The Reactivity of Thiols with 1,4-Naphthoquinone Derivatives
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The reactivity of 1,4-naphthoquinones, particularly at the 2- and/or 3-positions, toward a nucleophile such as a thiolate anion, is very important in many biological events. We study the reactivity of twelve 1,4-naphthoquinone derivatives with various substitutents at the 2- and/or 3-positions toward a thiolate anion to compare intra- and intermolecular reactions and examine the reactivity difference due to the proximity effect. Some substituents are easily replaced by a thiolatenucleophile, whereas others are not. We found that an alkylamino group is the substituent that most strongly resists the reaction with a nucleophileintermolecularly. Conversely, in the reaction with the anion from 1,2-ethanedithiol, the ethylamino group of 2-bromo-3-ethylamino-1,4-naphthoquinone is easily substituted by the thiolate, forming 1,2,3,4-tetrahydro-1,4-dithia-9,10-anthracenedione (1) through an intramolecular reaction. Such a remarkable difference between an intermolecular and an intramolecular reaction could be explained by the proximity effect of the second thiolatenucleophile, or the high effective concentration of the second thiolate anion after the first reaction at the bromine group in the intermolecular setting. Furthermore, we found a negative correlation between the distance in reacting groups to the percent yield by reacting 2-bromo-3-methoxy-1,4-naphthoquinone with dithiols with carbon chains of different lengths. The greater the distance, the lower the yield. This relationship can be explained by the proximity effect.