CHAPTER 2 AROMATICITY 85
prepared, for example, and the evidence supported its aromaticity.290 This study suggested that increasing benzoannelation of the parent, 116, led to a step-down in aromaticity, a result of competing ring currents in the annulenic system. [18]Annulene (115) is diatropic:291 the 12 outer protons are found at
d ¼ 9 and the 6 inner protons at
d ¼3. X-ray crystallography292 shows that it is nearly planar, so that interference of the inner hydrogens is not important in annulenes this large. Compound 115 is reasonably stable, being distillable at reduced pres- sures, and undergoes aromatic substitutions.293 The CC bond distances are not equal, but they do not alternate. There are 12 inner bonds of
1.38 A ˚ and 6 outer bonds of
1.42 A ˚ .292 Compound 115 has been estimated to have a resonance energy of
37 kcal mol1 (155 kJ mol1), similar to that of benzene.294 The known bridged [18]annulenes are also diatropic295 as are most of the known dehydro[18]annulenes.296 The dianions of open and bridged [16]annulenes297 are also 18-electron aromatic systems,298 and there are dibenzo[18]annulenes.299 [22]Annulene300 and dehydro[22]annulene301 are also diatropic. A dehydro- benzo[22]annulene has been prepared that has eight C C units, is planar and possesses a weak induced ring current.302 In the latter compound there are 13 outer protons at 6.25–8.45 d and 7 inner protons at 0.70–3.45 d. Some aromatic bridged
CHAPTER 2 AROMATICITY 85
prepared, for example, and the evidence supported its aromaticity.290 This study suggested that increasing benzoannelation of the parent, 116, led to a step-down in aromaticity, a result of competing ring currents in the annulenic system. [18]Annulene (115) is diatropic:291 the 12 outer protons are found at
d ¼ 9 and the 6 inner protons at
d ¼3. X-ray crystallography292 shows that it is nearly planar, so that interference of the inner hydrogens is not important in annulenes this large. Compound 115 is reasonably stable, being distillable at reduced pres- sures, and undergoes aromatic substitutions.293 The CC bond distances are not equal, but they do not alternate. There are 12 inner bonds of
1.38 A ˚ and 6 outer bonds of
1.42 A ˚ .292 Compound 115 has been estimated to have a resonance energy of
37 kcal mol1 (155 kJ mol1), similar to that of benzene.294 The known bridged [18]annulenes are also diatropic295 as are most of the known dehydro[18]annulenes.296 The dianions of open and bridged [16]annulenes297 are also 18-electron aromatic systems,298 and there are dibenzo[18]annulenes.299 [22]Annulene300 and dehydro[22]annulene301 are also diatropic. A dehydro- benzo[22]annulene has been prepared that has eight C C units, is planar and possesses a weak induced ring current.302 In the latter compound there are 13 outer protons at 6.25–8.45 d and 7 inner protons at 0.70–3.45 d. Some aromatic bridged
การแปล กรุณารอสักครู่..
