Hiickel-aromatic annulenes

Consequently, the reduction of [4n]annu-lenes should be observed at relatively positive potentials with small AE separations for the dianion formation, while the reduction of the [4n + 2]annulenes should occur at more negative potentials with large AE separations for the dianion formation. This is exactly what is observed in (see Table 1). Although benzene [37, 38], the classical Hiickel aromatic with 4 1 + 2 = 6 jr-electrons, is reduced at —3.42 V (vs. Ag/AgCl), the reduction of the [Sjannulene COT occurs at —1.81V. Similarly, the [16]annulene (3) [67, 68] is more easily reduced than the corresponding [ISjannulene (4) [69], although the reduction of the larger jr-systems should be more favorable for electrostatic reasons. 

Non-benzoid phanes have always played an important role in cyclophane chemistry and many structures have been prepared with other Hiickel aromatic rings such as azulene or tropone [49]. A cyclophane with two bridged annulene units was recently synthesized by Mitchell [50,51 ]. Dimethyldihydropyrene (76), an excellent NMR probe first introduced by Boekelheide [52], was converted into the dialcohol 77 in three steps. Reaction with adipoyl chloride afforded the large [10.10]cyclophane 78. Unfortunately the conversion of the dialcohol into the corresponding dibromide 79, an obvious precursor to the interesting phane 80, has failed so far. 

A Hiickel aromatic system with ten electrons (n = 2) can be established with the 9-membered azonine ting system involving the eight pi-electrons of the double bonds and the lone pair on nitrogen. The aU-cis ring (7.10) is known but not stable because of the angle strain as described in the aza-annulenes (Chapter 6). Its proton NMR spectrum... 

The [nlannulenes may be defined as cyclic polyaUcenes possessing a closed circuit of n Ti-conjugated pz-orbitals. The first three small annulenes are shown in Fig. 1 and include [4]annulene (cyclobutadiene), [6]annulene (benzene), and [8]annulene (cyclooctatetraene). The bracketed number, n, can be classified as either a 4n+2 (Hiickel aromatic/Mobous antiasomatic) or 4n (Huckel antiaromatic/ Mobius aromatic) delocalized n-electron species. That said, it is not surprising that the concept of aromaticity is closely associated with the annulenes. Indeed, the first three annulenes listed cover the concepts of Hiickel aromatic, antiaromatic, and nonaromatic properties (see Fig. 1). 

Fig. 1 Depicting the first three simple [n]annulenes along with their rc-electron count and Hiickel aromatic/antiaromatic/nonaromatic classification.

Beginning in the 1960s, Franz Sondheimer and his colleagues, first at the Weizmann Institute in Israel and later at the University of London, synthesized a number of larger annulenes, primarily to test the validity of HiickeTs criteria for aromaticity. They found, for example, that both [14] annulene and [18] annulene are aromatic, as predicted by Hiickel. [18]Annulene has a resonance energy of approximately 418 kJ (100 kcal)/mol. Notice that for these annulenes to achieve planarity, several of the carbon-carbon double bonds in each must have the trans configuration. 

The scope of aromaticity is thus much broader for Hiickel-type annulenes than for fullerenes, where it is essentially limited to delocalization evidenced by ring currents. The prediction that according to the computed NMR criteria C6o ° should the most aromatic fullerene derivative to date does not mean it will be easily, or ever, synthesized. Yet the classifica-... 

In agreement with the Hiickel rule those annulenes and dehydroannulenes which contain (4 n + 2) 77 electrons and a reasonably planar carbon skeleton appear to be aromatic. Aromaticity in annulenes is usually equated with positive resonance energy and the absence of bond alternation. The most direct method of measuring bond alternation is by single crystal X-ray diffraction. Unfortunately this method has been applied in only a few cases. 

Results from X-ray studies of three annulenes are presented In Table 8. According to Hiickel s rule [14]annulene (14-ANN) and [18]annulene (18-ANN) should be aromatic and most probably planar molecules, while [16]annulene (16-ANN), as a [4n]annulene, should be antiaromatic. The [14]annulene molecule is nonplanar, with a structure that approaches C2h symmetry. The cause of the nonplanarity is the steric overcrowding in the center of the molecule. While the spread of the individual bond lengths implies possible significant differences, there is no significant pattern to the values obtained. 

Annulene was the first macrocyclic annulene containing (4n -j- 2) zr-electrons to be synthesized. The compound is of considerable interest, since it is the type of annulene that was predicted to be aromatic by Hiickel.10 It proved to be aromatic in practice, as evidenced from the proton magnetic resonance spectrum,8-11 the X-ray crystallographic analysis,18 and the fact that electrophilic substitution reactions could be effected.13... 

Methano[10]annulene represents a Hiickel-type aromatie(4w. + 2)jr-system and is reminiscent of benzene or napthhalene in both its physical and chemical properties.14 The aromatic nature of the hydrocarbon is... 

Annulene satisfies the Hiickel (4n+2) it electron rule for aromaticity, and many of its properties indicate aromaticity (Section 11.20). As shown in Figure 13.10a, [18]annulene contains two different kinds of protons 12 lie on the ring s periphery ("outside"), and 6 reside near the middle of the molecule ("inside"). The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the 1H NMR spectrum. The outside protons have a chemical shift 8 of 9.3 ppm, which makes them even less shielded than those of benzene. The six inside protons, on the... 

The Hiickel theory [21, 22], which has become the standard description of annulene aromaticity, is one of the cornerstones of physical organic chemistry. Unfortunately, in its qualitative version, from which the famous Hiickel rules are derived, the theory does... 

The porphyrin ring contains 22 electrons in its n orbitals. As explained above, the Hiickel rule cannot be applied to this electron count, because the molecule is not monocyclic. However, the porphyrin ring can be formally derived from neutral [18]annulene, by introduction of appropriate heteroatoms and bridges (Fig. 3). The macrocycle is thus shown to be aromatic in the Hiickel sense, and is denoted [18]porphyrin. As we will show in subsequent sections, this approach is readily generalized to other porphyrinoids, whose aromatic character can be predicted by defining a neutral annulenoid pathway in the macrocycle. These pathways will be... 

Oxidation and reduction of all-carbon annulenes are known to result in cationic or anionic species with aromatic properties different from those of the neutral ring, offering a textbook example of the validity of Hiickel rules [87], hi pyrrole-containing porphyrinoids, such oxidations and reductions can be realized with retention of charge neutrality because the charge can be adjusted by protonation or... 

The general name annulene is sometimes given to rings that contain alternating single and double bonds in a single Lewis structure. Thus, benzene can be called [6]annulene, and cyclooctatetraene can be called [8]annulene. A number of larger annulenes have been prepared to determine whether they follow Hiickel s rule and are aromatic when they have 4n + 2 electrons in the cycle. 

Erich Hiickel developed a shortcut for predicting which of the annulenes and related compounds are aromatic and which are antiaromatic. In using Hiickel s rule, we must be certain that the compound under consideration meets the criteria for an aromatic or antiaromatic system. 

Benzene is [6]annulene, cyclic, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (41V+2) system, with N = 1. Hiickel s rule predicts benzene to be aromatic. 

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