Diatropic compounds

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)... 

A bispyridine derivative of dibenzo-hexaaza[18]-annulene 236 was prepared by Bell and Guzzo.275 Fur an building blocks were used in the syntheses of the diatropic compounds 237 and 238.276-278 The chemical shifts of the indicated hydrogens show that the diamagnetic ring current decreases in the order 237 > 238 (Scheme 83). 

Indolizines are diatropic compounds, i.e. the possession of a ring current may be deduced from the chemical shift of the ring protons. 

Unlike [3.3.3]cyclazine (230), the cyclopentacyclazines (285) are unequivocally diatropic compounds (see Table 11). The five-membered ring double bonds have been shown to be delocalized on the basis of the vicinal coupling constants of the corresponding protons. These observations suggest that the cyclazines (285) may be considered as [13]annulene anions weakly coupled to a localized azomethinium cross-link represented as in (286). The mutual interaction between the two closed shell systems may be small since the bonding MOs of the peripheral [13]annulene can interact only weakly with the antibonding MO of the azomethine group by reason of symmetry. 

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. 

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. ... 

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... 

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... 

Pyrrolo[ l, 2 -3,4]pyrimido[2,l,6-a/]pyrrolizine (40a) was obtained in 10% yield by a reaction of the sodium salt of pyrrole-2-aldehyde with vinylene-l,2-bistriphenylphosphonium iodide (39)in boiling toluene. The intermediate (yields shown in Scheme 1) were isolated from a reaction in boiling benzene. Compound 40a is diatropic and on refluxing in acetic anhydride it gives 40b. The position of substitution is in accordance with HMO calculations.44... 

Cycl[3,2,2]azines are diatropic. The proton magnetic resonance (PMR) spectrum of the parent compound (la) consists of an A2B multiplet from the protons of the six-membered ring and one AB quartet from the two pairs of protons of the five-membered rings. 

As a consequence, the properties of the ions 73 and 74 should differ much more than those of the known isoconjugates 71101 and 72.102 The cycl[4,2,2]azinium salts (73) presumably will be diatropic stable compounds. 

The PMR spectrum of 85a agrees well with theoretical considerations. It can be seen from the vicinal coupling constants that the seven-membered ring is planar, containing localized bonds. A paratropic effect of 1.7 ppm in the five- and six-membered rings and one of at least 2.7 ppm in the seven-membered ring follows from comparison with suitable reference compounds. The different paratropism can be explained by the superposition of two ring systems, a diatropic indolizine and a paratropic [ 12]annulene moiety.106,107... 

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