Subject Chemistry Paper No. and Paper 1: ORGANIC CHEMISTRY- I (Nature of Title Bonding and Stereochemistry) Module No. and Module 9: Homoaromaticity Title Module Tag CHE_P1_M9 CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity Table of Content 1. Learning Outcomes 2. Introduction 3. Homoaromaticity 4. Evidence of Homoaromaticity 5. Criteria for homoaromaticity 6. Classification of homoaromatic compounds 7. Examples of homoaromatic compounds 7.1. Cationic homoaromatics 7.2. Neutral homoaromatics 7.2.1. Monohomoaromatics 7.2.2. Bishomoaromatics 7.2.3. 1,2-diboretane 7.2.4. Trishomoaromatics 7.3. Anionic homoaromatics 7.3.1. 1,2-diboretanide 7.3.2. Bis-diazene 8. Summary CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity 1. Learning Outcomes After studying this module, you shall be able to Understand the difference between aromaticity and homoaromaticity Understand about homoaromatic compounds Know the criteria for the compound to be homoaromatic Classify homoaromatic compounds Examples of homoaromatic compounds which includes cationic, neutral and anionic homoaromatics 2. Introduction As you are already aware of aromaticity which is most commonly defines molecule to be planar and have a complete delocalized system of (4n+2)π electrons. This complete cyclic delocalization of π electrons leads to an extra thermodynamic stability and explains many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds compared to a localized resonance form. It also accounts for the reactivity of aromatic compounds which differs significantly from alkenes (which tend to undergo addition reactions as compared to substitution reactions). Several molecules display aromatic character but do not strictly follow the definition mentioned above, leading to extensions to the definition of aromaticity for example, Möbius aromatics have (4n) π electrons yet they exhibit aromatic character. Spherically aromatic structures such as C are aromatic in spite of being non-planar. 60 Cyclopropane displays aromaticity in sigma orbitals rather than pi orbitals and inorganic pi complexes such as ferrocene exhibit aromaticity in three dimensions. Similarly, homoaromatic compounds also exhibits aromatic behaviour but in different manner which we will discuss in this module. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity 3. Homoaromaticity In the previous module, we discussed about the compounds which contain alternate double and single bonds and follows the Hückel’s rule. But in 1959, Saul Winstein talked about a new class of aromatic molecules during his study of the “tris-homocyclopropenyl” cation. These molecules, were termed as a homoaromatic molecules which showed discontinuity in 3 the p orbital overlap due to the presence of a single sp hybridized atom, generally CH2 group, but larger alkyl residue or a heteroatom at one or several positions within the ring could also be present. So, we can say that “those compounds where the stable configuration systems with (4n + 2) e- can only be formed by bypassing one saturated sp3 hybridized carbon atom leads to homoaromaticity”. Such compounds are definitely less stable than the aromatic compounds (with complete delocalization of e-), as they show poor overlap of the p-orbitals. For example, Homotropylium ion (cyclooctatrienyl cation): This is formed when cyclooctatetraene is treated with H2SO4. The 6 e- can be assumed to be delocalized over seven carbon atoms, and the eighth saturated carbon atom undergo non-planarity. These homoaromatic compounds are generally exist as cationic and anionic species but some studies also give the evidences for 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. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity 4. Evidence of Homoaromaticity Winstein proposed the first homoaromatic compound i.e., tris-homocyclopropenyl cation (3) as shown. This evidence is collected from the acetolysis experiment of (1) in which its cis epimer reacts 35 times faster than the trans epimer. Evidences for homoaromatic structure was acquired from deuterium scrambling. It was found that 66% of deuterium was found on cyclopropane ring at position 1 and 5 and only 33% on carbinol carbon i.e., at position 3. This suggest that all the three positions 1,3 and 5 were equivalent. The structure (3) later on called as homoaromatic . After his discovery many cationic, neutral and anionic compounds have been suggested to show homoaromaticity. These compounds attain stability due to homoconjugation. Homoconjugation or in other words homoconjugative interactions accounts for the complete cyclic delocalization of 4n electrons and lowering of energy. But there are some other conditions which are responsible for the characteristic aromatic properties of these compounds. