Temperature Dependence of the Reduction of Phthalic Thioanhydrides by NaBH4: Competition between 3-Hydroxythiolactone and Phthalide Formation

The reduction of phthalic thioanhydrides with NaBH4 at between 0 and −20 °C leads to the selective formation of 3-hydroxy-2-thiophthalides, whereas higher temperatures favour the formation of the corresponding phthalide derivatives. A mechanism is proposed that can explain these observations. Kinetic control at low temperatures leads to an improved stability of the intermediate alkoxide, thus allowing isolation of the γ-hydroxythiolactone after subsequent acidification of the reaction mixture. In this way the formation of the thermodynamically favoured phthalide derivative is avoided. Further reduction of the γ-hydroxythiolactone with AcOH/57% aqueous HI yields 2-thiophthalides, useful precursors for conducting polymers. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)