Annulene diamagnetic ring currents

NMR data for aza[18]annulene 234 show a wide separation between the centers of the inner and outer proton lH NMR multiplets (Ad =11 ppm), indicative of a strong diamagnetic ring current.270 271 The bis-dehydro system 235 with 22 jr-electrons in the aromatic ring is diatropic.272... 

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). 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

Diprotonation of (97) with H2SO4 gives a species which is protonated at both carbonyl groups, not the delocalized annulene dication, presumably for steric reasons.140 Oxidation dications of perylenes, e.g. (98), are [4n + 2] species with diamagnetic ring currents, whereas the dibcnzo[ /. // ] derivative [two more benzene rings at the top and... 

Using the vinylogous aldehyde 192 in the place of 190 in the preparation of 208c, bisdehydroaza[2I]annulene (213) has been synthesized . The effect of the diamagnetic ring current was clearly observed in the H-n.m.r. spectrum of 213 which is a 2271 electron system, but the diatropicity was found to be less than that of the IStt system, 208c. 

The preparation of annelated tetrakisdehydro[18]annulenes 349 and 350 and the reference dehydroannulene 351 afforded further insight into the 7t-electron delocalization in annelated annulenes . When a dihydronaphthalene nucleus in 351 was replaced by naphthalene to form the annelated dehydroannulene 350, an appreciable suppression of diatropicity in 350 was observed. Further replacement of the dihydronaphthalene by naphthalene to give the dinaphtho-annelated derivative 349 resulted in a further suppression of the diatropicity in contrast to the increase of diamagnetic ring current in the case of transition from 342 to 340 and from 343 to 341. 

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... 

The aromatic nature of macrocycles 4.96a-4.99 was inferred from the large diamagnetic ring-current effect observed in their H NMR spectra. For instance, in the case of the octaethyl derivative 4.96a, the shift difference between the internal C-H protons and the external ones was found to be 25.3 ppm. This shift difference is over twice as large as that found for [18]annulene (A5 = 12.1 ppm). The UV-vis absorbance spectrum for 4.96a exhibits an intense Soret-like absorbance at 547 nm... 

Lai, Y. H., Chen, P. 2,5B,10b,11-Tetramethyldihydropyreno[5,6-c]furan the first furan-isoannulated [14]annulene that sustains as strong a diamagnetic ring current as the parent system. Tetrahedron Lett. 1988, 29, 3483-3486. 

The [14] annulenes are within the Hiickel criterion for aromaticity (m = 2) and a diamagnetic ring current is expected the spectrum of each compound so far investigated in this series tends to verify such a prediction (although a variety of temperature conditions are required to obtain satisfactory non-mobile spectra). 

Hexadehydro-[ 18]annulene 80>, 15, (see Table 4) possesses no inner protons and the six equivalent outer protons resonate at t 2.98. This represents a downfield shift of over 1 ppm from a model compound, cis-hex-3-ene-l,5-diyne, 9, (t 4.11). This model compound also served to clarify the interpretation of the singlet at t 5.55 in the case of l,5,9-tridehydro-[12]annulene, 1, a molecule for which aromaticity is not predicted. The downfield shift of 15 and the upfield shift of 1 from the acyclic model can then be attributed to a diamagnetic ring current in 15 and a paramagnetic ring current in 1. 

These few typical examples (41 to 48) indicate the dramatic effects operating in these systems, where the meso protons resonate about 5 ppm downfield from normal olefinic protons, and the imino protons, even when attached to a positively charged nitrogen atom, are shifted to very high fields. This behaviour is indicative of a substantial diamagnetic ring current in these molecules and lends credence to their inclusion as bridged [18]annulenes. 

---