Meisenheimer-type adducts from thiophene derivatives. Part 2. Kinetic, thermodynamic, and 13C n.m.r. studies of substituent effects in the reaction of sodium methoxide with some 2-methoxy-3-nitro-5-X-thiophenes in methanol

Abstract

The rate and equilibrium constants for the formation of the Meisenheimer-type adducts from some 2-methoxy-3-nitro-5-X-thiophenes and sodium methoxide in methanol have been measured at various temperatures. The absence of linear correlation between log Ke, or log k1 and σp– constants has been interpreted in the light of the hyper-ortho relation in the thiophene ring. 13C N.m.r. chemical shifts for substrates (1a–g) and related gem-dimethoxy adducts (2a–g) have also been determined. The comparison between thermodynamic and l3C n.m.r. data suggests that most of the studied substituents favour the formation of the adducts mainly by stabilizing a fractional negative charge on C-5 of the ring.