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 K_e, or log k₁ and σ_p^– 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.