Diatropicity

The criterion of ipsocentric ring current has been used to assess aromaticity in S-N heterocycles (and related inorganic ring systems). Current density maps indicate that the ten r-electron systems [SsNs], [S4N3] " and [S4N4] ", and the fourteen r-electron system [S5N5] " support diatropic k currents, reinforced by a circulations. 

Pyrrolo[l,2-a]azepin-5-one (11), prepared by cyclization of methyl 5-(pyrrol-2-yl)penta-2,4-dienoate (10) with sodium hydride in toluene, in trifluoroacetic acid solution forms the diatropic 5-hydroxypyrrolo[1,2-a]azepinium ion 12.216 6-Methyl-5//-pyrrolo[l,2-tf]azepin-5-one(mp41 -43 C), formed in low yield (20%) by the action of [(ethoxycarbonyl)methylene]triphenylphos-phorane on 4-(pyrrol-2-yl)but-3-en-2-one, behaves similarly. 

A diatropic cation 15 is also noted in strong acid solution for 7//-pyrrolo[l,2-a]azepin-7-one (14), prepared by dehydrogenation of its 5,6-dihydro derivative 13.217 It is presumed that the deep-red solution produced on treating 7-methylene-7//-pyrrolo[l,2- azepine (16), prepared from the ketone 13 as indicated, with strong acid is indicative of the formation of 7-methylpyrrolo[l,2-a]azepinium cation 17 however, the salt could not be isolated nor a satisfactory HNMR spectrum obtained.217... 

Azonine anions 1 (X = N M + ), the parent compound 1 (X = NH) and, to a lesser extent, alkyl-substituted derivatives (X = NR), however, can be classified as aromatic, as they are flat, display high thermal stability and exhibit diatropic properties. 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

It follows that aromaticity can be determined from an NMR spectrum. If the protons attached to the ring are shifted downfield from the normal alkene region, we can conclude that the molecule is diatropic, and hence aromatic. In addition, if the compound has protons above or within the ring (we shall see an example of the latter on p. 69), then if the compound is diatropic, these will be shifted upfield. One drawback to this method is that it cannot be applied to compounds that have no protons in either category, for example, the dianion of squaiic acid (p. 69). Unfortunately, NMR is of no help here, since these spectra do not show ring currents. ... 

The tropylium ion has an aromatic sextet spread over seven carbon atoms. An analogous ion, with the sextet spread over eight carbon atoms, is 1,3,5,7-tetra-methylcyclooctatetraene dication (47), which is stable in solution at -50°C, is diatropic and approximately planar and is not stable above about -30°C. ... 

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... 

Another way of eliminating the hydrogen interferences of [14]annulene is to introduce one or more triple bonds into the system, as in dehydro[14]annulene (87). All five known dehydro[14]annulenes are diatropic. Compound 79 can be nitrated or sulfonated. The extra electrons of the triple bond do not form part of the aromatic system but simply exist as a localized bond. The [18]annulene (88)... 

The known bridged [18]annulenes are also diatropic as are most of the known... 

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. ... 

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

Using similar methodology, macrocycle 126 was prepared, as well as the unusual monoene 127 [76]. Considerable debate in the literature over the last thirty years has focused on whether dehydrobenzoannulenes are able to sustain induced ring currents [5al. Although fusion of arenes to the annulenic core provides rigidity and stability, this also weakens the diatropicity/paratropicity of the macrocycle significantly. Until quite recently, the number of planar systems available for study was limited however, with the the addition of 123 and 126, the series of alkyne-linked, tribenzo-fused dehydroannulenes is complete from... 

Theoretical calculations indicate that the Hiickel (4 n + 2) rule should break down at higher values of n, with the onset of bond alternation9), and it has been predicted that the limit should lie between 22- and 26-membered ring compounds10). However doubt has been cast on this prediction by the synthesis of a diatropic mono-dehydro[26]annuleneu) and a diatropic bisdehydro[30]annulene12). 

The NMR spectra of the isomeric [1 ljannulenones 12-15 are more complex than that of 11 due to the lack of symmetry of these molecules. The same conclusions concerning the atropicity of the annulenones and the diatropicity of the hydroxyan-nulenium ions can, however, be drawn. 

Annulenones containing 15 carbon-atoms may be expected to be diatropic. Since the [13]annulenones were paratropic it was of interest to determine the NMR spectral properties of the next higher members. 

Only one example of the potentially diatropic [19]annulenones is known. Wittig reaction of 5-(/3-formylvinyl)-2-furaldehyde 135 with the bisphosphonium salt 120... 

That the [19]annulenone 136 is diatropic is evident from the high field resonance of H13 compared to the adjacent external protons Ht2 and H14, and to all the olefinic protons of the homoannulene 137. Further evidence is the significantly lower field resonance of H3, H4, H17, and Hlg when compared with the similar protons H3 and H4 of the atropic ketones 143,144, and 145 (see below). 

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