Carbon Dioxide Based N‐Formylation of Amines Catalyzed by Fluoride and Hydroxide Anions

Abstract

We described herein a simple approach for N‐formylation with CO2 and hydrosilane reducing agents. Fluoride and hydroxide salts efficiently catalyzed the reaction, principally through activation of the hydrosilanes, which led to hydrosilane reactivities comparable to those of NaBH4/LiAlH4. Consequently, the N‐formylation of amines with CO2 could be achieved at room temperature and atmospheric pressure. The mechanism of these anionic catalysts contrasts that of the currently reported systems, for which activation of CO2 is the key mechanistic step. Using tetrabutylammonium fluoride as a simple ammonium salt catalyst, the N‐formylated products of both aliphatic and aromatic amines could be obtained in excellent yields with high selectivities.

Delineation of the Critical Parameters of Salt Catalysts in the N‐Formylation of Amines with CO2

“…[30] In addition, NHCs were shown to be excellent catalysts for the N-formylation of amines with CO 2 and hydrosilanes. [11] H-bond donor groups can interact with halide ions, therebyi ncreasing the ion pairing energy,r esulting in the poor yields obtained with [HOEMIm]Cl and [BMIm]Cl. We postulated that the cation only affects the reaction indirectly by ionpairing, thereby decreasing the reactivity of the anion.…”

“…[11] In contrast, tetrafluoroborate (BF 4 À )a nd hexafluorophosphate (PF 6 À )s alts are catalytically inactive. F À .…”

“…F À . [10,11] An umber of salts including [TBA]F, [11] Cs 2 CO 3 , [19] glycine betaine, [40] zwitterionic P-ylide-CO 2 adducts [41] and various carboxylates [42] were proposed to act this way. [10] Based on these observations and the fact that [TBA]F can activate hydrosilanes in the reduction of carbonyls, [35][36][37][38][39] the activity of salt catalysts was attributed to the nucleophilicity of the anion and its ability to activate the hydrosilane reducing agent prior to CO 2 insertion.…”

“…[10] It was proposed that the nucleophilicity of the anion activates the hydrosilane reducing agent enabling the reduction of CO 2 [9,11,17] or,m ore recently,I Ls were shown to act as ab ase duringc arbamatea ssisted activation (vide infra). [10] It was proposed that the nucleophilicity of the anion activates the hydrosilane reducing agent enabling the reduction of CO 2 [9,11,17] or,m ore recently,I Ls were shown to act as ab ase duringc arbamatea ssisted activation (vide infra).…”

“…[10] It was proposed that the nucleophilicity of the anion activates the hydrosilane reducing agent enabling the reduction of CO 2 [9,11,17] or,m ore recently,I Ls were shown to act as ab ase duringc arbamatea ssisted activation (vide infra). [11] Te trabutylammonium fluoride ([TBA]F) [9,11] and Cs 2 CO 3 [19] were identified as particularly efficient catalysts for the N-formylation reaction with comparable activity to the best metal catalysts based on Fe, [20] Rh, [21] W [22] and Cu. F À .…”