Stability of pyridiniumylquinones to aqueous media: The formation of pyridinium-oxy zwitterionic quinones

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 H-0 technique, the aqueous pK(a)'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)-3,6-dioxy-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 E(1/2) = -0.69 and -1.10 V vs SCE.