Cyclic 7r-electron system

The term aromatic will be used in a strict non-historical sense to mean possessing a cyclic 7r-electron system (6 and 10 electrons for the mono- and bi-cyclic rings discussed in this review). Heteroaromatic compounds, like carboaromatics, have widely different degrees and types of electronic dissymmetry and polarizabihty. Consequently, their reactivity varies tremendously with any one reagent and their relative reactivity changes drastically with the type of reagent. In this sense, aromatic compounds show differences in reactivity but not in aromaticity. The virtues of this qiuilitative concept of aromaticity and the pitfalls of trying to use it as a quantitative concept in modern context have been ably presented by Peters and by Balaban and Simon. ... 

Only even functions of the linear case remain acceptable in the cyclic system and hence the quantum condition becomes L(= 27rr) = nX. The energy levels for a cyclic 7r-electron system follows immediately as... 

In general, linear 7r-electron systems with Z1r = 2N electrons at the lowest energy levels have closed-shell singlet states while cyclic systems reach closed shell structures only when ZT = 4N + 2. Cyclic 7r-electron systems with Zn 4N + 2 will therefore exhibit multiplet ground states according to Hund s rules, and should be chemically reactive because of the unpaired electrons. Hiickel s rule that predicts pronounced stability for so-called aromatic ring systems with 4jV + 2 7r-electrons is based on this shell structure. The comparison with cyclic systems further predicts that ring closure of linear 7r-electron systems should be exothermic by an amount... 

The action of V on atomic functions located at other centers in the molecule is more complicated [12]. It needs to be evaluated for the understanding of phenomena such as natural and magnetic circular dichroism and ring currents in cyclic 7r-electron systems, but is not of immediate interest presently. 

Reduction should result in l,2-dihydro-l,2-diazocine (2) which formally contains a cyclic, conj ugatcd 10 7r-electron system and potentially possesses aromatic character. To date, however, only the synthesis of a dibenzo derivative has been described (see Section 1.2.1.3.). 

Benzene, a cyclic conjugated polyene, absorbs at 260, 200, and 180 nm. All of these bands are associated with the 7r-electron system of benzene. The intense bands at 200 and 180 nm are assigned to transitions to dipolar excited states, and the weak band at 260 nm is ascribed to a forbidden transition to a homopolar excited state. 

In the case of vinylfurans and vinylpyrroles there is the possibility of cycloaddition involving either the cyclic diene system or the diene system including the double bond. 2-Vinylfuran reacts in high yield with maleic anhydride in ether at room temperature to form the adduct involving the exocyclic double bond. Similarly, 2- and 3-vinylpyrroles react with 7r-electron-deficient alkenes and alkynes under relatively mild conditions to give the corresponding tetrahydro- and dihydro-indoles (Scheme 51) (80JOC4515). 

If you compare the orbital energies of the Hiickel and Mobius cyclic 7r systems (Figures 21-13 and 21-16), you will see that the Hiickel systems have only one lowest-energy MO, whereas the Mobius systems have two. Hiickel systems have an odd number of bonding orbitals (which, when full, accommodate 2, 6, 10, 14, or An + 2 electrons) and the Mobius systems have an even number of bonding orbitals (which, when full, accommodate 4, 8, 12, or An electrons). The Hiickel molecular orbitals have zero or an even number of nodes (see, for example, the benzene MOs, Figure 21-5) the Mobius molecular orbitals are not shown, but they have one or an odd number of nodes. 

The greater stability of 59 compared to 60 also requires explanation. The ring is more nearly planar in 59 than in 60, which may marginally increase steric shielding of the silicon atom, but this seems an insufficient rationalization. Instead, it has been suggested that 59 is stabilized by aromatic resonance, since it forms a 6 7r-electron cyclic system, 59a98-367. 

Since 1-azirines are well known, it does not seem reasonable to attribute the nonexistence of 2-azirines exclusively to strain energy. An unfavorable electronic situation is most likely responsible for their alleged instability. As mentioned previously, the 2-azirine ring system is a cyclic conjugated necessarily planar n system containing four 7r electrons and Hiickel s rule would not predict it to be stabilized by delocalization. In fact, HMO theory predicts that delocalization results in a less stable 7r system than the open chain analog.5 Such systems are predicted to be relatively unstable and have recently been designated as antiaromatic.8,9... 

All these photocyclizations are well explained in terms of an electrocyclic mechanism of nitrogen-containing, six 7r-electron conjugated system, according to the Woodward - Hoffmann rule, by postulating the intermediacy of a common trans cyclic structure from which respective types of products are formed depending on the reaction conditions either a nonoxidative, oxidative, or reductive condition. 

Homoaromaticity as a chemical concept is based on the concepts of aromaticity and homoconjugation. In its simplest form aromaticity is an electron counting concept. Thus if there are 4 + 2 7r-electrons in a planar (or nearly planar) cyclic system, then this is considered to be aromatic. ... 

Setting out the requirements for homoaromaticity in this manner, it should be easy to distinguish homoaromatic from non-homoaromatic molecules. Clearly, an appropriate geometry or structure of the species in question is required. This pertains not only to the appropriate placement of the AOs at the homoconjugative centres but also to the structural changes associated with the cyclic delocalization of (Aq + 2) 7r-electrons. This cyclic delocalization should also be reflected by the stability of the system and its spectroscopic properties, including in particular its NMR spectrum. 

Roberts and coworkers, investigating the ionization reactions of cyclobutene derivatives, found that the resulting cyclobutenyl ions were unusually stable " They suggested that rather than regarding these ions as simple allyl cations, their properties were consistent with a C(1),C(3) interaction and cyclic delocalization of the 7r-electrons. As such, these 27c-electron systems were considered to be the homoaromatic counterparts of the well established, aromatic cyclopropenium ions. ... 

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