Homoaromaticity theoretical studies

A theoretical study on 5V-methylselenabenzene 192 and its oxygen and sulfur analogues has been carried out using ab initio calculations with the DFT method <2006M1791> (cf. Section 7.11.2, structure 3). The X-H compounds were also evaluated. These structures have 6ji electrons and are homoaromatic with ylide character. Structural parameters were also determined for these structures. The substituent on the chalcogen atom is calculated to be more stable in a pyramidal rather than a planar conformation. 

A theoretical study was carried out to evaluate some bicyclic heterocyclic systems related to the bicyclo[3.2.11-octane skeleton for evidence of neutral homoaromaticity <2005JOC1998>. Included in the study were phosphines 17 and 18. Consideration of a series of factors including NICS values, diamagnetic susceptibility exaltations, and stabilization energies led to the conclusion that, while bicyclic compound 17 was nonhomoaromatic, the analogue 18 displayed evidence of homoaromaticity. 

Recent theoretical studies have resulted in establishing appropriate criteria for homoaromaticity of carbocalions. These include aromatic ring-current shielding (ARCS), nucleus-independent chemical shifts (NICS), bond-length alternations, and NMR shielding. On the basis of such criteria, cations CgHg, C.J I],-), and were found to be the most homoaromatic. The... 

Alkorta I, Elguero J, Eckert-Maksic M et al (2004) Influence of the H/F replacement on the homoaromaticity of homotropylium ion a GIAO/DFT theoretical study. Tetrahedron 60 2259-2265... 

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... 

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... 

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.)... 

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

In many respects the homotropenylium ion can be considered to be the archetype or benzene of homoaromatic systems. It is not only one of the earliest examples of a homoaromatic system to be described, but well more than forty substituted derivatives of the homotropenylium cation have now been reported69. These substituted ions have been examined by a broad range of experimental techniques and theoretical methods. It is interesting to note that unlike the trishomocyclopropenium cation, the initial homoaromatic system to be studied, characterization of the homotropenylium ion did not rely on a... 

The concept of homoaromaticity29 1001-1006 was advanced by Winstein in 1960. It represented a challenge to experimental and theoretical chemists alike. - The question of homoaromatic overlap has been mainly studied in six-Jt-electron Hiickeloid systems,29 1001 1006 although several two-jt-electron homoaromatic systems have been discovered subsequently.230 1006 1010-1013... 

Scheme 4.17 presents a couple of other strange-looking cationic species which were discovered in studies in a related field. In connection with the problem of dodecahedrane synthesis via the isomerization of pagodane 34 (cf. data in Schemes 4.10 and 4.11), Olah s and Prinzbach s groups engaged in studies of the behavior of pagodane derivatives under superacid conditions. Their hope was to force the cationic isomerization of 34 to 3. Despite all attempts, this route was unworkable. As a reward for these apparently futile efforts they were able to observe the unexpected formation of a very stable cationic species, the pagodane dication 55 (Scheme 4.17). The pattern of its NMR spectra combined with the nature of its quenching adduct 56, and the theoretical analysis of possible alternatives, enabled the authors to ascribe to this dication the unprecedented four-center/two-electron delocalized bis-homoaromatic structure.

The question of two rings going-on one is also seminal to the study of homoaromaticity. For n odd, the set of [(CH) CH2] ions demonstrate a delicate balance between mono and bicyclic structures (see Ref 133 and numerous references cited therein to both the experimental and theoretical literature). In the case of n = 3, one can imagine a planar cyclobutenyl cation (63) and a markedly non-planar, highly puckered bicyclobutyl cation (64). Both calculational theory on the parent and experiment on derivatives show the latter geometry to be preferred. However, as in the case of the other purported bicyclobutane derivatives characterized by the 2,4-carbons trigonally coordinated that were discussed earlier in this section, formal theory shows there is no 1,3-bond. The ion is not homoaromatic and there is no cyclopropane ring. In the case of n = 5,... 

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