Alkali-metal-catalyzed synthesis of isoureas from alcohols and carbodiimides

Abstract

International audienceCurrent synthetic methods for the formation of isoureas rely on the addition of the alcohol (R'OH) to the corresponding carbodiimide (RN=C=NR). State-of-the-art catalysts rely on transition metal and actinide complexes. Copper (CuCl, CuCl$_2$, Cu$_2$O) and zinc (ZnCl$_2$) salts are reported to act as Lewis acids , able to enhance the electrophilicity of the carbodiimide reagent. Recently, in 2016, Eisen described new actinide complexes (U[N(SiMe$_3$)$_2$]$_3$ and [(Me$_3$Si)$_2$N]2An[κ$^2$ (N,C)-CH$_2$Si(CH$_3$)$_2$N(SiMe$_3$)] (An = Th or U)) able to catalyze the formation of isoureas under mild conditions. Capitalizing on our knowledge on the chemical reactivity of guanidine bases and alkali metal like Lewis acid, we have recently designed the first alkali metal catalysts able to facilitate the addition of alcohols to carbodiimides. The role and influence of the alkali metal has been investigated by controlling the Lewis acidity of the alkali metal with exogenous ligands. Experimental studies, combined with DFT calculations, offer a new vision on the active role of alkali metal cations in catalysis