A SIMPLE METHOD FOR PREPARING MAGNESIUM PORPHYRINS

The synthesis of magnesium tetraarylporphyrins has been investigated u sing magnesium halides in noncoordinating solvents with hindered amine bases. The rate of reaction increases in the series MgCl2 << MgBr2 < MgBr2 . O(Et)(2) < MgI2. Considerable latitude exists in selecting amo ng magnesium reagents (MgBr2, MgBr2 . O(Et)(2), MgI2), solvents (tolue ne, CH2Cl(2), CHCl3), and bases (triethylamine, diisopropylethylamine, 2,2,6,6-tetramethylpiperidine) for efficient metalation of tetrapheny lporphyrin at room temperature. Thus treatment of a toluene, CH2Cl2, o r CHCl3 solution of tetraphenylporphyrin with excess MgBr2, MgBr2 . O( Et)(2), or MgI2 and triethylamine at room temperature quantitatively a ffords the magnesium chelate in <10 min. Tetramesitylporphyrin is conv erted to the magnesium chelate with MgI2 and diisopropylethylamine in CH2Cl2 at room temperature in 10 min or by reaction with MgBr2 . O(Et) (2) and triethylamine in toluene at 60 degrees C for 1 h. akis(2,6-dim ethoxyphenyl)-porphinato]magnesium(II) was formed in similar fashion. The reaction conditions are compatible with porphyrins bearing (trimet hylsilyl)ethynyl groups, and the reactions can be performed in the pre sence of zinc tetraphenylporphyrin without transmetalation. This appro ach is fundamentally distinct from that with DMF-MgCl2, which is desig ned to achieve high mutual solubility of the metal ion and free base p orphyrin at elevated temperature. The facile magnesium insertion achie ved with MgBr2, MgBr2 . O(Et)(2), or MgI2 at room temperature is attri buted to the lability of their ligands, their partial organic solubili ty, and the limited stability of their crystal lattices relative to th e porphyrin magnesium chelate. A noncoordinating milieu is essential t o avoid forming octahedral complexes of magnesium that are more stable than the magnesium porphyrin. The ability to form magnesium tetraaryl porphyrins under gentle conditions enables biomimetic studies where zi nc porphyrins have previously been used.