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 1H, 29Si 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 °C. Increasing the reaction temperature to
60 °C enables a competitive, higher-energy pathway to 4 and 3, which requires direct reduction of CO2 with PhSiH3 to formoxysilane E. Reduction
of aminal 4, in the presence of CO2 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).