Dibenzo cyclooctatetraene

Von bemerkenswerter thermischer Stabilitat ist das aus 1,2,3,4-Dibenzo-cyclooctatetraen (Formel 140) durch UV-Bestrahlung darstellbare Va-lenzisomere 141 (313). Neben der bei 350° eintretenden Isomerisierung... 

The photochemical cycloaddition of 4-methyl-l,2,4-triazoline-3,5-dione to the dibenzo-cyclooctatetraene 289 yields 3.5% of the cycloadduct 290, together with 36% of 291, the product of a di-jr-methane rearrangement (equation 153. Anthrasteroids 293 (R = H, Ac or COPh) are produced in an oxidative rearrangement when the phenyltriazoline-dione adduct of 292 is treated with boron trifluoride etherate. The 5,7-diene system of ergosteryl acetate 294 can be protected by cycloadduct formation, allowing selective hydrogenation of the 22,23-double bond " ... 

A triplet-sensitized photoreaction in the crystalline state has been achieved by using crystalline organic salts which were prepared from cationic sensitizers and an anionic reactant (Scheme 40) [73]. Thus, upon selective excitation of the sensitizer these salts or hydrogen-bonded complex gave two dibenzosemi-bullvalenes through triplet-triplet energy transfer to the reactant, A dibenzo-cyclooctatetraene, which is a product of direct (singlet state) irradiation, was not produced. 

Dis.solution of 1,4-dimethyl-, 1,3,5,7-tetramethyl, 1,3,5,7-tetra-phenyl- or S /m.-dibenzo-cyclooctatetraenes in a mixture of antimony pentafluoride and sulphonyl chloride fluoride at -78 results in the formation of species which, judged from their n.m.r. spectra, are planar dications with delocalised sets of six ir-electrons, giving rise to diamagnetic ring currents [59]. In the case of the dibenzo--derivative it appears that all fourteen ir-electrons are delocalised over the sixteen carbon atom periphery. 

Crabtree described the use of dibenzo[a,e]cyclooctatetraene, a potent selective poison of homogeneous hydrogenation catalysts, as a tool to distinguish between homogeneous and heterogeneous catalysis in the hydrogenation of hexene with a range of catalysts [24]. 

Except for one recent example, all iridium-catalyzed allylic substitution reactions have been performed under an inert atmosphere with dry solvent and reagents. The iridium metalacycle is sensitive to protonation, which opens the metalacycle and results in the formation of a less-active complex containing a K -phosphoramidite ligand. A paper by Helmchen et al. addressed this issue [107]. Nearly all iridium catalysts used for allylic substitution consist of an iridium fragment chelated by COD. In the presence of a catalyst containing dibenzo[a,c]cyclooctatetraene (dbcot) in place of COD, allylic substimtion reactions occur in air with results that are comparable to those of reactions performed under an inert atmosphere (Scheme 35). 

A planar molecular structure in the solid state was observed for dilithiated dibenzo[a, e]cyclooctatetraene by Rabideau and coworkers. Dilithiated 233 is formed in THF by the reaction of compound 232 with an excess of metallic lithium at room temperature (Scheme 81). In the solid state, two lithium-TMEDA units are capping both sides of the central eight-membered ring plane . 

The syntheses of planar, benzo-fused cyclooctatetraenes embedded in small rings have always been my favorite research objectives. The major advances in this area have been the syntheses of the reportedly planar derivatives of dibenzo[a, c]cy-clooctene 35, namely 108, 109, and 110. ... 

Macrocyclic ligands have played an important part in the development of modern co-ordination chemistry. But what exactly is a macrocycle As far as a co-ordination chemist is concerned, the definition of Melson is probably the most useful. Melson stated that a macrocycle is a cyclic molecule with three or more potential donor atoms in a ring of at least nine atoms. Thus, ethylene oxide (6.1), 1,4-dithiane (6.2), cyclotetradecane (6.3) and cyclooctatetraene (6.4) are not commonly thought of as macrocycles (Fig. 6-1), whereas molecules such as cyclam (6.5), phthalocyanine (6.6), 1,4,7-trithiacyclononane (6.7) and dibenzo-18-crown-6 (6.8) fit the definition (Fig. 6-2). 

The nonplanar cyclooctatetraene (38, COT) and its planar aromatic dianion (382-) have been extensively studied because these systems represent a textbook example for the application of the Htickel 4w + 2 rule57. Recently, Rabideau and coworkers were able to establish the first crystal structure of a dilithiated COT system by reducing dibenzo[a,e]cyclooctatetraene (39) in TMEDA to its dianion (392 )58. The X-ray of 392-... 

Benzannelated cyclooctatetraene (COT) has attracted much interest due to the variability of isomers of the dibenzannelated COT and the possibility to prepare doubly and quadruply charged systems. Dibenzo[a,d]cyclooctatetraene dianion (392 ) was prepared in 1965 127). The H NMR spectrum of 392- was studied and compared with its isomer i.e. dibenzo[a,e]cyclooctatetraene dianion (402 ). The two dianions were prepared by potassium metal reduction of 39 and 40, respectively 128,129). These... 

Ru and Ir catalysts are poisoned by dibenzo[a,c]cyclooctatetraene, DCT, but DCT has little effect on heterogeneous Rh, Pd and Pd systems. This test is based on the idea that the homogeneous catalyst must have some substitution lability and will react with DCT to form a stable diene complex. The reactions of the Ir complex under hydrogenation conditions are shown in Scheme 9.3. ... 

The transition state for this reaction is a superantiaromatic (anti-Hiickel-dibenzo)-normal-Huckel-cyclooctatetraene. If the phase relationships in these reactions are examined, it is clear that there will be a phase inversion at both ring junctions. By proper choice of the basis set of AOs, these phase inversions can be eliminated, so the transition state is of Hiickel type. 

The palladium-catalyzed self-homocoupling of borylvinyl iodobenzene derivatives, which were available via the selective Suzuki-Miyaura cross-coupling of the gem-diboryl reagents with iodo compounds, resulted in a [4-1-4] cycloaddition, giving selectively dibenzo[a,c]cyclooctatetraenes in good yields (Scheme 8.41) [66]. 

Zhang, H., Wei, J., Zhao, E, Liang, Y., Wang, Z., Xi, Z. (2010). Selective synthesis of dibenzo[a,c]cyclooctatetraenes via palladium-catalyzed [4+4] eycUc homocoupling of boiylvinyl iodobenzene derivatives. Chemical Communications, 46, 7439—7441. 

Stiles M, Burckhardt U (1964) Thermal and photochemical rearrangement of substituted dibenzo[a, e]cyclooctatetraenes. J Am Chem Soc 86 3396-3397... 

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