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

Abstract

A mechanistic investigation into the guanidine catalyzed reductive amination of CO 2 , using a combination of 1 H, 29 Si 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 CO 2 with PhSiH 3 to formoxysilane E. Reduction of aminal 4, in the presence of CO 2 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 CO 2 with silane at 0.1 mol% catalyst loading (TON = 805 and TOF = 33.5 h-1).

Homogeneous Catalytic Reduction of CO2 with Silicon‐Hydrides, State of the Art

“…However, there is a lack of knowledge about the reaction mechanism of these catalytic processes. The proposed reaction paths are summarized in Scheme ,…”

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C−N Bond Forming Reactions of Amines with CO2

“…More nucleophilic amines also react faster with formoxysilane to form formamides. Silylcarbamates observed in the N-formylation reaction [15,20] should also be considered, but so far have not been investigated.…”

“…[19] Nevertheless,insome cases the reaction follows ad ifferent pathway and the formoxysilane intermediate is replaced by silylcarbamate (see below). [15,20] In contrast, the reaction with hydroboranes is less well understood and the involvement of formoxyboranes remains rather speculative. [21][22][23][24] In all cases,t he CO 2 reduction stage is regarded as rate determining and is the focus of catalyst development.…”

“…[40,41] Forcomparison, with glycine betaine as the catalyst the reaction yields formamides at 50 8 8Cand 10 bar of CO 2 ,at708 8Cand 1bar N-methylamines and at 50 8 8Cw ith 1equivalent of CO 2 aminals. [40] Aminals were also observed as intermediates in the synthesis of Nmethylamines with TMG, [20] CsCOOH, [55] K 2 WO 4 [48] and [NHex 4 ]ReO 4. [57] Aminals are easily reduced to N-methylamines,e ven under ambient conditions,but the reaction requires CO 2 .…”

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C−N Bond Forming Reactions of Amines with CO2

“…The rate‐determining step in the N‐formylations of these amines appears to be shifted from the CO 2 reduction to the aminolysis step. In addition, changes in the base strength showed limited effect on the reaction rate and silylcarbamate together with formic acid were observed as the reaction intermediates instead of, or together with, the formoxysilane . Extensive exchange of silane substituents, which included the simultaneously observed formic acid, was observed by EXSY NMR experiments (Scheme ).…”