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Chapter 2 Aromatic Heterocycles Chapter 2 Aromatic Heterocycles 2.1 Six-atom, six-π-electron heterocycles Aza derivatives of benzene N N N N N N N pyridine pyridazine pyrimidine pyrazine N N N N N N N N N N N N N 1,2,3-triazine 1,2,4-triazine 1,3,5-triazine 1,2,4,5-tetrazine π-Electron populations for azines 0.942 0.965 0.734 N 1.029 0.795 1.051 N N N N 0.816 0.533 N 0.787 N N 0.584 N N 1.368 1.467 1.426 1.240 1.449 Other heterobenzenes B B Si O S P H H borabenzene 1H-borata- silabenzene pyrylium thiopyrylium phospha- benzene cation cation benzene 2.2 Five-atom, six-π-electron heterocycles 2π-Electron populations for pyrrole, furan and thiophene 1.090 1.067 1.046 1.087 N 1.078 1.071 1.5647 O 1.710 S H 1.760 02-Aro-hetero Chinpiao Chen 2008/2/22 10:03:00 AM 7 Aromatic five-membered heterocycles with two or more heteroatoms N N N N N N N N N N N O S S H H H isothiazole oxazole thiazole 1H-pyrazole 1H-imidazole 1H-tetrazole 2.3 Benzo-fused ring systems Benzo-fused six-membered nitrogen heteroaromatics N N N N N N quinoline isoquinoline cinnoline quinazoline N N N N N N N N N N quinoxaline phthalazine 1,2,3-benzotriazine 1,2,4-benzotriazine Five-membered benzo-fused heterocycles NH N O S H indole benzofuran benzo[b]thiophene isoindole N N O N N N O N H H isobenzofuran benzimidazole 1,2-benzisoxazole 1H-benzotriazole Other fused heterocycles N N N N N N N N N H N N phenanthridine pteridine purine quinolizinium cation indolizine 02-Aro-hetero Chinpiao Chen 2008/2/22 10:03:00 AM 8 2.4 Some criteria of aromaticity in heterocycles Bond lengths Typical lengths (Å) of single and double bonds linking sp2-hybridized atoms CC 1.48 CC 1.34 CN 1.45 CN 1.27 CO 1.36 CO 1.22 CS 1.75 CS 1.64 NN 1.41 NN 1.23 Bond distances (Å) in some heteroaromatic compounds 1.394 1.375 1.393 1.384 1.348 1.307 N 1.395 1.341 1.373 1.333 N N 1.340 N 1.330 1.331 N 1.331 N 1.307 H 1.417 1.416 1.379 1.423 1.372 1.382 1.373 1.331 N 1.358 1.370 N N 1.326 1.367 1.304 N 1.370 N N S 1.714 S H H H 1.398 1.436 1.409 1.372 1.431 1.381 1.359 1.425 1.395 1.358 1.423 1.361 1.406 1.356 1.309 N 1.396 1.352 N O 1.362 N 1.291 N 1.380 1.389 1.372 1.369 O O H 1.384 1.386 1.395 1.374 Aromaticity indices of heterocycles based on structure (%) Benzene 100 Pyridine 82 Pyridazine 65 Pyrimidine 67 Pyrazine 75 Pyrrole 37 Thiophene 45 Furan 12 Pyrazole 61 Imidazole 43 1,2,4-Triazole 71 Tetrazole 80 02-Aro-hetero Chinpiao Chen 2008/2/22 10:03:00 AM 9 Chemical shifts of aromatic and nonaromatic heterocycles 7.10 6.41 6.78 7.13 7.46 8.56 7.35 N O S 5.50 5.63 Me 5.50 4.95 6.17 6.31 S 7.63 O NMe Ring current effects on methyl substituents Me O Me N S Me H δ (Me) observed 2.30 2.18 2.48 calculated 2.14 1.97 2.19 downfield shift 0.16 0.21 0.29 General conclusions 1. The degree of aromatic character of the monocyclic azabenzenes is slightly less than that of benzene and tends to decrease with increasing numbers of ring nitrogen atoms. 2. Other six-membered ‘heterobenzenes’ have substantial aromatic character. 3. The aromatic character of five-membered monocyclic heterocycles is lower than that of benzene. 4. Five-membered heterocycles containing oxygen tend to show less aromatic character than those containing nitrogen and sulfur. 5. In the five-membered heterocycles aromatic character tends to increase as the number of nitrogen atoms in the ring increases. 6. Benzo-fused heterocyclic ring systems have lower aromatic character than the corresponding monocyclic heterocycles. 02-Aro-hetero Chinpiao Chen 2008/2/22 10:03:00 AM 10 2.5 Reactivity of heteroaromatic compounds N HH 16 H H H N N H N H N H H H 17 H H H H H H N N H 18 H 17>18>16 2.6 Tautomerism of heteroaromatic compounds X XH NX NXH N N H H - OH O N N H H H N N N N N N N N N H OEt OEt N N H OH O X X 02-Aro-hetero Chinpiao Chen 2008/2/22 10:03:00 AM 11.
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