Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

Abstract

aThe simple perrhenate salt [NIJhexyl) 4 ]ij(ReO 4 )] acts as a catalyst for the reduction of organic carbonyls and carbon dioxide by primary and secondary hydrosilanes. In the case of CO 2 , this results in the formation of methanol equivalents via silylformate and silylacetal intermediates. Furthermore, the addition of alkylamines to the reaction mixture favours catalytic amine N-methylation over methanol production under certain conditions. DFT analysis of the mechanism of CO 2 reduction shows that the perrhenate anion activates the silylhydride forming a hypervalent silicate transition state such that the CO 2 can directly cleave a Si-H bond. IntroductionThe hydrosilylation of CO 2 to silylformates and silylethers is an attractive route for CO 2 utilisation as, unlike hydrogenation, this reaction is exergonic due to the comparative ease of Si-H bond activation and the strength of the Si-O bond, and the availability of relatively inexpensive and environmentally benign hydrosilanes. 66 Important to this chemistry is the formation of a lipophilic ion pair in which electrostatic and hydrogen-bonding interactions are enhanced in the hydrophobic phase. This catalyst has the advantages of ease of synthesis and manipulation and lack of air-sensitivity, and the recovery of the precious metal is driven by straightforward reverse phase-transfer. Given the efficacy of this new catalyst system and the use of high oxidation state rhenium oxo complexes in reduction catalysis, it is shown here that perrhenate can act as a catalyst for the reduction of carbon dioxide to methanol equivalents by hydrosilanes, that the methylation of alkylamines using CO 2 as the C 1 source occurs under similar catalytic conditions, and that this method can be used to reduce aldehydes and ketones to silylalcohols. Results and discussion Reduction of carbon dioxideInitially, the reaction between CO 2 (2.5 bar), PhSiH 3 (2.0 mmol), and pyridinium perrhenate 1 (2.0 mol%) at 80°C in C 6 D 6 in a Teflon-tapped NMR tube was monitored by 1 H NMR spectroscopy. However, only 15% consumption of hydrosilane is seen under these conditions after 16 h ( Fig. S2 and S3, ESI †). In contrast, when the reaction is carried out using the simple lipophilic quaternary ammonium salt [NIJhexyl) 4 ]ijReO 4 ] 2 (2.5 mol%) with PhSiH 3 in C 6 D 6 at 1 bar of

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

“…[48] Although the N-methylation reaction conditions have not been optimized for all Nformylation catalysts,s imilar observations were made with TBD, [27] BTMG, [20] thiazolium carbenes, [33] CsCOOH and KCOOH with 18-crown-6, [55] [TBA]F, [19] glycine betaine, [40,41] Cs 2 CO 3 [47] and many others ( Figure 8). [42,44,56,57] Anumber of N-methylation pathways has been proposed. However,experimental and computational evidence supports only two,and both proceed by af ormoxysilane intermediate (Scheme 5).…”

Iron‐Rich Natural Mineral Gibeon Meteorite Catalyzed N‐formylation of Amines using CO2 as the C1 Source

“…N ‐formylation of amines is an attractive way to fix CO 2 into value‐added formamides using a hydride donor such as hydrosilanes in the presence of a suitable catalyst. Metal‐containing catalysts based on Pt, Re,, Ni, W, Cs,, Ru,, Rh, Ir, Pd, Au, Cu, Zn, Co and Fe,, have been reported for this reaction. In addition, a number of organocatalysts have also been reported, including 1,3,2‐diazaphospholene, N ‐heterocyclic carbenes (NHCs), N ‐heterocyclic olefins (NHOs), γ ‐valerolactone, triazabicyclodecene (TBD), glycine betaine, ionic liquids (ILs) and alkali carbonates ,.…”

In Situ Generation of an N‐Heterocyclic Carbene Functionalized Metal–Organic Framework by Postsynthetic Ligand Exchange: Efficient and Selective Hydrosilylation of CO2

“…[1] However, this process is energy-intensive, owing to the thermodynamically inert nature of CO 2 .A variety of homogeneous catalytic systems are used to transform CO 2 into methanol, including those based on organometallic catalysts, [2] organocatalysts, [3] frustrated Lewis pairs, [4] perrhenate anions, [5] and alkali-metal carbonates. [1] However, this process is energy-intensive, owing to the thermodynamically inert nature of CO 2 .A variety of homogeneous catalytic systems are used to transform CO 2 into methanol, including those based on organometallic catalysts, [2] organocatalysts, [3] frustrated Lewis pairs, [4] perrhenate anions, [5] and alkali-metal carbonates.…”

“…[7] Ying and co-workers reported the first example of NHC-catalyzed reduction of CO 2 with silanes to form methoxide products along with siloxane (Ph 2 (MeO)SiO-) n via formoxysilane intermediates. [2][3][4][5][6] However, their recent attempts to immobilize the NHC moieties onto polymeric materials significantly lowered the yields of silyl methoxide,and the deactivation of the catalyst was observed after three cycles. [2][3][4][5][6] However, their recent attempts to immobilize the NHC moieties onto polymeric materials significantly lowered the yields of silyl methoxide,and the deactivation of the catalyst was observed after three cycles.…”

“…[10] Accordingly,t he high degree of control over the structural and chemical features makes MOFs extremely promising as next-generation CO 2 capture materials, [11] and their ability to promote catalytic conversion of CO 2 into high-value chemicals has been demonstrated. [12,13] To date,o nly two MOF-based catalysts have been reported to successfully realize chemical reduction of CO 2 to either methanol [14] or other related intermediates (HCO 2 H), [15] but both require harsh reaction conditions, including elevated operating temperature (up to 250 8 8C) and high CO 2 pressure (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). [12,13] To date,o nly two MOF-based catalysts have been reported to successfully realize chemical reduction of CO 2 to either methanol [14] or other related intermediates (HCO 2 H), [15] but both require harsh reaction conditions, including elevated operating temperature (up to 250 8 8C) and high CO 2 pressure (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15).…”