Synthesis of 2,3-dialkyl-6,7-dichloro- and 2,3-dialkyl-6,7-dibromo-1,4-naphthoquinones

Thermal treatment of 1,4-benzoquinone and its 2-methyl and 2,3-dimethyl homologues with 3,4-dichlorothiophene 1,1-dioxide followed by oxidation affords the corresponding 6,7-dichloro-1,4-naphthoquinones; the parent compound and its 2-methyl homologue can be alkylated at the free quinonoid positions using the alkanoic acid–persulphate–silver ion system. Nitration of 2,3-dibromonaphthalene yields 2,3-dibromo-5-nitronaphthalene, which via sequential reduction, diazotisation, hydrolysis, and oxidation with Fremy's salt gives 6,7-dibromo-1,4-naphthoquinone, which can be similarly alkylated.