Odourless Strategy for Deep Eutectic Solvent-Mediated Ring Opening of Epoxides with In Situ Generated S-Alkylisothiouronium Salts

Abstract

A general, straightforward and odourless ring-opening reaction allows the preparation of β-hydroxy sulfides from in situ generated S-alkylisothiouronium salts in urea–choline chloride-based deep eutectic solvent (DES). In addition, reaction of epoxides with thiourea in DES yields the corresponding thiiranes.

Key words

deep eutectic solvent - epoxide - odourless thiol chemistry

• References and Notes

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• 9 Deep Eutectic Solvent Preparation: The choline chloride–urea-based deep eutectic solvent was prepared according to the literature (see ref. 1). Urea (200 mmol) and choline chloride (100 mmol) were mixed, stirred and heated until a clear liquid was formed. This was used without any further purification.
• 10 General Procedure for Ring Opening of Epoxides with In Situ Generated Thiol in DES: A test tube, equipped with a magnetic stir bar, was charged with alkyl halide (1.0 mmol), thiourea (1.0 mmol), and the DES (0.5 mL), and the mixture was heated at 60 °C until the reaction was complete (monitored by TLC, or GC; usually 20 min). Next, epoxide (1.0 mmol), and aq NaOH (8 M, 0.4 mL) were added and the reaction was stirred at r.t. for 60–120 min. The crude reaction mixture was then diluted with EtOAc (10 mL), washed with aq NH₄Cl and H₂O to remove DES. The organic layer was dried over Na₂SO₄, filtered and evaporated under reduced pressure. The crude reaction products were purified by flash chromatography on silica gel column to afford the corresponding pure materials. All the products gave analytical data identical with those in the literature reports.
• 11 General Procedure for Conversion of Epoxides to Thiiranes: A mixture of epoxide (1.0 mmol) and thiourea (1.2 mmol), in DES (0.5 mL), was stirred at 60 °C for 80–120 min and, after completion of the reaction as indicated by GC, the reaction mixture was diluted with H₂O (5 mL) and extracted with Et₂O (10 mL). After drying over Na₂SO₄, filtering and evaporation of the solvent, the crude product was purified by silica gel column chromatography, eluting with EtOAc–n-hexane (1:10) to give the pure product.