Reactions of Various Thione-compounds with Epoxides. A New Method for the Transformation of the C=S Bond into the C=O

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The reactions of propylene oxide (PO) with various thione-compounds containing two α-hetero atoms including dithiocarbamates, thioncarbamates, trithiocarbonates and thioureas, and their cyclic analogs were studied in the presence of triethylamine or boron trifluoride etherate as a catalyst. It was found that most of these thione-compounds reacted with PO at 170–180°C to afford the corresponding one-compounds in good yields. This type of reaction may provide a new method for the transformation of the C=S bonds in thione-compounds into the C=O bonds. Boron trifluoride etherate appeared to be more effective than triethylamine as the catalyst. The mechanism which involves initial cycloaddition of PO to the C=S bond followed by elimination of propylene sulfide was proposed for the transformation. The reactivities of these thione-compounds were significantly affected by whether they are cyclic or open-chain and by the nature of the α-hetero atoms: the cyclic thione-compounds were more reactive then the open-chain analogs except for a pair of thioureas and the enhancement by the α-hetero atoms was in the order: N,O≈N,N>N,S>S,S. Both observations may be interpreted in terms of basicity of the thione-compounds. These results also support the proposed mechanism for the formation of 2-oxazolidones from 2-substituted imino-1,3-dithiolanes with epoxides in our preceding paper.
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