly indicate it as being bishomoaromatic with a C(2)- C(8) distance of 2.03 A (MP2). These results confirm the earlier expectations based on MO theory and semi-empirical calculations. 

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... 

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. 

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