Didehydro annulenes

Dieckmann cyclization thiophenes from, 4, 874-875 Thiaannulenes structure, 7, 715 twice-bridged structure, 7, 718 Thiaannulenes, didehydro-structure, 7, 718 Thia[17]annulenes restricted structure, 7, 720... 

FIGURE 35. Polymerization schemes of octatetrayne derivatives and cycloaromatization of 1,6-didehydro[10]annulene (98). Reprinted with permission from Reference 53. Copyright (1994) American Chemical Society... 

The energetics of the structures of l,2-didehydro[10]annulene differs markedly from that of [lOJannulene. The lowest energy conformation has one trans double bond and has the heart-like conformation (45). The next lowest energy isomer, lying... 

It was found that soUd-state polymerization of these monomers always proceeds by 1,4-addition. Furthermore, polydiacetylene 96 could be thermally reacted and the structure of the final polymer was proposed to be that of the ladder polymer 97 where the repeating unit is l,6-didehydro[10]annulene (98), i.e. two conjugated polydiacetylenes. However, the annulene 98 is expected to be unstable. In fact, its cycloaromatization reaction to the... 

A positional isomer of 5, l,7-didehydro-[14]annulene 66,90), 6, (see Table 4) also exhibits chemical shifts characteristic of an aromatic compound, the inner protons (t 10.7) and the outer protons (t 1.4 to 2.7) covering a somewhat narrower range (9.3 ppm) than the 15 ppm separation observed for 5. 

For l,3-didehydro-[16]annulene 96>, 13 (see Table 4), analogous interconversion between non-equivalent conformers occurs. The n.m.r. spectrum 98> was investigated at 0°C, as the compound is very unstable. Double irradiation again aided in the interpretation of this spectrum. At this temperature it appears that rotation of only two of the three double bonds occurs, leading to four possible conformers, 15a-d "). Thus... 

Of the various [20]annulenes prepared 115>116> only one compound has as yet been investigated by n.m.r. This is l,ll-didehydro-[20]-annulene, 24, whose n.m.r.66) indicates the presence of a paramagnetic ring current. The room temperature spectrum is of the mobile type arising from the rotation of two pairs of trans double bonds, which leads to an averaged "symmetrical molecule (a similar case was seen for l,9-didehydro-[16]annulene, 12). When the bond rotation is "frozen out at — 80 °C, one sees the deshielded inner protons (t —1.60 to —0.45) and shielded outer protons (t4.40 to 4.93) characteristic of an antiaromatic compound. 

Myers has also studied the cycloaromatization of the related aromatic hydrocarbon l,6-didehydro[10]annulene (65, Scheme 7-19), demonstrating that the reaction has a half-life of 25 min at -51 C and making it one of the most rapid diradical-forming reactions known [138, 139]. 

Strictly these compounds should be described as trisdidehydro-[18]annulene and bisdidehydro[12]annulene, but common usage is as given with the absence of two hydrogens described by dehydro rather than didehydro.) ... 

In an effort to overcome steric interactions due to -substituents in cyclotetra(2,3-thienylene)s 4.6 (see above), Marsella et al. synthesized expanded thiophene-fused didehydro [12]annulene 4.32, which incorporated two additional ethynylene groups between the -positions of the cycUc stmcture (Scheme 1.44) [408]. Regiospeciflc halogenation of a 2,2 -bithiophene and Sonogashira-type cross-coupling with the ethynylated counter part were key elements of the synthetic strategy, which was also used to prepare a further extended macrocycle 4.33 (Chart 1.54), profiting from quadrupolar interactions of the central phenylene units [409]. 

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