Guanidine-Catalyzed Reductive Amination of Carbon Dioxide with Silanes: Switching between Pathways and Suppressing Catalyst Deactivation

A mechanistic investigation into the guanidine-catalyzed reductive amination of CO2, using a combination of H-1, Si-29 NMR, FT-IR, MS, and GC profiling, is reported. Inexpensive and readily available N,N,N',N'-tetramethylguanidine (TMG) was found to be an equally effective catalyst compared to more elaborate cyclic guanidines. Different catalytic pathways to formamide 2, aminal 4, and N-methylamine 3 were identified. A pathway to formamide product 2 dominates at 23 degrees C. Increasing the reaction temperature to 60 degrees C enables a competitive, higher-energy pathway to 4 and 3, which requires direct reduction of CO, with PhSiH3 to formoxysilane E. Reduction of aminal 4, in the presence of CO, and the catalyst, led to formation of a 1:1 ratio of 2 and 3. The catalyst itself can be formylated under the reaction conditions, resulting in its deactivation. Thus, alkylated TMGs were found to be more stable and more active catalysts than TMG, leading to a successful organocatalyzed reductive functionalization of CO2 with silane at 0.1 mol % catalyst loading (TON = 805 and TOF = 33.5 h(-1)).