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V. Cyclic Thionocarbonates

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    24067
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    Two basic procedures for the synthesis of cyclic thionocarbonates are in com­mon use. The first involves reacting a compound containing adjacent ­hydroxyl groups with N,N-thio­carbonyl­di­imidazole (eq 11).50 Most cyclic thionocarbonates are synthesized by this pro­cedure. The sec­ond approach involves initial formation of a stannylene complex and then treat­ment of this complex with thiophosgene (Scheme 8)51 or phenoxy­thio­car­bonyl chlor­ide52–54 (Scheme 9)54.

    II11(11).png

    II11s8.png

    II11s9.png

    A third, but seldom used, reaction for cyclic thionocarbonate for­ma­tion is one ­conducted under phase-transfer conditions. This synthesis is cap­able of producing either a bisxanthate55 or a cyclic thionocarbonate (Scheme 10).56 The critical factors in determining which type of product will be pro­duced are the timing of reagent addition and the relative amounts of the reagents used. To maximize the cyclic-thionocarbonate yield, methyl iodide needs to be added to the reaction mixture after the other reagents; also, the phase-transfer catalyst, and the remaining reagents, need to be limited to molar amounts equal to that of the substrate (Scheme 10).56

    II11s10.png

    Synthesis of a cyclic thionocarbonate by initial stannylene complex formation can be com­pli­cated if more than one complex is possible because a dynamic equi­librium will exist between the pos­sible structures.57–59 The equilibrium population of the various com­plexes is determined by their stability, which is a function of fac­tors such as ring strain, steric hindrance, and inductive effects. The relative amounts of the various complexes do not by themselves deter­mine final product dis­tribution because "the steric inaccessibility of the acti­vated oxygen atoms may retard or prevent a major complex from reacting, thus allowing a minor com­plex to determine the pro­duct".57 An illustration of how these fac­tors can cause quite different cyclic thionocarbonates to form from struc­tur­ally similar compounds is provided by the reactions shown in equa­tions 12 and 13.57

    II11(12).png

    II11(13).png

    Although there can be uncertainty about which cyclic thionocarbonate will form from com­pounds where more than one stannylene complex is pos­sible, this uncertainty disappears for mole­cules with cis-related, vicinal hydroxyl groups. For such compounds the major (some­times ex­clu­sive) pro­duct will come from a complex involving these cis-related groups (eq 1256).51,52,56

    In some situations a competition exists between formation of cyclic and noncyclic thiono­car­bon­ates. In the reaction shown in Scheme 11 there is such a competition between the cyclic thionocarbonate 27 and the noncyclic thionocarbonate 26.57 Complete cyclic thiono­car­bonate formation is only temporarily delayed if reaction is allowed to continue because compound 26 is converted into 27 under the reaction conditions.

    II11s11.png


    This page titled V. Cyclic Thionocarbonates is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.

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