Homoaromatic 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... 

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]... 

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... 

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. 

Cyclic organoboranes containing mnltiple boron atoms within the ring stracture have been extensively stndied as precursors to (multidecker) transition-metal complexes and to nonclassical organoboranes (see Boron Metallacarbabo-ranes and Boron Polyhedral Carboranes). Earlier efforts have been thoroughly reviewed. Recently, a series of triboracyclopentane derivatives were reported by Bemdt and Schleyer (Scheme 6). " Reduction of these boracy-cles gave the nonclassical bishomoaromatic anions (28) and homoaromatic dianions (29). Related trishomoaromatic dianions obtained via reduction of 1,3,5-triboracyclohexane derivatives with lithium were studied by Siebert and Schleyer. i 5> ... 

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... 

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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