Atropic, annulenes

The NMR and UV characteristics of 58a and b were shown to be consistent with a heavily buckled atropic frame and one whose hetero-atomic unit is directed syn to the methano bridge.60 It is also interesting to note in this connection that, whereas thia-annulene (58b) does differ rather significantly in some aspects of its NMR and UV spectra from... 

The tris heterobridged thia[17]annulene (120) is available in unsubstituted as well as diester form from base-induced ring-forming condensation between dialdehyde 117 and 118 or 119, respectively.79 Analysis of the unsubstituted system 120a and b by XH NMR revealed both members to be atropic. Rationalizations of this result center chiefly on (i) the... 

The H-n.m.r. spectra of237 and 238 do not indicate any conformational mobility, in contrast to bisdehydrothia[13]- (227) and [17]annulenes (223 and 224) . It was found that the sulphides 237 and 238 are clearly paratropic 16tc electron systems. The H-n.m.r. spectrum of mono-cw-bisdehydrooxa[15]annulene (239) shows that it is atropic molecular models suggest that 239, unlike a -trans analogues (237 and 238), cannot be planar. 

Hetero[ll]annulenes are known either in sterically rigid bridged forms (25) (72AG(E)217) and (26) (72TL4129) or as the more flexible triannulated varieties (27) (70JA5284), (28) and (29) (71CC1434). Both types of system are strictly atropic by NMR which is not unexpected... 

In contrast, the r-excessive hetero[ll]annulenes 21 [10] and the pyridine-like hetero[12]annulene 22 [11] are atropic molecules. As shown by their NMR spectra, they avoid antiaromatic destabilization of a paratropic 12 r-perimeter by assuming nonplanar structures. 

With these larger rings the difference in energy between the localised and delocalised structures is likely to be small so that perturbation by, for example, an extra triple bond, is likely to be of importance. This has been thought [77] to explain the difference between raonodehydro[26]annulene, which is diatropic [77], and tris-dehydro[26]annulene, which is atropic [78],... 

The diatropicity or paratropicity of the bisdehydroannulenones is lowered when cyclohexene rings are fused to the annulene ring [94,95]. For example compounds (XXVI) and (XXVII) are both atropic. 

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