Alkylation orientation rules in conjugate addition of Grignard reagents to nitropyrrole and nitrothiphene systems

Conjugate addition of various Grignard reagents to 1-alkyl-2-nitropyrroles and 2-nitrothiophene has been investigated. 1-Alkyl-2-nitropyrroles undergo alkylation at 3 and 5 positions with prebalence of the latter isomer. On the contrary, in 2-nitrothiophene system, formation of the 3-isomer prevails. In both systems, a bulkier Grignard reagents favours the 5-isomer formation. This reaction allows 2-alkyl-3-nitrothiophenes and pyrroles to be synthesized with a one-pot procedure instead of the classical multi-stage reaction.