Органикалық молекулалардың гомоароматтылығы

Organic molecular structure
Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability. The concept of homoaromaticity was pioneered by Saul Winstein in 1959, prompted by his studies of the “tris homocyclopropenyl” cation. Since the publication of Winstein's paper, much research has been devoted to understanding and classifying these molecules, which represent an additional class of aromatic molecules included under the continuously broadening definition of aromaticity. To date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common. The 'homotropylium' cation (C8H9+) is perhaps the best studied example of a homoaromatic compound.

The term "homoaromaticity" derives from the structural similarity between homoaromatic compounds and the analogous homo conjugated alkenes previously observed in the literature. Winstein subsequently observed that this non classical model of the 3 bicyclo[3.1.0]hexyl cation is analogous to the previously well studied aromatic cyclopropenyl cation. Like the cyclopropenyl cation, positive charge is delocalized over three equivalent carbons containing two π electrons. This electronic configuration thus satisfies Huckel's rule (requiring 4n+2 π electrons) for aromaticity. Indeed, Winstein noticed that the only fundamental difference between this aromatic propenyl cation and his non classical hexyl cation was the fact that, in the latter ion, conjugation is interrupted by three units. The group thus proposed the name "tris homocyclopropenyl"—the tris homo counterpart to the cyclopropenyl cation.

The criterion for aromaticity has evolved as new developments and insights continue to contribute to our understanding of these remarkably stable organic molecules. The required characteristics of these molecules has thus remained the subject of some controversy. Classically, aromatic compounds were defined as planar molecules that possess a cyclically delocalized system of (4n+2)π electrons, satisfying Huckel's rule. Most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability relative to predictions based on localized resonance structures. Succinctly, three important features seem to characterize aromatic compounds:
molecular structure (i. e. coplanarity: all contributing atoms in the same plane)
molecular energetics (i. e. increased thermodynamic stability)
spectroscopic and magnetic properties (i. e. magnetic field induced ring current)

A number of exceptions to these conventional rules exist, however. Many molecules, including Möbius 4nπ electron species, pericyclic transition states, molecules in which delocalized electrons circulate in the ring plane or through σ (rather than π) bonds, many transition metal sandwich molecules, and others have been deemed aromatic though they somehow deviate from the conventional parameters for aromaticity. Consequently, the criterion for homoaromatic delocalization remains similarly ambiguous and somewhat controversial. The homotropylium cation, (C8H9+), though not the first example of a homoaromatic compound ever discovered, has proven to be the most studied of the compounds classified as homoaromatic, and is therefore often considered the classic example of homoaromaticity. By the mid 1980s, there were more than 40 reported substituted derivatives of the homotropylium cation, reflecting the importance of this ion in formulating our understanding of homoaromatic compounds. Much of the early evidence for homoaromaticity comes from observations of unusual NMR properties associated with this molecule.

While characterizing the compound resulting from deprotonation of cyclooctatriene by 1H NMR spectroscopy, the group observed that the resonance corresponding to two protons bonded to the same methylene bridge carbon exhibited an astonishing degree of separation in chemical shift. From this observation, Pettit, et al. concluded that the classical structure of the cyclooctatrienyl cation must be incorrect. Instead, the group proposed the structure of the bicyclo[5.1.0]octadienyl compound, theorizing that the cyclopropane bond located on the interior of the eight membered ring must be subject to considerable delocalization, thus explaining the dramatic difference in observed chemical shift. Upon further consideration, Pettit was inclined to represent the compound as the "homotropylium ion," which shows the "internal cyclopropane" bond totally replaced by electron delocalization. This structure shows how delocalization is cyclic and involves 6 π electrons, consistent with Huckel's rule for aromaticity. The magnetic field of the NMR could thus induce a ring current in the ion, responsible for the significant differences in resonance between the exo and endo protons of this methylene bridge. Pettit, et al. thus emphasized the remarkable similarity between this compound and the aromatic tropylium ion, describing a new "homo counterpart" to an aromatic species already known, precisely as predicted by Winstein. Subsequent NMR studies undertaken by Winstein and others sought to evaluate the properties of metal carbonyl complexes with the homotropylium ion. Comparison between a molybdenum complex and an iron complex proved particularly fruitful. Molybdenum tricarbonyl was expected to coordinate to the homotropylium cation by accepting 6 π electrons, thereby preserving the homoaromatic features of the complex. By contrast, iron tricarbonyl was expected to coordinate to the cation by accepting only 4 π electrons from the homotropylium ion, creating a complex in which the electrons of the cation are localized. Studies of these complexes by 1H NMR spectroscopy showed a large difference in chemical shift values for methylene protons of the Mo complex, consistent with a homoaromatic structure, but detected virtually no comparable difference in resonance for the same protons in the Fe complex.

