B Benzo-Derivatives of Mono-enes

If the isomerization of enediones to unsaturated cyclobutanediones described in the preceding were to occur with their benzo-derivatives, migration of a double bond out of the aromatic ring would be required, as illustrated below. Not surprisingly, this has not been observed. Instead, bisdecarbonylation and a-cleavage have been described in a number of preliminary reports. 

In the first case reported 71 benzonorbomedione (57), wavelength dependent behaviour was observed. Ketoaldehyde 58 and minor amounts of indene (59) were obtained above 300 nm while 59 was the major product at 254 nm. Indene was shown not to be formed from 58 under these conditions although the presumed precursor 

Analogous mono- and bisdecarbonylations were observed with benzobicyclo-[2.2.2]octenedione 65 where monoketone 66 was obtained in low yield and a number of products derived from presumed o-quinodimethane 67 were isolated the material balance was incomplete. In contrast to the behaviour observed with 63, sensitization by benzophenone gave the same product mixtures as direct irradiation. 

Mechanisms of these reactions have not been established. It should be noted that all reported experiments were performed in the notoriously poor hydrogen-donating solvents benzene or acetone with excitation into S2 of the dione. Reactions at long wavelengths in good H-donating solvents or in the presence of olefins might exhibit normal a-diketone photochemistry as has been observed 73 with 68. 

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