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Protochemical and crystallographic studies on Dimethyl 9,10-Dihydro-9-Methyl-9,10-Ethenoanthracene-11,12-Dicarbosylate Pokkuluri, Phani Raj


The photochemistry of dimethyl 9,10-dihydro-9-methyl-9,10-etheno-anthracene-11,12-dicarboxylate (5) was studied in solution as well as in the solid state. Upon direct irradiation in solution, three products were obtained, two of which were identified as regioisomeric products of the di-π-methane rearrangement (derivatives of semibull-valene) and the other a derivative of dibenzocyclooctatetraene. When sensitized (by acetone, xanthone or benzophenone) in solution, only the di-π-methane rearrangement products were obtained, suggesting that the other photoproduct obtained in direct irradiation might be a singlet-derived product. In acetonitrile approximately 65% of the photochemical reaction observed was di-π-methane rearrangement. Solid state irradiations mainly resulted in di-π-methane rearrangement but with a reversal of product selectivity from that observed in solution. The X-ray crystal structure of the starting material indicates that one of the ester groups is conjugated to the central double bond while the other is non-conjugated. Radical stabilization through conjugation with the ester carbonyl group is consistent with the major product formed in the solid state. A regular decrease in the photoproduct selectivity was observed with the extent of grinding of the crystals, and this may be attributed to a lower regioselectivity at the surface than that within the bulk of the crystal. Irradiation in a KBr matrix resulted not only in producing more presumed singlet product, but also in the enhancement of the rate of the reaction. Also the singlet photoproduct was found to be produced more when pressure was applied in the solid state. Thus, the photochemical reaction of 5 was found to be sensitive to the environment in which the reaction is taking place. Irradiation of a solid solution of 1% xanthone and the starting material demonstrated the possibility of solid-state sensitization. The sensitized irradiations in the solid state were found to produce more of the presumed singlet product, and triplet-triplet annihilation was suggested as one of the possible explanations.

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