Aromatic ring current

Something interesting happens when we go beyond benzene to apply the aromatic ring current test to annulenes... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Porphyrin, 5,10,15,20-tetraphenyl-, 4, 386 Porphyrin, vinyl-synthesis, 4, 278, 279 Porphyrin coenzymes in biochemical pathways, 1, 258-260 Porphyrinogen, mcso-tetraaryl-synthesis, 4, 230 Porphyrinogens, 4, 378, 394 pyrazoles, 5, 228 synthesis, 4, 231 Porphyrins, 4, 377-442 acetylation, 4, 395 aromatic ring current, 4, 385 basicity, 4, 400 biosynthesis, reviews, 1, 99... 

The spectrum also reveals a significant diamagnetic (aromatic) ring current. The signals of the internal hydrogens (C-3, C-6, C-10, and C-13) are very far upfield = —0.61 ppm). ... 

Figure 15.14 The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized tt electrons circulating in the molecular orbitals of the aromatic ring.

Calculations show that only 60% of the chemical shift difference between and H is the result of the aromatic ring current, and that even Ha is shielded it would appear at 5 = w5.5 without the ring current Childs, R.F. McGlinchey, M.J. Varadarajan, A. J. Am. Chem. Soc., 1984, 106, 5974. 

Recrystallization of 76 (R=H) from CH2CI2 provided crystals adequate for X-ray structural determination. The molecule was found to be saddle shaped with a phenyl ring at each vertex and nadir. The alkynyl bonds were found to be essentially linear and to possess a mean length of 1.194 A, typical for the length of triple bonds in free butadiyne. Although 76 is a dehydrobenzoannulene possessing a 4n TT-electron circuit, the nonplanarity of the macrocycle alleviated much of the strain associated with a flat structure and thus precluded the possibility of anti-aromatic ring currents. 

Various techniques are suitable for the study of binding locations of organic solubi-lizates in micelles and vesicles. Typically, these methods have included changes in NMR chemical shifts as a result of aromatic ring current effects, paramagnetic relaxation... 

Nondestructive reactions of trisacetylacetonates of chromium(lll), cobalt(lll), and rhodium(lll) are reviewed. Halogenation, nitration, thiocyanation, acylation, formylation, chloromethylation, and aminomethylation take place at the central carbon of the chelate rings. Trisubstituted chelates were obtained in all cases except acylation and formylation. Unsymmetrically and partially substituted chelates have been prepared. Substitutions on partially resolved acetylacetonates yielded optically active products. NMR spectra of unsymmetrically substituted, diamagnetic chelates were interpreted as evidence for aromatic ring currents. Several groups were displaced from the chelate rings under electrophilic conditions. The synthesis of the chromium(lll) chelate of mal-onaldehyde is outlined. 

NMR has been widely invoked in assessing aromaticity. Comparison of the chemical shifts of furan, H-2 7.46 and H-3 6.41, with those observed for 4,5-dihydrofuran, H-2 6.31 and H-3 4.95 (66JCS(B)127), indicates there is ca. 1-1.5 ppm downfield shift attributable to the presence of an aromatic ring current in furan. The same effect is observed for thiophene, H-2 7.35 and H-3 7.13, and 4,5-dihydrothiophene, H-2 6.17 and H-3 5.63 ppm. The similar range of chemical shifts observed for all of the parent heterocycles may be compared with that for benzene, 7.27 8, and further attests to their possessing appreciable ring currents. 

A special magnetic behaviour of aromatic compounds is believed to result from the occurrence of aromatic ring currents. The following magnetic criteria can be used to determine if a molecule is aromatic. 

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