Investigation of a ‘reverse’ approach to extended porphyrin systems. Synthesis of a 2,3-diaminoporphyrin and its reactions with α-diones
Abstract
A 2,3-diaminoporphyrin has been synthesised for the first time and its reaction with α-diones has been examined. Two regioselective routes to the precursor 2-amino-3-nitroporphyrins have been established. 2-Aminoporphyrins are directly nitrated on the porphyrin ring in the 3-position while 2-nitroporphyrins react with acylamide ions regioselectively at the 3-position and can be converted to the required 2-amino-3-nitroporphyrins by hydrolysis of the amide bond. 2,3-Diamino-5,10,15,20-tetraarylporphyrins are prepared by transfer hydrogenation of the corresponding 2-amino-3-nitroporphyrins but are relatively unstable. Electrochemical measurements show that 2,3-diaminoporphyrins are easily oxidised and this probably accounts for their instability. Condensation of the 2,3-diaminoporphyrin 29 with the α-diones benzil and cyclohexane-1,2-dione occurs readily and in good yield to give the ring annulated systems 31 and 32, respectively. Reaction with o-benzoquinone, however, causes decomposition of the diaminoporphyrin 29 making ‘reverse’ synthesis of quinoxalinoporphyrins and related polyporphyrin systems much less attractive than the alternate approach involving condensation of a porphyrin-2,3-dione with o-phenylenediamine and related diamines.