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

Abstract A novel phosgene‐free route to different isocyanates starts from CO 2 and aminosilanes (cf. silylamines) to form so‐called carbamoyloxysilanes ( O ‐silylcarbamates), i. e., compounds with the general motif R 1 R 2 N−CO−O−SiR 3 R 4 R 5 as potential precursors. We focused on the insertion reaction of CO 2 into Si−N bonds of substrates with cyclic (mostly aromatic) amine substituents, i. e., PhNHSiMe 3 , (PhNH) 2 SiMe 2 , PhCH 2 NHSiMe 3 , p‐(MeO)C 6 H 4 NHSiMe 3 , o‐C 6 H 4 (NHSiMe 3 ) 2 , 1,2‐C 6 H 10 (NHSiMe 3 ) 2 , o‐C 6 H 4 (NHSiMe 3 )(CH 2 NHSiMe 3 ) and 1,8‐C 10 H 6 (NHSiMe 3 ) 2 . Compared to previously investigated aminosilanes these reactions are hindered due to the reduced nucleophilicity/basicity of the N‐atoms. Whereas slightly increased CO 2 pressure (8 bar) and prolonged reaction times (24 h) were sufficient to overcome hindrance of the insertion into, e. g., PhNHSiMe 3 , intermolecular effects in some two‐fold NHSiMe 3 functionalized substrates led to partial mono‐insertion (e. g., into o‐C 6 H 4 (NHSiMe 3 )(CH 2 NHSiMe 3 )) or intra‐molecular condensation of the intermediate insertion product in case of 1,8‐C 10 H 6 (NHSiMe 3 ) 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) 2 CO, 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 1 H, 13 C and 29 Si NMR spectroscopy, along with X‐ray diffraction analysis for crystallized solids.