A NHC-Silyliumylidene Cation for Catalytic N-Formylation of Amines Using Carbon Dioxide

Abstract

This study describes the use of a silicon(II) complex, namely, the NHC-silyliumylidene cation complex [(I Me ) 2 SiH]I (1, I Me = :C{N(Me)C(Me)} 2 ), to catalyze the chemoselective N-formylation of primary and secondary amines using CO 2 and PhSiH 3 under mild conditions to afford the corresponding formamides as a sole product (average reaction time: 4.5 h; primary amines, average yield: 95%, average TOF: 8 h −1 ; secondary amines, average yield: 98%, average TOF: 17 h −1 ). The activity of 1 and product yields outperform the currently available non-transition-metal catalysts used for this catalysis. Mechanistic studies show that the silicon(II) center in complex 1 catalyzes the C−N bond formation via a different pathway in comparison with non-transitionmetal catalysts. It sequentially activates CO 2 , PhSiH 3 , and amines, which proceeds via a dihydrogen elimination mechanism, to form formamides, siloxanes, and dihydrogen gas.

An NHC‐Stabilised Phosphinidene for Catalytic Formylation: A DFT‐Guided Approach

“…Upon the formation of the N‐silylated amide, it reacts with the phenylsilyl formate. Subsequently, the formyl transfer takes place from 5 to 4 and the reaction proceeds through various transition states as depicted in Figure 4 leading to the formation of desired product 3 along with siloxanes as the by‐product, also reported previously [44] . Furthermore, in a control experiment, triphenylsiloxane Ph 3 Si−O−SiPh 3 was identified as the by‐product by 1 H and 29 Si NMR spectroscopy when Ph 3 SiH was used as the reducing agent [46] …”

“…Such formylation of amines using CO 2 has been accomplished with various low‐valent metal‐free main group compounds in recent time [42–43] . For example, reports by So and co‐workers demonstrated the use of NHC‐stabilised silylene [44] and N‐phosphinoamidinato N‐heterocyclic carbene‐diborene [45] for catalytic formylation of amines using CO 2 . On the contrary, the formylation of primary amides by metal‐free catalyst has rarely been explored except in a recent report [46] .…”

Carbene and N-phosphinoamidinate stabilized diboron derivatives for catalytic hydroboration

“…It is noteworthy that compound 2.36 is one of the rare main-group catalysts that mediate N-formylation using carbon dioxide and pinacolborane, including the NHC-silyliumylidene cation [( Me NHC)2SiH]I. [41] Moreover, 2.36 exceeds the latter in view of both reaction time and TOF (example of [( Me NHC)2SiH]I, 2.43a: yield 68%, TOF 6.8 h −1 ). Other main-group-element catalysts, namely proazaphosphatrane [42] and carbodicarbene, [43] can only use the potent 9-BBN for the N-methylation of amines with carbon dioxide.…”

“…[40] Data processing, correction, structural solution and refinement were done similar with the SHELXTL suite of programs. [41] Selected crystallographic data is provided in section 4.5.1.…”

“…The yields of products were calculated on the basis of the integration of 1 H NMR signals of R1R2NC(= O)H at ca. 8 ppm relative to that of cyclohexane (CH2, 1.37 ppm) [41]. (0.013 g, 0.02 mmol) and C6D6 (0.5 mL) were mixed in a J.…”