Photochemical behavior of doubly bridged Dewar benzenes. Synthesis of prismanes.
The photochem. of Dewar benzenes bridged at the 1,4- and 5,6-positions and substituted at the 2,3-positions is reported. Simple alkyl substitution leads to prismanes only to a minor extent. Introduction of Ph groups into the 2- or 2,3-positions gives the corresponding doubly bridged prismanes in yields ?80%. Pentamethylene-bridged bis-Dewar benzenes rearrange upon irradn. to compd. I. Irradn. of the doubly bridged Dewar benzene derivs. with one ester group at the 5-position does not yield isolable products. Introduction of one SO2CMe3 group such as in dewar benzene II (R = H, Me, PhCH2, Ph) gives almost quant. yields of isolable prismanes if pentamethylene or hexamethylene bridges are used. The non-bridged species, 2-tert-butylsulfonyl-1,4,5,6-tetramethylbicyclo[2.2.0]hexa-2,5-diene, aromatizes upon irradn. into 1-(tert-butylsulfonyl)-2,3,4,5-tetramethylbenzene. In contrast to the pentamethylene-bridged monosulfone, the corresponding disulfone II (R = SO2CMe3) undergoes no photochem. reaction. This is explained by the x-ray structure, which reveals a strong interaction between one of the tert-Bu groups and the pentamethylene chain bridging the double bond in II (R = SO2CMe3).
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