Homoaromaticity
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Homoaromaticity, in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, refers to a special case of
aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturate ...
in which
conjugation Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the chang ...
is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of
p-orbital In atomic theory and quantum mechanics, an atomic orbital is a Function (mathematics), function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electr ...
s, 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 In chemistry, chemical stability is the thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibri ...
. The concept of homoaromaticity was pioneered by
Saul Winstein Saul Winstein (October 8, 1912 – November 23, 1969) was a Jewish Canadian chemist who discovered the '' Winstein reaction.'' He argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debat ...
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 An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
and
anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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.


Overview


Naming

The term "homoaromaticity" derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugated
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s previously observed in the literature. The
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
Gold Book The International Union of Pure and Applied Chemistry publishes many books which contain its complete list of definitions. The definitions are divided into seven "colour books": Gold, Green, Blue, Purple, Orange, White, and Red. There is also an e ...
requires that Bis-, Tris-, etc. prefixes be used to describe homoaromatic compounds in which two, three, etc. sp3 centers separately interrupt conjugation of the aromatic system. :


History

The concept of homoaromaticity has its origins in the debate over the non-classical
carbonium ion In chemistry, a carbonium ion is any cation that has a pentavalent carbon atom. The name carbonium may also be used for the simplest member of the class, properly called methanium (), where the five valences are filled with hydrogen atoms. The ...
s that occurred in the 1950s.
Saul Winstein Saul Winstein (October 8, 1912 – November 23, 1969) was a Jewish Canadian chemist who discovered the '' Winstein reaction.'' He argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debat ...
, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclo .1.0exyl cation. : In a series of acetolysis experiments, Winstein et al. observed that the
solvolysis In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
reaction occurred empirically faster when the
tosyl In organic chemistry, a toluenesulfonyl group (tosyl group, abbreviated Ts or Tos) is a univalent functional group with the chemical formula –. It consists of a Toluene, tolyl group, –, joined to a sulfonyl group, ––, with the open vale ...
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited t ...
was in the
equatorial Equatorial may refer to something related to: *Earth's equator **the tropics, the Earth's equatorial region **tropical climate *the Celestial equator ** equatorial orbit **equatorial coordinate system ** equatorial mount, of telescopes * equatorial ...
position. The group ascribed this difference in reaction rates to the
anchimeric assistance In organic chemistry, neighbouring group participation (NGP, also known as anchimeric assistance) has been defined by the International Union of Pure and Applied Chemistry (IUPAC) as the interaction of a reaction centre with a lone pair of electro ...
invoked by the "cis" isomer. This result thus supported a non-classical structure for the cation. Winstein subsequently observed that this non-classical model of the 3-bicyclo .1.0exyl cation is analogous to the previously well-studied aromatic cyclopropenyl cation. Like the cyclopropenyl cation, positive charge is
delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
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 Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the chang ...
is interrupted by three -- units. The group thus proposed the name "tris-homocyclopropenyl"—the tris-homo counterpart to the cyclopropenyl cation.


Evidence for homoaromaticity


Criterion for homoaromaticity

The criterion for
aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturate ...
has evolved as new developments and insights continue to contribute to our understanding of these remarkably stable
organic molecule In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
s. 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 In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
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 In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all. For example, three points are always coplanar, and if the points are distinct and non-collinear, the plane they determine is unique. How ...
: all contributing atoms in the same plane) #molecular energetics (i.e. increased
thermodynamic stability In chemistry, chemical stability is the thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibriu ...
) #
spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
and
magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
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 In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted fashion, and the bond orbitals involved in the reaction overlap ...
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
s, molecules in which
delocalized electron In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly di ...
s 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.


Early evidence for homoaromaticity

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. in 1962, when the group reacted cyclooctatraene with strong acids. Much of the early evidence for homoaromaticity comes from observations of unusual NMR properties associated with this molecule.


NMR spectroscopy studies

While characterizing the compound resulting from deprotonation of cyclooctatriene by 1H
NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
, the group observed that the
resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
corresponding to two
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s bonded to the same methylene bridge
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
exhibited an astonishing degree of separation in
chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...
. : 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 .1.0ctadienyl compound, theorizing that the
cyclopropane Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
bond located on the interior of the eight-membered ring must be subject to considerable
delocalization In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
, 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 Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
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 Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
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 Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
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.