An important piece of early evidence in support of the homotropylium cation structure that did not rely on the magnetic properties of the molecule involved the acquisition of its UV spectrum. Winstein et al. determined that the absorption maxima for the homotropylium cation exhibited a considerably shorter wavelength than would be precited for the classical cyclooctatrienyl cation or the bicyclo[5.1.0]octadienyl compound with the fully formed internal cyclopropane bond (and a localized electronic structure). Instead, the UV spectrum most resembled that of the aromatic tropylium ion. Further calculations allowed Winstein to determine that the bond order between the two carbon atoms adjacent to the outlying methylene bridge is comparable to that of the π bond separating the corresponding carbon atoms in the tropylium cation. Although this experiment proved to be highly illuminating, UV spectra are generally considered to be poor indicators of aromaticity or homoaromaticity.

Homoaromaticity can better be explained using Perturbation Molecular Orbital Theory (PMO) as described in a 1975 study by Robert C. Haddon. The homotropenylium cation can be considered as a perturbed version of the tropenylium cation due to the addition of a homoconjugate linkage interfering with the resonance of the original cation.

The most important factor in influencing homoaromatic character is the addition of a single homoconjugate linkage into the parent aromatic compound. The location of the homoconjugate bond is not important as all homoaromatic species can be derived from aromatic compounds that possess symmetry and equal bond order between all carbons. The insertion of a homoconjugate linkage perturbs the π electron density an amount δβ, which depending on the ring size, must be greater than 0 and less than 1, where 0 represents no perturbation and 1 represents total loss of aromaticity (destabilization equivalent to the open chain form).

A significant second order effect on the Perturbation Molecular Orbital model of homoaromaticity is the addition of a second homoconjugate linkage and its influence on stability. The effect is often a doubling of the instability brought about by the addition of a single homoconjugate linkage, although there is an additional term that depends on the proximity of the two linkages. In order to minimize δβ and thus keep the coupling term to a minimum, bishomoaromatic compounds form depending on the conformation of greatest stability by resonance and smallest steric hindrance. The synthesis of the 1,3 bishomotropenylium cation by protonating cis bicyclo[6.1.0]nona 2,4,6 triene agrees with theoretical calculations and maximizes stability by forming the two methylene bridges at the 1st and 3rd carbons. In addition to the homotropylium cation, another well established cationic homoaromatic compound is the norbornen 7 yl cation, which has been shown to be strongly homoaromatic, proven both theoretically and experimentally. An intriguing case of σ bishomoaromaticity can be found in the dications of pagodanes. In these 4 center 2 electron systems the delocalization happens in the plane that is defined by the four carbon atoms (prototype for the phenomenon of σ aromaticity is cyclopropane which gains about 11.3 kcal mol−1 stability from the effect). The dications are accessible either via oxidation of pagodane or via oxidation of the corresponding bis seco dodecahedradiene:

Reduction of the corresponding six electrons dianions was not possible so far.

