Substituent effects and structural limitations in the conversion of 3,3-diarylphthalides to 4,4-diaryl-3,4-dihydro-1(2H)-phthalazinones.

Abstract

Heating of crystal violet lactone (CVL) (1a) and its analogs with hydrazine hydrate afforded a single product in each case, to which the 4, 4-diaryl-3, 4-dihydro-1 (2H)-phthalazinone structure (2) was assigned. Kinetic data showed that the introduction of a dimethylamino substituent in 3, 3-diarylphthalides (1) led to an increase in the rate of formation of the dihydrophthalazinone (2). Thus, the reaction seems to occur by a mechanism which involves a carbonium intermediate. Replacement of 3, 3-diaryl groups on the phthalide by a xanthyl group resulted in the formation of an N-aminophthalimidine derivative on similar treatment. The dihydrophthalazinone (2a) exhibited a greenish-blue color on exposure to a weak acid, indicative of heterolytic ring cleavage to form the dye cation, whereas treatment of 2a with a strong acid resulted in a facile conversion to the N-aminophthalimidine (6) and CVL (1a) in a ratio of ca. 2 : 1.