Benzobarrelene

The photolysis of benzobarrelene. A, has been studied in considerable detail. Direct photolysis gives C, but when acetone is used as a photosensitizer, the di-rc-methane rearrangement product B is formed. 

HIGH TEMPERATURE BROMBVATION IVC f D. BROMINATION OF BENZONORBORNADIENE AND BENZOBARRELENE... 

In connection with our continuing interest in the temperature bromination reactions (ref. 2) we have been interested in the bromination reactions of benzonorbornadiene 1 and benzobarrelene 2. 

In the second part of this work we studied the bromination of benzobarrelene at room temperature and at 150°C. Some dipolar addition reactions to the benzobarrelene have been reported by Paquette and al (ref. 7). Surprisingly, there is no report in the literature on bromination of benzobarrelene. However, Barkash and al. (ref. 8) has reported bromination of tetrafluorobenzobarrelene 2 and isolated from complex reaction mixture only compounds 8 and 9 (Scheme 2). 

Scheme 4. Mechanism of formation of products in the bromination of benzobarrelene.

Next we studied high temperature bromination of benzobarrelene at 150 C. NMR analysis indicated that the reaction mixture was very complex and consisted of at least ten products. After repeated column chromatography combined with fractional crystallization we have been able to separate 18 compounds (Scheme 6). Four of them were bromoalcohol compounds 18, 12, 22 and 2fl. After high temperature bromination we expected three isomeric non-rearranged products with benzobarrelene skeleton and isolated 22, 22, and 24 in yields of 34, 9.3, and 6.2 %, respectively. Because of the very close structural similarity we were not able to make a clear-cut differentiation between the stereochemistry of 22 and 24-Therefore, we carried out an X-ray analysis (ref. 9) of the isomer 22-... 

Finally, we would like to conclude that high temperature bromination of bicyclic systems gives more non-rearranged products. If the molecule is more strained, the tendency to rearrange decreases as in the case of benzonorbornadiene. On the other hand, substituents at double bond of benzobarrelene retards also rearrangement (ref. 1). 

B. Benzobarrelene. A dry 1-1., three-necked, round-bottomed flask is equipped with a magnetic stirring bar, a combined gas-inlet tube and rubber septum, a 500-ml. pressure-equalizing dropping funnel, and a reflux condenser connected to a Nujol bubbler. The flask is charged with 500 ml. of dry tetrahydrofuran (Note 14) and 17 g. (0.74 mole) of sodium wire with a diameter of ca. 0.5 mm. The mixture is stirred and heated at reflux under an atmosphere of argon or nitrogen, and 50 ml. of freshly distilled tert-butyl alcohol is added. Immediately afterward a solution of 15 g. (0.051 mole) of... 

The spectral properties of benzobarrelene are as follows infrared (potassium iodide) cm. strong peaks 1460, 1325, 790, 750, 690, 660 proton magnetic resonance (chloroform-d) S (multiplicity, number of protons, assignment) 4.9 (multiple , 2, bridgehead H), 6.8-7.3 (multiple , 8, aryl and vinyl H). The reported melting point is 65.5-66°.From 20 g. of tetrachlorobenzo-barrelene the submitters obtained 8.3-8.8 g. (79-83%) of benzobarrelene, m.p. 64-65°. 

Although benzobarrelene has been used in a number of recent studies, the best available published synthesis" starts with the Diels-Alder reaction of j8-naphthol and maleic anhydride, affording benzobarrelene in ca. 1% yield after five further steps. Minor improvements allow small quantities of benzobarrelene to be prepared in an overall yield of ca. 10%. The reaction of benzyne with benzene is relatively inefficient, giving benzobarrelene in ca. 2% yield. When benzyne is generated by decomposition of benzenediazonium-2-carboxylate at high dilution in benzene, the yield of benzobarrelene is raised to 14%. The reactions of benzyne with other aromatic substrates are equally inefficient. 

The generation of pentachlorophenyllithium by the reaction of butyllithium with hexachlorobenzene has been reported previously by Rausch, Tibbetts, and Gordon. The present procedure for the preparation of benzobarrelene is based on the submitters previously published note." By this method 10-g. quantities of benzobarrelene may be obtained in ca. 3 working days without the use of large-scale apparatus. The generality of the procedure is shown by the examples given in Table I. 

An aliquot may be removed at this stage and analyzed by either gas chromatography or thin-layer chromatography to ensure that the reaction is complete. Benzobarrelene has a retention time of ca. 5 minutes in a gas chromatographic analysis under the conditions stated in Note 6, but with a column temperature of 104°. The completion of the reaction is also indicated by a purple coloration of the precipitated sodium chloride. 

Benzobarrelene is similar to barrelene in that rearrangement to benzo-cyclooctatetraene takes place from the singlet state while benzosemibullvalene is formed from the triplet state<44> ... 

The hydrogen-labeled benzobarrelene indicates that the formation of benzo-cyclooctatetraene from the direct photolysis takes place predominantly via benzovinyl bridging<44) j... 

Poly(l,4-naphthylenevinylenes) have been prepared by metathesis polymerization of benzobarrelenes [181,182] and the photoluminescence properties of homopolymers and block-copolymers have been studied in some detail [183]. PPV also has been prepared via ROMP of [2.2]paracyclophane-l,9-diene [184] and ROMP of a paracyclophene that contains a solubilizing leaving group [185]. The resulting polymer is converted to PPV upon acid catalysis at room temperature. ADMET of 2,5-dialkyl-l,4-divinylbenzenes using Mo or W catalysts has... 

Benzobarrelenes are an important class of molecule where the di-jr-mcthanc rearrangement is operative. Clearly, within this class there is a possibility that benzo-vinyl interactions can be in competition with vinyl-vinyl processes. The direct irradiation (X > 330 nm) of the benzobarrelene 313 in a variety of solvents (benzene, acetonitrile, methanol or hexane) affords two principal products 314 and 315 in a ratio of 1 1. The products are formed via a di-7r-methane process involving vinyl-vinyl bridging which... 

Both thermal (120 °C at 0.005r) and photochemical decomposition of the tosylhy-drazone salt (21) were proposed to proceed by initial formation of the cyclopropyl-carbene followed by fragmentation to biradicals (22) and (23), which proceeded to 1- and 2-vinylnaphthalene and benzobarrelene (24). ... 

Benzobarrelene derivative 4 rearranges to biphenyl 5 when heated under reflux in ethanol for 20 hours.2... 

It was noted earlier that the lithium carbenoids on reaction with alkyl halides afford 1-alkyl-1-halo-cyclopropanes. It is possible to effect endo selective monoalkylation, as illustrated with a benzobarrelene... 

Di-ir-Methane Rearrangement of Barrelene, Benzobarrelene, Dibenzobarrelene, and Related Derivatives... 

Barrelene, benzobarrelene and dibenzobarrelene derivatives are readily synthesized by Diels-Alder reactions of arenes with alkenes. For barrelene and related derivatives, the DPM rearrangement proceeds solely from the triplet state, giving semibullva-lenes (Scheme 4.1) [3, 4]. 

The acetophenone-sensitized irradiation of benzobarrelene 5 gives benzosemi-bullvalene 6 in a quantitative yield (Scheme 4.3) [6]. However, the direct irradiation... 

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