Semibullvalene, homoaromaticity

This is perhaps the area where there is the most optimism of attaining the elusive goal of neutral homoaromaticity. It has been suggested that semibullvalene [83], which undergoes degenerate Cope rearrangements through a homoaromatic transition state [83b] (2) with extremely low... 

The general conclusion from the foregoing is that the semibullvalene nucleus provides the most promising candidate for neutral homoaromaticity. 

Earlier dynamic NMR studies had already demonstrated that the substituted semibullvalenes [87], [88] and [89] were indeed rapidly equilibrating Cope systems and not ground-state homoaromatics (Anet and Schenck, 1970 Moriarty et al., 1972 Russell et al., 1973). The bis(trifluoromethyl)semi-bullvalenes [90] and [91] had also been formulated as non-equilibrating Cope systems (Liu, 1968 Liu and Krespan, 1969). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

A selective and brief survey of some of the more recent studies follows. Askani et al. (1984), using isotope perturbation methods (Siehl, 1987), showed that both the dimethyl [92] and the tetracarbomethoxy [93] semibullvalenes are Cope systems. In a related study, mild perturbation of the symmetrical semibullvalenes [94] and [95] is achieved by replacing one of the methyl groups with an ethyl group [94a] and [95a]. Again these systems were shown to be rapidly equilibrating tautomers and not neutral homoaromatics (Gompper et al., 1985). 

Medium ring annelations across the 2,8-positions of semibullvalene have been extensively studied by the Paquette group (see Paquette and Burson, 1978, and references cited therein). These annelations result in the favouring of one tautomer or the other but do not lead to a homoaromatic species. The previously described elassovalenes [82] (page 298) may also be... 

Following the calculations (Williams and Kurtz, 1988) which led to the prediction that the bisannelated semibullvalene [108] would be homoaromatic, the Mullen group prepared the first (and, to date, only) example of a bisannelated semibullvalene ([109]) (Kohnz et al., 1989). The Cope rearrangement in [109] is extremely facile the estimated upper limit for the free energy of activation is 3.6 kcal mol-1. 

Numerous semiempirical studies have been made of semibullvalene and a wide range of derivatives (see Williams and Kurtz, 1988 Dewar and Jie, 1988, and references cited therein). Agreement with experimental data, where available, is excellent. Several target molecules are predicted to exhibit the elusive goal of neutral homoaromaticity which provides an enticing challenge for synthetic efforts. 

The primary piece of information obtained from most theoretical calculations is the molecular structure. If homoaromatic interactions are important in a molecule, they may cause the molecule to adopt an unusual geometry. In suitable radicals, ESR evidence has been taken to indicate systems of high symmetry which in turn has been interpreted in terms of homoaromatic interactions (Dai et al., 1990). A computational example of this effect is shown in the semiempirical calculations of Williams and Kurtz (1988) on the bisannelated semibullvalene [108]. Here simple configuration interaction... 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

In view of the clear homoaromatic character of 120 as demonstrated by the recent work on 123-127, the question arises as to whether cyclic homoconjugation is important for the ground state of semibullvalene, 120. 

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. 

In contrast, CIDNP results indicate that the radical cations of barbaralane (157, X = C = 0) and semibullvalene (157, X = —) correspond to the elusive structure type with a single minimum [391, 424]. The spin density resides primarily on the termini (C-2,4,6,8) of the twin allyl moieties, whereas the remaining (internal) carbons of the 5 jr-electron perimeter have negative spin density. This spin density distribution reflects the coefficients of orbital 158, the HOMO of a bis-homoaromatic structure (Fig. 32) [424], More recently, ESR results have confirmed this assignment [392, 393],... 

Suitable substitution can also decrease the isomerization barrier. As predicted using essentially qualitative theory the semibullvalene system is markedly susceptible to such modulation. For example 3,7-dicyano substitution increases the barrier in 1,5-dimethylsemibullvalene from 6 to 10 kcalmol" while 2,6-dicyano substitution decreases the barrier to ca. 4 kcal mol" L (Strictly speaking, the comparison is being made between data on the isomeric dimethyldicyano semibullvalenes and that of the parent hydrocarbon. ) Recently, it has been shown that l,5-dimethyl-2,6-dicyanosemibullvalene has a thermally accessible isomer that corresponds to a delocalized, homoaromatic structure (70), i.e. the cyclopropane has disappeared It is perhaps not so obvious when a cyclopropane is a cyclopropane. 

Williams RV (2001) Semibullvalenes and related molecules ever closer approaches to neutral homoaromaticity. Eur J Org Chem 2 227-235... 

Williams RV, Gadgil VR, Chauhan K et al (1996) l,5-Dimethyl-2,4,6,8-semibullvalene tetracarboxylic Dianhydride a close approach to a neutral homoaromatic semibullvalene. J Am Chem Soc 118 4208-4209... 

In 1971, based on the theoretical study, Dewar predicted that the introduction of heteroatoms like nitrogen into the semibullvalene skeleton (e.g., 2,6-dia-zasemibuUvalene, NSBV) could further reduce or even eliminate the barrier of the Cope rearrangement and thus result in a delocalized, homoaromatic ground state [16]. However, the difficulties in the synthesis, isolation, and structural characterization of NSBV hampered chemists to find an experimental probe as real model to prove the theoretical assumption. In 1982, Mullen reported the only experimental... 

Childs. Cremer, and Elia maintain that there are several examples of neutral homoaromatics. They suggest that 1,2-dihydroborete (70) and some of its derivatives are neutral homoaromatics. This electron-deficient system is isoelectronic with the mono-homocyclopropenium cation (17). It has been investigated by both theory and experiment, and its classification as homoaromatic is justified. In addition to 70, Childs, Cremer, and Elia propose that certain semibullvalenes, barbaralanes, bridged an-nulenes, cycloheptatrienes, and norcaradienes are homoaromatic. They draw particular attention to the semibullvalenes 71-75 and consider that 74 and 75 are the best candidates for experimental work. 

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