UNUSUAL PRODUCTS IN THE PHOTOCHEMICAL REACTIONS OF DIBENZOBARRELENES. James Trotter & John R. Scheffer. Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1.

Photochemical reactions frequently give products with unusual molecular structures. Photolyses of derivatives of dibenzobarrelene (9,10-dihydro-9,10-ethenoanthracene) give two types of photoproduct: (i) dibenzosemi-bullvalenes, produced via the triplet excited state by the well-established di-pi-methane mechanism; (ii) dibenzocyclooctatetraenes (COTs), via the singlet excited state and a (2+2) intramolecular cycloaddition mechanism. Photolysis of a 9,10-dimethyl-11,12-dicarboxylate derivative in solution in acetone (a triplet sensitizer) gives the expected dibenzosemibullvalene photoproduct. In benzene solution a COT is formed, but this photoproduct has an abnormal substitution pattern which is not explicable in terms of a (2+2)-cycloaddition mechanism. In solid-state photolysis the major product is a new type, with a pentalene structure, which provides an important clue in deriving a mechanism for its formation, involving a pentalene-like biradical intermediate; this mechanism also accounts for the formation of the unexpected COT. Further studies indicate that derivatives with 3 or 4 substituents give abnormal COTs, while those with 2 substituents give (2+2)-cycloaddition COTs, so that the differences in behaviour probably result from differences in steric crowding. Derivatives with only one substituent give no COTs, but new hydrogen-abstraction products.