There are many classes of neutral homoaromatic compounds although there is much debate as to whether they truly exhibit homoaromatic character or not. One class of neutral homoaromatics are called monohomoaromatics, one of which is cycloheptatriene, and numerous complex monohomoaromatics have been synthesized. One particular example is a 60 carbon fulleroid derivative that has a single methylene bridge. UV and NMR analysis have shown that the aromatic character of this modified fulleroid is not disrupted by the addition of a homoconjugate linkage, therefore this compound is definitively homoaromatic. Substituted neutral barbaralane derivatives (homoannulenes) have been disclosed as stable ground state homoaromatic molecules in 2023. Evidence for the homoaromatic character in this class of molecules stems from bond length analysis (X Ray structural analysis) as well as shifts in the NMR spectrum. The homoannulenes also act as photoswitches by which means a local 6π homoaromaticity can be switched to a global 10π homoaromaticity.

It was long considered that the best examples of neutral homoaromatics are bishomoaromatics such as barrelene and semibullvalene. First synthesized in 1966, semibullvalene has a structure that should lend itself well to homoaromaticity although there has been much debate whether semibullvalene derivatives can provide a true delocalized, ground state neutral homoaromatic compound or not. In an effort to further stabilize the delocalized transition structure by substituting semibullvalene with electron donating and accepting groups, it has been found that the activation barrier to this rearrangement can be lowered, but not eliminated. However, with the introduction of ring strain into the molecule, aimed at destabilizing the localized ground state structure's through the strategic addition of cyclic annulations, a delocalized homoaromatic ground state structure can indeed be achieved. Of the neutral homoaromatics, the compounds best believed to exhibit neutral homoaromaticity are boron containing compounds of 1,2 diboretane and its derivatives. Substituted diboretanes are shown to have a much greater stabilization in the delocalized state over the localized one, giving strong indications of homoaromaticity. When electron donating groups are attached to the two boron atoms, the compound favors a classical model with localized bonds. Homoaromatic character is best seen when electron withdrawing groups are bonded to the boron atoms, causing the compound to adopt a nonclassical, delocalized structure.

As the name suggests, trishomoaromatics are defined as containing one additional methylene bridge compared to bishomoaromatics, therefore containing three of these homoconjugate bridges in total. Just like semibullvalene, there is still much debate as to the extent of the homoaromatic character of trishomoaromatics. While theoretically they are homoaromatic, these compounds show a stabilization of no more than 5% of benzene due to delocalization.

Unlike neutral homoaromatic compounds, anionic homoaromatics are widely accepted to exhibit "true" homoaromaticity. These anionic compounds are often prepared from their neutral parent compounds through lithium metal reduction. 1,2 diboretanide derivatives show strong homoaromatic character through their three atom (boron, boron, carbon), two electron bond, which contains shorter C B bonds than in the neutral classical analogue. These 1,2 diboretanides can be expanded to larger ring sizes with different substituents and all contain some degree of homoaromaticity. Anionic homoaromaticity can also be seen in dianionic bis diazene compounds, which contain a four atom (four nitrogens), six electron center. Experiment results have shown the shortening of the transannular nitrogen nitrogen distance, therefore demonstrating that dianionic bis diazene is a type of anionic bishomoaromatic compound. Peculiar feature of these systems is that the cyclic electron delocalization is taking place in the σ plane defined by the four nitrogens. These bis diazene dianions are therefore the first examples for 4 center 6 electron σ bishomoaromaticity. The corresponding 2 electron σ bishomoaromatic systems were realized in the form of pagodane dications (see above).

There are also reports of antihomoaromatic compounds. Just as aromatic compounds exhibit exceptional stability, antiaromatic compounds, which deviate from Huckel's rule and contain a closed loop of 4n π electrons, are relatively unstable. The bridged bicyclo[3.2.1]octa 3,6 dien 2 yl cation contains only 4 π electrons, and is therefore "bishomoantiaromatic." A series of theoretical calculations confirm that it is indeed less stable than the corresponding allyl cation. Similarly, a substituted bicyclo[3.2.1]octa 3,6 dien 2 yl cation (the 2 (4' Fluorophenyl) bicyclo[3.2.1]oct 3,6 dien 2 yl cation) was also shown to be an antiaromate when compared to its corresponding allyl cation, corroborated by theoretical calculations as well as by NMR analysis.