Synthesis of O-Ethyl Thioformate: A Useful Reagent for the Thioformylation of Amines

Abstract

O-Ethyl thioformate has been synthesized from triethyl­orthoformate and hydrogen sulfide gas using a Brønstead acid catalyst. The product can be isolated as a neat liquid in 83% overall yield. Both the crude and purified thiolate can be used to thioformylate a variety of amines in good to excellent yields.

Key words

hydrogen sulfide - thioformate - sulfur - amines - condensation

References and Notes

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16

In our hands, the addition of H₂S to 7 g triethylorthoformate with 2 mol% HClO₄ (70 wt% aq solution) using a balloon (as described in ref. 6) yielded 75% O-ethyl thioformate with 25% ethyl formate. Using this procedure, the formation of O-ethyl thioformate decreased with an equal increase in the formation of ethyl formate as the scale of the reaction increased.

19

Hydrogen sulfide was charged to the reaction mixture at 2.1 bar. Consumption of H₂S was monitored as a decrease in pressure, and additional H₂S was added to maintain reaction pressure between 1.7 and 2.1 bar. The reaction was judged to be complete when the pressure change over 30 min was <6.9˙10^-² bar.

20

All excess H₂S was scrubbed by venting through a solution of aq NaOH and NaOCl.

21

O-Ethyl thioformate purified by distillation typically contains 3 wt% ethyl formate as an impurity. The presence of ethyl formate does not affect the thioformylation chemistry described herein.

22

When stored under nitrogen, 1 is stable for 1 month at r.t. and for 5 months at -20 ˚C. See Supporting Information.

23

Details available in Supporting Information.