UV spectroscopy studies

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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
. Winstein et al. determined that the absorption maxima for the homotropylium cation exhibited a considerably shorter
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
than would be precited for the classical cyclooctatrienyl cation or the bicyclo .1.0ctadienyl 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 In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...
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.


More recent evidence for homoaromaticity

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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. 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 Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
molecule, while the external bonds appear to be shorter, indicating involvement of the internal cyclopropane bond in charge delocalization.


Molecular orbital description

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. R.C. Haddon initially proposed a Mobius model where the outer electrons of the sp3 hybridized methylene bridge carbon(2) back-donate to the adjacent carbons to stabilize the C1-C3 distance.


Perturbation molecular orbital theory

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. :


First-order effects

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). It is believed that with increasing ring size, the resonance stabilization of homoaromaticity is offset by the strain in forming the homoconjugate bridge. In fact, the maximum ring size for homoaromaticity is fairly low as a 16-membered
annulene Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds (' mancude'). They have the general formula CnHn (when ''n'' is an even number) or C''n''H''n''+1 (when ''n'' is an odd number). Th ...
ring favours the formation of the aromatic dication over the strained bridged homocation.


Second-order effects


=Second homoconjugate linkage

= 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 .1.0ona-2,4,6-triene agrees with theoretical calculations and maximizes stability by forming the two methylene bridges at the 1st and 3rd carbons. :


=Substituents

= 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. :


Examples of homoaromatic compounds

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.


Cationic homoaromatics

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 norbornen-7-yl cation, which has been shown to be strongly homoaromatic, proven both theoretically and experimentally. : An intriguing case of
σ-bishomoaromaticity Horst Prinzbach (20 July 1931 in Haslach im Kinzigtal – 18 September 2012 in Freiburg im Breisgau) was a German chemist and professor emeritus. Prinzbach studied chemistry at the University of Freiburg and received his PhD under Arthur L ...
can be found in the dications of
pagodane Pagodane is an organic compound with formula whose carbon skeleton was said to resemble a pagoda, hence the name. It is a polycyclic hydrocarbon whose molecule has the ''D''2''h'' point symmetry group. The compound is a highly crystalline solid ...
s. 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 Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself ...
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.


Neutral homoaromatics

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.


Bishomoaromatics

It was long considered that the best examples of neutral homoaromatics are bishomoaromatics such as
barrelene Barrelene is a bicyclic organic compound with chemical formula C8H8 and systematic name bicyclo .2.2cta-2,5,7-triene. First synthesized and described by Howard Zimmerman in 1960, the name derives from the resemblance to a barrel, with the staves ...
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. :


Trishomoaromatics

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. :


Anionic homoaromatics

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 Horst Prinzbach (20 July 1931 in Haslach im Kinzigtal – 18 September 2012 in Freiburg im Breisgau) was a German chemist and professor emeritus. Prinzbach studied chemistry at the University of Freiburg and received his PhD under Arthur L ...
. The corresponding 2 electron σ-bishomoaromatic systems were realized in the form of
pagodane Pagodane is an organic compound with formula whose carbon skeleton was said to resemble a pagoda, hence the name. It is a polycyclic hydrocarbon whose molecule has the ''D''2''h'' point symmetry group. The compound is a highly crystalline solid ...
dications (see above). :


Antihomoaromaticity

There are also reports of antihomoaromatic compounds. Just as
aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
compounds exhibit exceptional stability,
antiaromatic Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised (π or lone pair) electrons in it, as opposed to aromaticity. Unlike aroma ...
compounds, which deviate from Huckel's rule and contain a closed loop of 4n π electrons, are relatively unstable. The bridged bicyclo .2.1cta-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 .2.1cta-3,6-dien-2-yl cation (the 2-(4'-Fluorophenyl) bicyclo .2.1ct-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. :


External links

*Homoaromaticity
Gold Book The International Union of Pure and Applied Chemistry publishes many books which contain its complete list of definitions. The definitions are divided into seven "colour books": Gold, Green, Blue, Purple, Orange, White, and Red. There is also an e ...
br>Link


References

{{Organic chemistry Physical organic chemistry