HIGH-INTENSITY LASER-JET PHOTOCHEMISTRY - FORMATION OF BENZOCYCLOBUTENEDIONE FROM 1,2,3-INDANTRIONE VIA TRANSIENT TARGETING

The photochemistry of 1,2,3-indantrione (1) under high-intensity (laser-jet) conditions has been investigated. Irradiation of trione 1 in a benzene solution under laser-jet conditions produces a complex mixture in which benzocyclobutenedione (2) is observed to be the sole high-intensity product (ca. 11% yield, six cycles, 3.5 W MLUV argon ion laser) as compared to conventional irradiation (dispersed laser beam, 20 min, 3.5 W MLUV argon ion laser). Under similar conditions, done 2 was not observed to undergo photolysis in the laser-jet (mass balance ca. 90%). Additionally, phthalic anhydride, (E)-1,1'-bis-isobenzofuranvlidene-3,3'-dione ((E)-4), and 2-benzopyrano[4,3c]benzopyran-6,12-dione (5) were identified also in both the laser-jet and conventional irradiations of both trione 1 and dione 2. In view of the previously reported product studies, it is proposed that the formation of dione 2 under laser-jet conditions occurs by a photochemical electrocyclization of the intermediary cyclohexadiene-1,2-bisketene (B). The decarbonylation of trione 1 to bisketene B and the electrocyclization of the transient bisketene B to dione 2 constitute an unprecedented sequential multiple-photon transformation of polyketones in solution, which has been uncovered through product studies by using the novel laser-jet technique.