Radical formation during autoxidation of 4-dimethylaminophenol and some properties of the reaction products

Abstract

4-Dimethylaminophenol (DMAP), after intravenous injection, rapidly forms ferrihaemoglobin and has been successfully used in the treatment of cyanide poisoning. Since DMAP produces many equivalents of ferrihaemoglobin, it was of interest to obtain further insight into this catalytic process. DMAP autoxidizes readily at pH regions above neutrality, a process which is markedly accelerated by oxyhaemoglobin. The resulting red-coloured product was identified as the N- N,-dimethylamino) phenoxyl radical by EPR spectroscopy. The same radical was also produced by pulse radiolysis and oxidation with ferricyanide. The 4-( N- N-dimethylamino)phenoxyl radical is quite unstable and decays in a pseudo-first order reaction ( k = 0.4 sec −1 at pH 8.5,22°) with the formation of p-benzoquinone and dimethylamine. This observed decay rate is identical with the rate of hydrolysis of N, N-dimethylquinonimine. When a solution containing the phenoxyl radical was extracted with ether, half the stoichiometric amount of DMAP was recovered. Hence it is apparent that the phenoxyl radical decays by disproportionation yielding DMAP and N, N-dimethylquinomine. The latter product then quickly hydrolyses. The equilibrium of this disproportionation reaction is far towards the radical side, and the pseudo-first order hydrolysis controls the radical decay rate. p-Benzoquinone rapidly reacts with DMAP ( k 2 = 2 × 10 4M −1sec −1) with the formation of the 4- N, N-dimethylamino)phenoxyl and the semiquinone radicals. This reaction explains the autocatalytic phenoxyl radical formation during the autoxidation of DMAP. DMAP is not oxidized by H 2O 2 or O 12 ⊘ but the 4-( N- N-dimethylamino)phenoxyl radical is very rapidly reduced by O 2 ⊘ ( k 2 = 2 × 10 8M −1 sec −1. In addition, the phenoxyl radical is quickly reduced by NAD(P)H or GSH with the formation of NAD(P) + or GSSG. Since DMAP is also able to reduce two equivalents of ferrihaemoglobin (provided that the ferrohaemoglobin produced is trapped by carbon monoxide), electrophilic addition reactions of the phenoxyl radical seem unimportant in contrast to N, N-dimethylquinonimine. Hence, during the catalytic ferrihaemoglobin formation, DMAP is oxidized by oxygen which is activated by haemoglobin, and the phenoxyl radical oxidizes ferrohaemoglobin. This catalytic process is terminated by covalent binding of N, N-dimethylquinonimine to SH groups of haemoglobin (and GSH in red cells).