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[6]Radialene (2d) and its hexa- (2a) and dodecamethyl (2c) derivatives were subjected to several novel chemical reactions. Pyrolysis of 2a under flash vacuum conditions provided a mixture of 1,5-hydrogen shift products 9 and 10, and the benzocyclobutenes 5 and 6, produced by electrocyclization of an intermediate o-xylylene derivative 7. At the higher end of the investigated temperature range (200−400 °C) the formation of cyclization products 11 was also observed. The hydrocarbon 2c isomerized to the benzocyclobutene 12 under these conditions. Photolysis at 254 nm converted 2c into a novel isomer that, according to X-ray structural analysis, has the unusual twist configuration 14. Compound 2c was photoisomerized through photoinduced hydrogen shift reactions to 15, and the stable p-xylylenes 16 and 17. Dichloro- and dibromocarbene add to 2d with formation of the rotaradialene anti adducts 19a and 19b, respectively, the structures of which were also established by X-ray structural analysis. With dichlorocarbene, 2a provided the three dichlorocarbene adducts 20, 22, and 23, whereas methylenation with diiodomethane/trimethylaluminium afforded a complex product mixture from which the monoadduct 21 could be isolated. The analogous product 24 and the bisadduct 25 were obtained from 2c under the same conditions. Epoxidation of 2a with m-chloroperbenzoic acid gave the monoadduct 26, together with higher epoxidation products, which, however, could not be separated. Depending on the reaction conditions, 2c could be oxidized with m-chloroperbenzoic acid to give the epoxides 27, 28, and 29. Hydrochlorination of 2a gave a complex mixture of addition products, which was converted into the olefins 9 and 10 by base treatment, showing that the addition step takes place less regioselectively than previously assumed. With Fe2(CO)9, 2a was converted into the iron tricarbonyl complex 33 in poor yield. Repetition of the literature procedure for the preparation of 2a allowed the isolation of novel diastereomers of this oldest hexaradialene; according to NMR experiments the methyl substituents of this hydrocarbon are arranged as shown in the structure cccaca-2a. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) |
