Annulenes, bridged protonation

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

A homoaromatic structure also best explains the n.m.r. characteristics of the anion 75 (see Table 15), which was derived from 1,e-methanols 10] annulene 219>. This 10r9C system has peripheral protons which absorb from t 3.19 to 5.28 (or ca. t 2.1 to 4.2 on correction for charge density), and bridge protons which are about 2.5 ppm more shielded than the corresponding protons in 65, the compound formed on protonation of the anion. 

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

The [12]annulene (96) has been prepared. In solution this molecule exhibits rapid conformational mobility (as do many other annulenes), so that above a certain temperature, in this case — 150°C, all protons are magnetically equivalent. However, at — 170°C the mobility is greatly slowed and the three inner protons are found at 85 while the nine outer protons are at 68. Annulene 96 suffers from hydrogen interference and is certainly not planar. It is very unstable and above —50°C rearranges to 97. Several bridged and dehydro[12]annulenes are known. 

The results for [16] annulene are similar. The compound was synthesized in two different ways, both of which gave 103, which in solution is in equilibrium with 104. Above -50°C there is conformational mobility, resulting in the magnetic equivalence of all protons, but at — 130°C the compound is clearly paratropic there are 4 protons at 10.565 and 12 at 5.35 5. In the solid state, where the compound exists entirely as 103, X-ray crystallography shows that the molecules are nonplanar with almost complete bond alternation The single bonds are 1.44-1.47 A and the double bonds are 1.31-1.35 A. A number of dehydro and bridged... 

The PMR spectra of substituted derivatives, however, provide additional evidence for the structure of bridged [12]annulenes. A comparison of 92a and 92b reveals a normal control of the cyano group over the chemical shift of the neighboring protons (A = 1.0 ppm), the influence on the remote pyrrolic protons being small (A = 0.11-0.39 ppm). Hence diazapyracylenes should be composed of loosely connected closed-shell moieties (formula 97a) the influence of the substituents on the chemical shift being limited to the substituted part of the molecule.112... 

Steric interactions generally predominate relative to ring current effects in carbon-13 NMR. Proton shifts of bridged [10]- and [14]annulenes, for example, clearly identify these compounds to be aromatic due to the typical deshieldings of about 2 ppm relative to comparable non-aromatic cyclopolyenes [401a]. In contrast, carbon-13 shifts of bridged annulenes (Table 4.58) are more closely related to those of comparable non-aromatic cyclopolyenes with similar steric interactions than to those of other annulenes [401 b]. 

Protonation of bismethano[14]annulene and its bridged analogs allowed to generate and characterize the corresponding 109 and 110 cations.286 Protonation always occurs at C(7). 

Like normal porphyrin, the a-P -linked porphyrin isomers 3.160 and 3.161 may be considered as being heteroatom-bridged [18]annulenes. They display, for instance, properties consistent with a 4n + 2 aromatic formulation. In particular, H NMR spectra of both 3.160 and 3.161 indicate the presence of a sustained diamagnetic ring current. " This is evident from the upheld shift of the internal NH and CH protons as well as from the corresponding downfield shift of the... 

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