By the mids, 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. After initial reports of a "homoaromatic" structure for the tris-homocyclopropenyl cation were published by Winstein, many groups began to report observations of similar compounds.
One of the best studied of these molecules is the homotropylium cation, the parent compound of which was first isolated as a stable salt by Pettit, et al. While characterizing the compound resulting from deprotonation of cyclooctatriene by 1 H 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. Instead, the group proposed the structure of the bicyclo[5.
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. 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. 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. Studies of these complexes by 1 H 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. Instead, the UV spectrum most resembled that of the aromatic tropylium ion.
More recently, work has been done to investigate the structure of the purportedly homoaromatic homotropylium ion by employing various other experimental techniques and theoretical calculations. One key experimental study involved analysis of a substituted homotropylium ion by X-ray crystallography.
These crystallographic studies have been used to demonstrate that the internuclear distance between the atoms at the base of the cyclopropenyl structure is indeed longer than would be expected for a normal cyclopropane molecule, while the external bonds appear to be shorter, indicating involvement of the internal cyclopropane bond in charge delocalization. The molecular orbital explanation of the stability of homoaromaticity has been widely discussed with numerous diverse theories, mostly focused on the homotropenylium cation as a reference.
Haddon initially proposed a Mobius model where the outer electrons of the sp 3 hybridized methylene bridge carbon 2 back-donate to the adjacent carbons to stabilize the C1-C3 distance. 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.
In fact, the maximum ring size for homoaromaticity is fairly low as a membered annulene ring favours the formation of the aromatic dication over the strained bridged homocation. 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.
The synthesis of the 1,3-bishomotropenylium cation by protonating cis-bicyclo[6.
The addition of a substituent to a homoaromatic compound has a large influence over the stability of the compound. Depending on the relative locations of the substituent and the homoconjugate linkage, the substituent can either have a stabilizing or destabilizing effect. This interaction is best demonstrated by looking at a substituted tropenylium cation. If an inductively electron-donating group is attached to the cation at the 1st or 3rd carbon position, it has a stabilizing effect, improving the homoaromatic character of the compound. However, if this same substituent is attached at the 2nd or 4th carbon, the interaction between the substituent at the homoconjugate bridge has a destabilizing effect.
Therefore, protonation of methyl or phenyl substituted cyclooctatetraenes will result in the 1 isomer of the homotropenylium cation.
Following the discovery of the first homoaromatic compounds, research has gone into synthesizing new homoaromatic compounds that possess similar stability to their aromatic parent compounds. There are several classes of homoaromatic compounds, each of which have been predicted theoretically and proven experimentally. The most established and well-known homoaromatic species are cationic homoaromatic compounds. As stated earlier, the homotropenylium cation is one of the most studied homoaromatic compounds. Many homoaromatic cationic compounds use as a basis a cyclopropenyl cation, a tropylium cation, or a cyclobutadiene dication as these compounds exhibit strong aromatic character.
In addition to the homotropylium cation, another well established cationic homoaromatic compound is the norbornenyl cation, which has been shown to be strongly homoaromatic, proven both theoretically and experimentally. The dications are accessible either via oxidation of pagodane or via oxidation of the corresponding bis-seco-dodecahedradiene: .
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.www.cantinesanpancrazio.it/components/cojocima/649-come-localizzare-un.php
One particular example is a 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. It was long considered that the best examples of neutral homoaromatics are bishomoaromatics such as barrelene and semibullvalene. First synthesized in ,  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. Showell , Julie B. Warneck , Reinhold Tacke. Their aromaticity is revealed by density-functional computations of their structures and the nucleus-independent chemical shifts NICS.
Besides the formerly used endohedral inclusion strategy, spherical homoaromaticity is another way to stabilize silicon and germanium clusters.
Polyhedron , 10, Science , , Pure Appl. Tetrahedron , 56, Special issue on aromaticity, May, Organometallics , 16, Phosphorus, Sulfur Silicon Relat.
What is "homoaromaticity"
Organometallics , 18, Organometallics , 19, However, homoaromaticity,15 well-established in organic chemistry, has received much less attention in silicon16 and germanium17 counterparts. We are 5 a Tokitoh, N. Organometallics , 21, Organometallics , 20, Organometallics , 22, Internet J. The reported geometries are local minima, unless otherwise stated.
WOA1 - Single-site catalysts containing homoaromatic ligands - Google Patents
All of the calculations were carried out with the Gaussian 98 program. Replacing the 18 a Hirsch, A. Spherical aromaticity - an overview. Chem, Int.
Recent theoretical studies, see: c Kumar, V. B , 66, B , 65, B , 67, D , 24, A , , Gaussian 98; Gaussian, Inc. Molecular skeletons of four-center two-electron 4c-2e homoaromatic systems. Table 1. Their homoaromaticity is confirmed by the computed highly negative nucleus-independent chemical shift NICS 26 values at the cage center, Most recently, the aromaticity of 2 has also been characterized by Laali et al.
The homoconjugative SiSi and Ge-Ge distances are 2.
Its aromaticity is evidenced by the homoconjugative Si-Si distance of 2. However, we failed to get SCF convergence for the Ge analogue.
- Viscosimetry of Polymers and Polyelectrolytes (Springer Laboratory)!
- Homoaromaticity | Aromaticity | Nuclear Magnetic Resonance Spectroscopy.
- Discrete Optimization for TSP-Like Genome Mapping Problems (Genetics-Research and Issues)!
Molecular skeletons of eight-center eight-electron 8c-8e homoaromatic systems. Figure 4. Molecular skeletons of the strain-free systems.