Homoaromatic anions

Grutzner and Winstein (1968) proposed one of the more interesting homoaromatic anionic systems — the bicycloaromatic system [147]. Their evidence was equivalences in the lH NMR spectrum that could not be separated over a temperature range of -35°C to 100°C. If this molecule is undergoing a rapid conversion, then the barrier is less than 11.8 kcal mol-1. 

Anionic systems represent a problematical area with respect to homoaromaticity. Williams and Kurtz in their review summarize the position as there being no anions which are currently recognized as being homoaromatic21. In our view, the situation is not as simple as this and there well may be examples of homoaromatic anions. Certainly, this is an area of considerable scrutiny at present and the issues are far from being fully settled. 

Williams and Kurtz, in their 1994 review, found no evidence to support the unequivocal assignment of homoaromaticity to an anionic system. Childs, Cremer, and Elia considered that there may be examples of anionic homoaromatics. They favored the bicyclo[3.2.1]octadienyl anion 130 as the most likely contender for homoaromaticity (130c). Indeed, 130 has been the best studied of the potential homoaromatic anions, and interest in and controversy over this system continues today (vide infra). Other intriguing molecules, rich in heteroatoms, have recently and convincingly been advanced as candidates for anionic homoaromaticity. 

Two-/r-electron (n = 0) bis-homoaromatic compounds, such as bis(homotriborirane) anions 68, were... 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

A recent very good review of many potential anionic homoaromatic species was given by Balaban et al. (1987). Unlike the cationic systems discussed previously, where the homoaromaticity of several species has been firmly established, no anionic systems are currently recognized as being homoaromatic. 

By analogy with the homoaromatic homotropylium cation, a prototype anionic system would be the homocyclopentadienyl system [138a]. However, Olah et al. (1978) demonstrated by H and 13C NMR that this species is not the homoaromatic [138a] but exists in the form of the planar [138b]. Theoretical MINDO/3 (Olah et al., 1978) and STO-3G RHF (Birch et al., 1980) results are in agreement with the experimental results. 

Early PMO theory by Haddon (1975) predicted that the homophenalenyl anions [143], [144] and [145] should be good candidates for anionic homoaromaticity. Murata and Nakasuji (1980) attempted to synthesize these systems. They were unable to obtain any anions similar to [143] and [144]. However, NMR analysis of the potential product [145] indicated that anion [146] was formed instead. 

The first derivative of carba-nido-tetraboranes(7), 16a, was prepared by reaction of anionic 17 with iodomethane and characterized by NMR spectroscopy and by model computations (Scheme 3.2-10) [28]. The structurally analyzed 16b is obtained by deuteration of the dianion 10a2 [20] mentioned in Section 3.2.2.2. The results of an X-ray structural analysis of its dilithium salt are discussed in Section 3.2.8.3. The lithium cations are coordinated side-on to the B-B 2c2e bonds just as predicted for the aromatic U2B3H3 [6]. Obviously, (Li+)210a2 is a 2e homoaromatic. Since the positions of the lithium cations resemble those of the deuter-... 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

Thiin dioxides form anions which appear to have some homoaromatic stability. The anions react with aldehydes (Scheme 54). 

We start with an examination of some examples of acyclic systems in which there is evidence or the possibility of cyclopropyl homoconjugation. We then move on to a broader examination of homoaromatic systems, treating cationic, neutral and anionic systems in separate sections. The results of experimental work and theoretical examinations are integrated so as to provide a cohesive overview of each system. In order to limit the size of the chapter, we refrain from reviewing in detail systems such as the bridged annulenes and radical species. The chapter concludes with a reflective section that seeks to draw together theory with experiment and point out new directions for future work. 

Homoaromatic delocalization in 133 was initially invoked in order to account for its enhanced stability and NMR properties273. However, this explanation was challenged in 1981 by two different groups. On the basis of calculations Grutzner and Jorgensen24 (MINDO/3) as well as Mayr, Schleyer and colleagues273 (MNDO and STO-3G) concluded independently that the properties of 133 could be accounted without resort to homoaromatic delocalization. Moreover, they also stated more generally that homoaromatic stabilization was not expected to be an important phenomenon in anions. 

There are other anions for which the claim of homoaromatic delocalization has been made. Work on these systems is relatively old and has been reviewed extensively14 21. Overall, it is not clear there are any good examples of anions which are homoaromatic. Perhaps, with futher work, 133 will be demonstrated to be an example however, it is clear that homoaromatic delocalization is not generally going to be an important phenomenon in carbanions. 

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

---