N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO₂ Functionalizations

Abstract

The first acceptor-free heavier germanium analogue of an acylium ion, [RGe(O)(NHC) 2 ]X (R = Mes Ter= 2,6-(2,4,6-Me 3 C 6 H 2 ) 2 C 6 H 3 ; NHC = IMe 4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = (Cl or BArF= {(3,5-(CF 3 ) 2 C 6 H 5 ) 4 B}), was isolated by reacting [RGe(NHC) 2 ]X with N 2 O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh 3 )] and corresponding heavier analogues ([RGe(S)(NHC) 2 ]X and [RGe(Se)(NHC) 2 ]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO 2 and silane, with the former found to be an example of reversible activation of CO 2 , thus mimicking the behavior of transition metal oxides. Furthermore, its transition metal like nature is demonstrated as it was found to be an active catalyst in both CO 2 hydrosilylation and reductive N-functionalization of amines using CO 2 as C 1 source. Mechanistic studies were undertaken both experimentally and computationally, which revealed the reaction proceeds via a NHC-siloxygermylene [(NHC)RGe(OSiHPh 2 )].

Germyliumylidene: A Versatile Low Valent Group 14 Catalyst

“…This suggests a cooperative silane/CO 2 mechanism, and therefore, the mechanism was investigated computationally (Figure S50). In a similar fashion to the previous case, [7] the SiÀ H bond in PhSiH 3 is activated by the germanium lone pair in 1M + and the hydride transfer from the hypercoordinate silane to the free CO 2 occurs in a concerted process via the transition state TS-1 (Figure 1). IRC calculations confirm the direct formation of the transition state from the separated species (1M + + PhSiH 3 + CO 2 ) following a three-component mechanism.…”

Heavier Tetrylenes as Single Site Catalysts

“…[59] It is noteworthy to mention that there are only few reports on catalytic amination of CO 2 and CÀ N bond formation by using main group compounds e. g. carbodicarbene, [60] proazaphosphatrane, [61] 1,3,2-diazaphospholene [62] and NHCgermaacylium cation. [63] The catalytic reaction of primary amines with CO 2 and PhSiH 3 was performed using 5 mol% of 9 at 60°C (Scheme 12). [59] Anilines were found to be formylated very efficiently (> 99%, TOF = 10.0 h À 1 ) and their derivatives with various functional groups/steric substituents were converted in very good to moderate yields.…”

“…Expanding the scope of the catalyst 9 , So and coworkers further used it for a chemoselective N‐formylation of primary and secondary amines using CO 2 and PhSiH 3 under mild conditions to afford the corresponding formamide as a sole product [59] . It is noteworthy to mention that there are only few reports on catalytic amination of CO 2 and C−N bond formation by using main group compounds e. g. carbodicarbene, [60] proazaphosphatrane, [61] 1,3,2‐diazaphospholene [62] and NHC‐germaacylium cation [63] . The catalytic reaction of primary amines with CO 2 and PhSiH 3 was performed using 5 mol% of 9 at 60 °C (Scheme 12).…”

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

“…It may be noted that in literature, metal‐free catalytic formylation of amines is quite well‐studied [38–41] . 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 .…”