Homoaromaticity
Encyclopedia
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.
alkene
s previously observed in the literature. The IUPAC Gold Book
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.
carbonium ion
s that occurred in the 1950s. Saul Winstein
, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclo[3.1.0]hexyl cation.
In a series of acetolysis experiments, Winstein et al. observed that the solvolysis
reaction occurred empirically faster when the tosyl
leaving group
was in the equatorial position. The group ascribed this difference in reaction rates to the anchimeric assistance 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[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 methylene
groups. The group thus proposed the name "tris-homocyclopropenyl" -- the tris-homo counterpart to the cyclopropenyl cation.
has evolved as new developments and inisights 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:
A number of exceptions to these conventional rules exist, however. Many molecules, inclduing Möbius
4nπ electron species, pericylclic transition state
s, molecules in which delocalized electron
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.
with strong acids. Much of the early evidence for homoaromaticity comes from observations of unusual NMR properties associated with this molecule.
NMR spectroscopy, the group observed that the resonance
corresponding to two proton
s bonded to the same methylene
carbon
exhibited an astonishing degree of separation in chemical shift
.
From this observation, Pettit, et al. concluded that 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 group. 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.
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 group 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, it should be noted that UV spectra are generally considered to be poor indicators of aromaticity or homoaromaticity.
. 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 bond
s appear to be shorter, indicating involvement of the internal cyclopropane bond in charge delocalization.
where the outer electrons of the sp3 hybridized methylene carbon(2) back-donate to the adjacent carbons to stabilize the C1-C3 distance.
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. 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.
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.
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.
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.
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 is a true ground state neutral homoaromatic compound or not. By substituting semibullvalene with electron donating and accepting groups, it is believed that a homoaromatic state can be reached, however this has yet to be established.
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.
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.
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.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
refers to a special case of aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
in which conjugation
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
is interrupted by a single sp3 hybridized
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
, 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
Chemical stability
Chemical stability when used in the technical sense in chemistry, means thermodynamic stability of a chemical system.Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms...
.
The concept of homoaromaticity was pioneered by Saul Winstein
Saul Winstein
Saul Winstein was the Canadian chemist who discovered the Winstein reaction, in which he argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debate with Herbert C. Brown over the existence of delocalized cations such as this. Richard F...
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
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
and anionic
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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.
Naming
The term "homoaromaticity" derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugatedHomoconjugation
In chemistry, homoconjugation has two meanings, one in acid-base chemistry and one in structural organic chemistry.-Meaning in acid-base chemistry:...
alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s previously observed in the literature. The IUPAC Gold Book
Gold Book
The Compendium of Chemical Terminology is a book published by the International Union of Pure and Applied Chemistry containing internationally accepted definitions for terms in chemistry...
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-classicalNon-classical ion
Non-classical ions in organic chemistry are a special type of carbonium ions displaying delocalization of sigma bonds in 3-center-2-electron bonds of bridged systems. The term non-classical ion was first used by John D...
carbonium ion
Carbonium ion
A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+.- Methanium:The parent compound methanium or CH5+ is protonated methane and a superacid. This ion exists as a reactive intermediate in the interstellar medium and can be produced in...
s that occurred in the 1950s. Saul Winstein
Saul Winstein
Saul Winstein was the Canadian chemist who discovered the Winstein reaction, in which he argued a non-classical cation was needed to explain the stability of the norbornyl cation. This fueled a debate with Herbert C. Brown over the existence of delocalized cations such as this. Richard F...
, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclo[3.1.0]hexyl cation.
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
reaction occurred empirically faster when the tosyl
Tosyl
A tosyl group is CH3C6H4SO2. This group is usually derived from the compound 4-toluenesulfonyl chloride, CH3C6H4SO2Cl, which forms esters and amides of toluenesulfonic or tosylic acid...
leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
was in the equatorial position. The group ascribed this difference in reaction rates to the anchimeric assistance 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[3.1.0]hexyl cation is analogous to the previously well-studied aromatic cyclopropenyl
Cyclopropene
Cyclopropene is an organic compound with the formula 34. It is the simplest isolable cycloalkene. It has a triangular structure. Because the ring is highly strained, cyclopropene is both difficult to prepare and interesting to study. Like cyclopropane, the carbon ring of cyclopropene is planar...
cation. Like the cyclopropenyl cation, positive charge is delocalized
Delocalized electron
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond....
over three equivalent carbons containing two π electrons. This electronic configuration thus satisfies Huckel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
(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
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
is interrupted by three methylene
Methylene
Methylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....
groups. The group thus proposed the name "tris-homocyclopropenyl" -- the tris-homo counterpart to the cyclopropenyl cation.
