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 cation 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.
Overview
Naming The term "homoaromaticity" derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugated alkenes 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.
History The concept of homoaromaticity has its origins in the debate over the non-classical carbonium ions 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 Hückel'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 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.
Evidence for homoaromaticity
Criterion for homoaromaticity The criterion for aromaticity has evolved as new developments and insights continue to contribute to our understanding of these remarkably stable organic molecules.le Noble, W. J. "Aromaticity" in Highlights of Organic Chemistry: an advanced textbook; Marcel Dekker, Inc.: New York, 1974. 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 Hückel's rule. Most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability relative to predictions based on localized resonance structures. 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.
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 cyclooctatetraene 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, 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.
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 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.
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. 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.
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 ring favours the formation of the aromatic dication over the strained bridged homocation.
Second-order effects
Second homoconjugate linkage