Formation of Aromatic O‐Silylcarbamates from Aminosilanes and Their Subsequent Thermal Decomposition with Formation of Isocyanates

A novel phosgene‐free route to different isocyanates starts from CO2 and aminosilanes (cf. silylamines) to form so‐called carbamoyloxysilanes (O‐silylcarbamates), i. e., compounds with the general motif R1R2N−CO−O−SiR3R4R5 as potential precursors. We focused on the insertion reaction of CO2 into Si−N bonds of substrates with cyclic (mostly aromatic) amine substituents, i. e., PhNHSiMe3, (PhNH)2SiMe2, PhCH2NHSiMe3, p‐(MeO)C6H4NHSiMe3, o‐C6H4(NHSiMe3)2, 1,2‐C6H10(NHSiMe3)2, o‐C6H4(NHSiMe3)(CH2NHSiMe3) and 1,8‐C10H6(NHSiMe3)2. Compared to previously investigated aminosilanes these reactions are hindered due to the reduced nucleophilicity/basicity of the N‐atoms. Whereas slightly increased CO2 pressure (8 bar) and prolonged reaction times (24 h) were sufficient to overcome hindrance of the insertion into, e. g., PhNHSiMe3, intermolecular effects in some two‐fold NHSiMe3 functionalized substrates led to partial mono‐insertion (e. g., into o‐C6H4(NHSiMe3)(CH2NHSiMe3)) or intra‐molecular condensation of the intermediate insertion product in case of 1,8‐C10H6(NHSiMe3)2 to form 1H‐perimidin‐2(3H)‐one and other side products. Thermal treatment of mono‐silylated O‐silylcarbamates RHN−CO−O−SiR’3 resulted mainly in the formation of substituted ureas (RHN)2CO, whereas desired isocyanates could not be detected in these cases. Therefore, we continued our studies focussing on N,O‐bissilylated precursors, which were obtained by an additional N‐silylation of the O‐silylated carbamates. This allowed the successful formation of isocyanates. As a sole byproduct hexamethyldisiloxane is formed. In all cases, known as well as yet unknown substances were characterised by 1H, 13C and 29Si NMR spectroscopy, along with X‐ray diffraction analysis for crystallized solids. A novel phosgene‐free approach towards isocyanates using aminosilanes and carbon dioxide is reported. The four‐step overall synthesis of isocyanates includes a pressure supported CO2 insertion into the Si−N bonds to form O‐silylcarbamates and their transformation into isocyanates by thermal treatment.