Mechanistic Insights on the Functionalization of CO₂ with Amines and Hydrosilanes Catalyzed by a Zwitterionic Iridium Carboxylate‐Functionalized Bis‐NHC Catalyst

Abstract

The zwitterionic complex [Cp*IrCl{(MeIm)2CHCOO}] (1) efficiently catalyzes the selective hydrosilylation of CO2 to afford the corresponding silylformate. The best reaction performance has been achieved in acetonitrile at 348 K using HSiMe2Ph. The 1‐catalyzed reaction of pyrrolidine with CO2 and HSiMe2Ph strongly depends on the CO2 pressure. At low concentration of CO2 (1 bar) formation of the corresponding silylcarbamate, by insertion of CO2 into the Si−N bond of the in situ generated silylamine was observed, while at higher pressure (3 bar) the formamide derivative was obtained as major reaction product. The unexpected formation of pyrrolidin‐1‐ium formate as intermediate of the reaction the 1‐catalyzed of CO2 with pyrrolidine and HSiMe2Ph has been observed, and its role in the formation of 1‐formylpyrrolidine rationalized. Moreover, a mechanism for the reaction of CO2 with hydrosilanes, in the presence and in the absence of amines, based on theoretical calculations has been proposed.

Carboxylate-Assisted β-(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by a Zwitterionic Bis-NHC Rhodium(III) Complex

“…This process requires to overcome an energy barrier of 17.0 kcalꞏmol -1 , dictated by transition structure TS-AB, which is feasible under the reaction conditions (RT and 333 K). Remarkably, within TS-AB the Cp* ligand changes its coordination mode form  5 to  1 , as we previously proposed for the related complex [Cp*IrCl{(MeIm)2CHCOO}], 22 and which is enabled by the flexibility in the coordination modes of the pentamethylcyclopentadiene ligand. 32,33,34 The next step involves the hydrosilane activation through a ligand assisted Si-H bond cleavage process, in line with other activation proposals for similar Ir-based catalysts.…”

“…In this sense, we have shown that the carboxylate moiety in 6 [Cp*IrCl{(MeIm)2CHCOO}] is involved in the precatalyst activation step leading to the active species for the reduction of CO2 with hydrosilanes to silylformates. 22 These precedents and the potential of the uncoordinated carboxylate moiety to assist nonclassical hydrosilylation pathways such as a silane-shuttle or proton-shuttle, facilitating the alkyne-vinylidene tautomerization, 23…”

β-(Z) Selectivity Control by Cyclometalated Rhodium(III)–Triazolylidene Homogeneous and Heterogeneous Terminal Alkyne Hydrosilylation Catalysts

“…Notice that the aforementioned process is facilitated by the coordination flexibility of the Cp* ligand, which changes its coordination mode form  5 to  1 , 39,40 as we previously reported for structurally related catalysts. 37 Then, the hydrosilane H-Si bond is activated by means of a -complex assisted metathesis (-CAM) mechanism involving the H-Si and Rh-CAr bond, thus forming new Rh-Si and C-H bons. 41,42 This step proceeds via TS-CD (see Figure S34 in the Supporting Information), and requires to surmount an energy barrier of 14.5 kcal•mol -1 (with respect to A, feasible under the working conditions), leading to the silyl intermediate D, whose relative energy is 1.2 kcal•mol -1 higher than that of C. We note that we also explored the classical inner-sphere mechanisms that involve the hydrosilane oxidative addition to the metal center, 43 but we could not find any stable intermediate.…”

Heterogeneous Organocatalysts for the Reduction of Carbon Dioxide with Silanes

“…Formamides, aminals, and N ‐methylated amines are the products of the two‐, four‐, and six‐electron reduction of CO 2 with amines, respectively. It was also found that under nickel [46] and iridium [47] complex catalysis silylcarbamate can be formed at lower CO 2 pressure than that of the typical formamide synthesis [48] …”

“…It was also found that under nickel [46] and iridium [47] complex catalysis silylcarbamate can be formed at lower CO 2 pressure than that of the typical formamide synthesis. [48] These new synthetic methods toward amine derivatives offer more direct and atom-efficient transformations under reaction conditions that are milder than those of existing processes.…”