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity 5. Criteria for Homoaromaticity To explain the characteristic aromatic properties of such compounds. We should first learn some of the conditions for the molecule, to show homoaromaticity are: 1) The molecule should possess one or more homoconjugative interactions (either through- bond or through-space) closing cyclic conjugation. 2) A closed cyclic system must show electron delocalization through. a) Effective overlap between the π -orbitals of the cyclic system. b) Bond orders and π -character indices should be close to that of an aromatic π -system c) Positive or negative charge should be delocalized throughout the cyclic system in case of charged molecules. d) There should be large degree of bond equalization resulting in difference of bond lengths from those of normal single or double bonds. 3) For either cyclopropyl or no-bond homoaromatic systems, the number of π –electrons participating in cyclic electron delocalization should be close to 4n +2. 4) Homoaromaticity should have resonance energy greater than 2 kcal mol-1 which leads to stabilization. 5) No-bond homoaromatic systems should possess exceptional magnetic properties that should lead to a) Significant equalization of 13C chemical shifts in the cyclic system. b) The magnetic susceptibility, χ, adopting a maximum value for an unconstrained homoaromatic system, indicating homoaromatic electron delocalization. c) A large chemical shift difference between the endo and exo oriented protons when the molecule possesses a CH2 group properly located above the ring. 6. Classification of homoaromatic compounds Homoaromatic compounds get stabilized due to some interactions, which results in delocalization. Therefore, depending on the type of interactions homoaromatics are classified into different categories: 1) No bond homoaromatic, where the cyclic delocalization is achieved through space. 2) Sigma bond homoaromatic, where the cyclic delocalization is achieved through bond interactions. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity 3) Homoaromatics having transannular homoconjugative interactions, it perturbs the aromatic delocalization, for example the bridged annulenes 15(C1-C6) and 16 (C1-C5) also termed as homonaphthalene and homoannulene due to the transannular homoconjugation. No-bond and bond homoaromatics are related by rearrangement as illustrated in the valence tautomerism between 1,3,5-cycloheptatriene and nor-caradiene. Both of them are well set up for homoconjugation. In Winstein’s terminology, they are mono homoaromatic and termed as monohomobenzene as there is only one interruption to the direct aromatic conjugation of benzene. Multiple homoconjugative insertions into an aromatic may result in bis-, tris-, tetrahomoaromatics, etc. Now, let us work out on some examples to understand the homoaromaticity more clearly. 7. Examples of Homoaromatic compounds 7.1 Cationic Homoaromatics The cationic homoaromatic compounds are most studied species for homoaromaticity. For example, the homotropenylium cation. Many homoaromatic cationic compounds generally use a cyclopropenyl cation, a tropylium cation, or a cyclobutadiene di-cations as their basis since these compounds exhibit strong aromatic character. CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 9: Homoaromaticity Another well-established homoaromatic cation which shows strong homoaromaticity is norbornene-7-yl cation. An interesting example of pagodanes which is in dicationic form shows σ- bishomoaromaticity termed as 2 electron σ-bishomoaromatic systems. In these 4-center-2- electron systems the delocalization happens in the plane that is defined by the four carbon atoms (like σ-aromaticity is shown in cyclopropane which gains about 11.3 kcal mol−1 stability from the effect). The dications are formed either via oxidation of pagodane or via oxidation of the corresponding bis-seco-dodecahedradiene. But the reduction of the corresponding six electrons dianions was not possible so far. 7.2 Neutral Homoaromatic Compounds Some studies also account for less common neutral homoaromatic compounds. There are many classes which include in neutral homoaromatic compounds which are as follows: 7.2.1. Monohomoaromatics