Annulene anti-aromaticity

The role of a-electrons in the stabilization of aromatics has been repeatedly emphasized by Shaik and Hiberty48 and has recently received experimental confirmation after a study of the effects of deuterium substitution of annulenes with internal hydrogens, such as the anti-aromatic [16] annul ene and the aromatic [18]annulene.49 50... 

The interesting hydrocarbon (104), having both aromatic and anti-aromatic character, could be isolated in low yield by treatment of a bis-ylide with freshly distilled glyoxal.96 Precursors for the preparation of helicenes, e.g. (105), have been synthesized in one-step reactions. The yield of the double Wittig reaction is better using a di-aldehyde rather than a bisphosphonium salt.97 The [12]-annulene (106) is obtained from the reaction of the bis-ylide derived from (107) and a di-aldehyde.98 The condensation of (107) with two moles of 3-methylpent-2-en-4-yn-l-al (108) was also successful.99... 

Figeys, H.P. (1970). Quantum-Chemical Studies on teh Aromaticity of Conjugated Systems - II. Aromatic and Anti-Aromatic Annulenes the (4n+2)-3i Electron Rule. Tetrahedron, 26,5225-5234. 

The stability and thus the structure of heterocycles 28, 30 and 34 deserves some comment. They are annulenes possessing (4n)-7t-electrons and are therefore anti-aromatic according to the so-called Hiickel rules. The aromatic and antiaromatic character is perturbed when one or more second-row... 

Figeys, H.P. (1970) Quantum-chemical studies on the aromaticity of conjugated systems-II. Aromatic and anti-aromatic annulenes the (4n -I- 2)-Jt electron rule. Tetrahedron, 26, 5225-5234. 

In the next section we take up a discussion of the magnetic properties of the annulenes. In striking analogy to Huckel s rule, an opposite directionality of these properties is found for the [4w]- and [4n + 2]-annulenes. For this reason magnetic properties have played the major role in the investigation of aromatic, non-aromatic and anti-aromatic character in the annulenes. By default, n.m.r. has been the standard experimental probe of annulene magnetism. 

We therefore conclude that n.m.r. will differentiate between the [4 ]- and [4n + 2]-annulenes, for it is quite apparent that proton chemical shifts are sensitive to the magnetic properties of the tr-electrons therein. Insofar as this property parallels the Hiickel predictions regarding aromatic and anti-aromatic character (which we now identify with diamagnetic and paramagnetic ring currents) then n.m.r. is a qualitative8) criterion of aromatic character in the annulenes. 

The [12]annulenes, being 4mz systems, (n = 3), are expected to be anti-aromatic. This prediction has been substantiated by the n.m.r. investigation of three compounds in this series. 

In the [16]annulene series, those compounds examined show typically anti-aromatic spectra in agreement with the prediction for 4ms systems (n = 4). 

Thus, from a consideration of both bridged and unbridged annulenes ranging from 6 to 36 peripheral iv-electrons, it has been seen that, in planar systems, n.m.r. spectroscopy can distinguish clearly between those aromatic (4n +2) it systems and the anti-aromatic Ann cases, until, at very larger ring size where Hiickel s rule becomes invalid, both series merge into non-aromatic behaviour. 

A contrasting conversion from an anti-aromatic to an aromatic system is illustrated by the addition of 2 7t-electrons to [16]annulene, and the results are equally remarkable. It was seen that in [16]annulene itself a dynamic equilibrium exists between the two configurations 16a... 

In anti-aromatic systems, on the other hand, a paramagnetic ring current is produced, in which the magnetic field strengthens the outer magnetic field inside, above and below the loop, and weakens it on the periphery. [16]Annulene, an example of an antiaromatic substance, therefore displays absorption signals at 5.33 ppm for the outer protons, and 9.44 ppm for the inner protons. 

Molecular orbital (MO) diagrams show that when a [4n]annulene is reduced to a [4n- -2]dianion, the LUMO level becomes fully populated, resulting in a stable ground state and an aromatic system. Conversely, the reduction of a [4n- -2]annulene affords a [4n] dianion, that possesses a low-lying LUMO, which can undergo a pseudo-Jahn-Teller distortion. These [4n]annulenes are predicted to be anti-aromatic [lb, 56-61]. 

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