PHOTOCHEMICAL BEHAVIOR OF DOUBLY BRIDGED DEWAR BENZENES - SYNTHESIS OF PRISMANES

In this paper we report on the photochemistry of Dewar benzenes bridge d at the 1,4- and 5,6-positions and substituted at the 2,3-positions. It was found that simple alkyl substitution leads to prismanes only to a minor extent. Introduction of phenyl groups into the 2- or 2,3-posi tions gives the corresponding doubly bridged prismanes in yields up to 80%. Pentamethylene-bridged bis-Dewar benzenes 19 rearrange upon irra diation to 22. Irradiation of the doubly bridged Dewar benzene derivat ives with one ester group at the 5-position does not yield isolable pr oducts. However, introduction of one tert-butylsulfonyl group such as 30-33 gives almost quantitative yields of isolable prismanes if pentam ethylene or hexamethylene bridges are used. The nonbridged species, yl -1,4,5,6-tetramethylbicyclo[2.2.0]hexa-2,5-diene (44), aromatizes upon irradiation into 1-(tert-butylsulfonyl)-2,3,4,5-tetramethylbenzene (4 5). In contrast to the pentamethylene-bridged monosulfone 47, the corr esponding disulfone 38 undergoes no photochemical reaction. This could be explained by the X-ray structure, which reveals a strong interacti on between one of the tert-butyl groups and the pentamethylene chain b ridging the double bond in 38.