Criterion for Homoaromaticity
The criterion for aromaticityAromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
has evolved as new developments and inisights 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
Delocalized electron
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond....
system of (4n+2)π electrons, satisfying Huckel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
. Most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability
Chemical stability
Chemical stability when used in the technical sense in chemistry, means thermodynamic stability of a chemical system.Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms...
relative to predictions based on localized resonance structures. Succinctly, three important features seem to characterize aromatic compounds:
- 1.) molecular structure (i.e. coplanarityCoplanarityIn geometry, a set of points in space is coplanar if all the points lie in the same geometric plane. For example, three distinct points are always coplanar; but a fourth point or more added in space can exist in another plane, incoplanarly....
: all contributing atoms in the same plane) - 2.) molecular energetics (i.e. increased thermodynamic stabilityChemical stabilityChemical stability when used in the technical sense in chemistry, means thermodynamic stability of a chemical system.Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment. This may be a dynamic equilibrium, where individual atoms...
) - 3.) spectroscopicSpectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
and magneticMagnetismMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
properties (i.e. magnetic field induced ring current)
A number of exceptions to these conventional rules exist, however. Many molecules, inclduing Möbius
Möbius aromaticity
In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of MO theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps which reveals the...
4nπ electron species, pericylclic transition state
Transition state
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 energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
s, molecules in which delocalized electron
Delocalized electron
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond....
s circulate in the ring plane or through σ
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
(rather than π
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
) 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 begain 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 cyclooctatraeneCyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
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 1HHydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force...
NMR spectroscopy, the group observed that the resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
corresponding to two proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s bonded to the same methylene
Methylene
Methylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....
carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
exhibited an astonishing degree of separation in chemical shift
Chemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
.
From this observation, Pettit, et al. concluded that 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
Cyclopropane
Cyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
bond located on the interior of the eight-membered ring must be subject to considerable delocalization
Delocalized electron
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond....
, 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 group. 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
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
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
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
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
Hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force...
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.
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. Winstein et al. determined that that the absorption maxima for the homotropylium cation exhibited a considerably shorter wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Tropylium ion
In organic chemistry, the tropylium ion is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropine . Salts of the tropylium cation can be stable, e.g. tropylium tetrafluoroborate...
. Further calculations allowed Winstein to determine that the bond order
Bond order
Bond order is the number of chemical bonds between a pair of atoms. For example, in diatomic nitrogen N≡N the bond order is 3, while in acetylene H−C≡C−H the bond order between the two carbon atoms is also 3, and the C−H bond order is 1. Bond order gives an indication to the stability of a bond....
between the two carbon atoms adjacent to the outlying methylene group is comparable to that of the π-bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
separating the corresponding carbon atoms in the tropylium cation. Although this experiment proved to be highly illuminating, it should be noted that 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 crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. These crystallographic studies have been used to demonstrate that the internuclear distance
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
between the atoms at the base of the cyclopropenyl structure is indeed longer than would be expected for a normal cyclopropane
Cyclopropane
Cyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
molecule, while the external bond
Bond
Bond, bonds, bonded, and bonding may refer to:* Peace-bonding, something which makes a weapon unusable as a weapon- Fiduciary :* Bond , in finance, a type of debt security...
s 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 modelMöbius aromaticity
In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of MO theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps which reveals the...
where the outer electrons of the sp3 hybridized methylene 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 annuleneAnnulene
Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn or CnHn+1...
ring favours the formation of the aromatic dication over the strained bridged homocation.
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[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.
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.
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 fulleroidBuckminsterfullerene
Buckminsterfullerene is a spherical fullerene molecule with the formula . It was first intentionally prepared in 1985 by Harold Kroto, James Heath, Sean O'Brien, Robert Curl and Richard Smalley at Rice University...
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 barreleneBarrelene
Barrelene is a bicyclic organic compound with chemical formula C8H8 and systematic name bicyclo[2.2.2]octa-2,5,7-triene. First synthesized and described by H. E. Zimmerman in 1960 the name derives from the obvious resemblance with a barrel, with the staves being three ethylene units attached to two...
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 is a true ground state neutral homoaromatic compound or not. By substituting semibullvalene with electron donating and accepting groups, it is believed that a homoaromatic state can be reached, however this has yet to be established.
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.
Antihomoaromaticity
There are also reports of antihomoaromatic compounds. Just as aromaticAromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
compounds exhibit exceptional stability, antiaromatic
Antiaromaticity
Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the pi electron energy of antiaromatic compounds is higher than that of its open-chain counterpart. Therefore antiaromatic compounds are unstable and highly reactive; often antiaromatic compounds...
compounds, which deviate from Huckel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
and contain a closed loop of 4n π electrons, are relatively unstable. The bridged
Bicyclic molecule
A bicyclic molecule is a molecule that features two fused rings. Bicyclic molecules occur widely in organic and inorganic compounds.Fusion of the rings can occur in three ways:...
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.