Stability of Pyridiniumylquinones to Aqueous Media: The Formation of Pyridinium−Oxy Zwitterionic Quinones

Abstract

The stability of perpyridiniumyl- and halopyridiniumylbenzoquinones was investigated. Reactions of p-chloranil and p-fluoranil with pyridines, followed by hydrolysis under mild conditions, gave 2,3-dihalo-5-oxy-6-(pyridinium-1‘-yl)-1,4-benzoquinones 1 and 2,5-dioxy-3,6-bis(pyridinium-1‘-yl)-1,4-benzoquinone 2. Using the H0 technique, the aqueous pKa's for two of these pyridinium−oxy zwitterions were determined to be −3 for 2,5-bis(4‘-tert-butylpyridinium-1‘-yl)-3,6-dioxy-1,4-benzoquinone (2) and −2 for 2,3-dichloro-5-oxy-6-(4‘-tert-butylpyridinium-1‘-yl)-1,4-benzoquinone (1a). In addition, it was found that a pyridinium-substituted anthraquinone, 1-[4‘-(dimethylamino)pyridinium-1‘-yl]anthraquinone (3), is quite stable toward hydrolysis conditions and shows two reversible reductions at E1/2 = −0.69 and −1.10 V vs SCE.