Dipyrromethane + dipyrromethanedicarbinol routes to an electron deficient meso-substituted phlorin with enhanced...

Two dipyrromethane + dipyrromethanedicarbinol routes to a meso-substituted phlorin bearing electron-withdrawing pentafluorophenyl substituents (TpFPPhl) were investigated in an attempt to obtain a phlorin with enhanced stability toward light and air and to explore the application of dipyrromethanecarbinol chemistry to the preparation of phlorins. For each route, a systematic survey of reaction parameters for the two-step, one-flask reaction leading to TpFPPhl was performed. The analytical-scale reactions were monitored for yield of TpFPPhl by HPLC. Sharp differences were observed in the yield of TpFPPhl afforded by the two synthetic routes. The most promising reaction condition (TFA catalysis, 100 mM) was performed on a preparative scale providing TpFPPhl in a yield of 45% (189 mg). The stability of the electron-deficient phlorin in dilute solution upon exposure to light and air was probed in a number of solvents, and decomposition was monitored by UV-vis spectroscopy and HPLC. Many of the solutions of TpFPPhl were found to be quite stable for periods of approximately 8 h, with decomposition requiring exposure periods of several days. Taken together, this work contributes an efficient synthesis of a meso-substituted phlorin of practical stability and provides further insights toward the adaptation of dipyrromethanecarbinol chemistry to the preparation of diverse porphyrinoids.