Dienediones and their Benzo-Derivatives

it might be possible to observe intermediate 72 by irradiating at low temperature. This result has been obtained in preliminary studies55 although results have not been nearly so clear-cut as might be desired. Compound 73, which is converted quantitatively into l,2,3,4-tetrachloro-5,6-diphenylbenzene (76) upon irradiation in the visible at room temperature, was irradiated in a glass at 436 nm and 77 K. A new 

0 Identical results in benzophenone-sensitized reactions. b Similar results were obtained with a monoimine. 

The same question arises in photolysis of benzo-bicyclo[2.2.2]octadienediones (77) which are summarized in Table 5. In all cases where long wavelength light was used at room temperature, a new maximum at about 500 nm was observed. As with 73, intensity of this maximum built up to a low level at the beginning of irradiation, remained constant, and finally decreased. The simplest explanation for the results is that isomerization of 77 to cyclobutanedione 78 competes with direct bisdecarbonyla-tion to naphthalene (79). This is supported by an nmr-study 61 in which it was shown that, within the limits of sensitivity of the method, naphthalene (R = n-propyl) was 

Finally, dibenzobicyclo[2.2.2]octadienediones (80) under all conditions of irradiation 39,82), even at 77 K and 436 nm23), gave only anthracene without evidence for any intermediate. 

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