Nucleophilic Addition Reactions of 2-Methyl-1,4-naphthoquinone Epoxide
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The reactions of 2-methyl-1,4-naphthoquinone epoxide with nucleophiles are studied here. In our study, 2-methyl-1,4-naphthoquinone was reacted with sodium cyanate and sodium thiocyanate in methanol. In the reaction of sodium thiocyanate and 1,4-naphthoquinone epoxide in acidic conditions, 2-methyl-3-thiocyano-1,4-naphthoquinone (1) was found to be formed as the major product along with 2-hydroxy-3-methyl-1,4-naphthoquinone (2) in a 4:1 ratio. The purity of 2-methyl-3-thiocyano-1,4-naphthoquinone was found to increase with an increase of the acidity of the reaction mixture. In non-acidic condition, sodium thiocyanate and 1,4-naphthoquinone epoxide formed a dimer, bis-3-(2-methyl-1,4-naphthoquinoyl) sulfide (3), and bis-3-(2-methyl-1,4-naphthoquinoyl)-4,4-dimethyl acetal sulfide (4). We propose that the dimer forms by the formation of 2-methyl-3-thiocyano-1,4-naphthoquinone which further reacts with 2-methyl-1,4-naphthoquinone epoxide using the excess sodium thiocyanate in the reaction medium as a base catalyst. The reaction of sodium cyanate and 1,4-naphthoquinone epoxide in a microwave reactor at 65˚C in both acidic and non-acidic conditions produces 2-amino-3-methyl-1,4-naphthoquinone (5). When an excess of acid was used (10:1), 2-chloro-3-methyl-1,4-naphthoquinone (6) was formed, with hydrochloric acid acting as a nucleophile (50% yield). With a longer reaction time, in excess acid, 2-methoxy-3-methyl-1,4-naphthoquinone (7) was formed at a 40% yield. Thus, our results show pH to be an important factor in determining the products of these reactions. Also, we can conclude that between sulfur and nitrogen, sulfur is a stronger nucleophile and that between nitrogen and oxygen, nitrogen is a stronger nucleophile.