Substitution Reactions of Nitrothiophens. III. Radical and Ionic Reactions of 4- and 5-Nitro-2-thienyl-methyl and -ethyl Chlorides and Acetates

Abstract

The reactions of 4-nitro-2-thienyl-methyl and -ethyl chlorides and acetates with lithium 2-nitropropan-2-ide (1) give excellent yields of the C-alkylates, 2-(2-methyl-2-nitropropyl)- and 2-(1,2-dimethyl-2-nitropropyl)-4-nitrothiophen. These reactions proceed by a novel ionic substitution process involving initial nucleophilic attack on the thiophen ring. The 5-nitro derivatives react to give complex mixtures of products, which arise from S_RN1, S_N2, elimination, electron transfer and radical coupling processes. For example, 2-(1-chloroethyl)-5-nitrothiophen (23) reacts with the salt (1) under nitrogen to give the C-alkylate, 2-(1,2-dimethyl-2-nitropropyl)-5-nitrothiophen (24), formedby an S_RN1 reaction, the alkene, 2-(1,2-dimethylprop-1-enyl)-5-nitrothiopen (25), derived from (24) by elimination of nitrous acid, 1-(5-nitro-2-thienyl)ethanone (26) and 1-(5-nitro-2-thienyl)ethanol(27) produced by S_N2 O-alkylation, and 2,2'-(1,2-dimethylethene-1,2-diyl)bis(5-nitrothiophen) (28) formed by electron transfer and radical coupling reactions.