From 3,4-dinitrothiophene to 1,2-diaryl-4-nitrobenzenes through (E,E,E)-1,6-diaryl-3-nitro-1,3,5-hexatrienes

The reactions between 1,2-bis(diethylamino)-2,3-dinitro-1,3-butadiene, easily obtainable by ring-opening of 3,4-dinitrothiophene with diethylamine, and arylmethylmagnesium chlorides in THF at 0°C furnished good yields of (E,E,E)-1,6-diaryl-3-nitro-1,3,5-hexatrienes. To explain the formation of the hexatrienes, a mechanism is advanced which involves, in particular, tautomerisation of the intermediate 1,6-diaryl-3,4-dinitro-2,4-hexadienes followed by base-induced elimination of nitrous acid. The electrocyclic disrotatory conversion of the hexatrienes into the corresponding 5,6-diaryl-2-nitro-1,3-cyclohexadienes was analysed by 1H NMR spectroscopy and exploited for the high-yielding synthesis of 1,2-diaryl-4-nitrobenzenes employing, as oxidants, either DDQ or iodine in the presence of cyclohexene oxide as HI scavenger.