Benzo-fused annulenes

Dibenzo[a,e]cyclooctene (19) has a tub-shaped structure in the crystalline state with dihedral angles of 99 between the two benzene rings [21]. The investigation of its flexibility employing dynamic NMR spectroscopic studies showed that the activation energy for the ring inversion in 19 is AG = 12.3 0.2 kcal mol at -5°C [22]. 

Moreover, a series of tub-shaped benzo[8]annulenes 43-45 with two diben-zocyclooctatetraene units were recently synthesized by lyoda and coworkers 

Their relevant binding behavior with guest molecules such as 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) and 1,2,4,5-tetracyanobenzene (TCNB) in solution and soUd state was also investigated by analyzing UV-vis and NMR spectra. Interestingly, solutions of 43a, 44b, and 45 showed thermochromism 

Indeed, equilibrium of the syn and anti conformers of 48 was found in solution, whereas the anti conformer existed only in the solid state. However, 48 showed almost no emission because of their remaining structural flexibility in solid state. Further study showed that their emissions were effectively enhanced in the crystalline state in the presence of solvent molecules. In particular, 48b exhibited pseudopolymorphism, thermochromism, and vapochromism, depending on the exposure to different saturated solvent vapors. 

Generally, all these multifaceted properties of benzo-fused COTs arise from the combination of the flexible tub-shaped jt-system core and the planar rigid arene scaffold. Their dynamic motion would change not only the conformation and aromaticity but also the optical, electrochemical, magnetic, and carrier-transporting properties for potential applications. 

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Youngs and Tessier synthesized one of the few [2,l-c]lF derivatives not already discussed [96]. Treatment of tribenzocyclotriyne 176 with 4 equiv. n-BuLi collapsed the annulene core to furnish the [2,l-c]lF dianion 177 subsequent quenching with MeOH as the proton source afforded the benzo-fused dihydro-lF 178 (Scheme 49). 

These proposed benzo-fused dehydro [8] annulenes can generally be accessed via a bromination-dehydrobromination synthetic pathway. As shown in Scheme 5.1, upon bromination under irradiation, dibenzo[a,e]COT (19) was converted into dibromo and tetrabomo compounds 20 and 21, which then underwent dehydro-bromination with f-BuOK to give stable dibenzo[ ,e]cyclocta-l,3,5-trien-7-yne... 

Scheme 5.1 Synthesis of benzo-fused dehydro[8]annulenes 11 and 12.

Benzo-fused [8]annulenes containing twisted % systems have also received great attention lately (Figure 5.5). In this section, we will review the synthesis and properties of some bent jt-conjugated systems comprised of benzo-fused[8]annulenes, excluding those with COT rings coordinated to metals. Tetrabenzo[a,c,e, ]cyclooctene (tetraphenylene) and its derivatives will be discussed in the next section. 

Although most of the fundamental studies of cycl[3.2.2]azines were reported in CHEC(1984) (see Section 12.16.6.3 for leading references), there is continuing interest - synthetic, spectroscopic, and theoretical - in these and their benzo- and dibenzo-fused analogues, all of which may be considered as bridged analogues of [10]-, [14]-, and [18]-annulenes, respectively. The same level of theoretical interest continues to apply to those />m -fused systems with a hypervalent sulfur or selenium at the 5 5 ring junction (Section 12.16.6.6). 

Aside from the l,4-dihydro-l,4-diazocines (l,4-diaza[8]annulenes) discussed in Section 5.19.4.4.1, completely unsaturated 1,4-diazocines are represented in the literature only by benzo and dibenzo fused systems. 

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