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

Abstract

In recent years, the applications of low‐valent main group compounds have gained momentum in the field of catalysis. Owing to the accessibility of two lone pairs of electrons, NHC‐stabilised phosphinidenes have been found to be excellent Lewis bases; however, they cannot yet be used as catalysts. Herein, an NHC‐stabilised phosphinidene, 1,3‐dimethyl‐2‐(phenylphosphanylidene)‐2,3‐dihydro‐1H imidazole (1), for the activation of CO2 is reported.A closer inspection of the CO2 activation process by DFT calculations along with intrinsic bond orbital analysis shows that phosphinidene is associated with phenylsilane through a noncovalent π‐π interaction between two phenyl rings which activates the Si−H bond facilitating hydride transfer to the CO2 molecule. Detailed DFT studies along with spectroscopic experiments were combined to understand the mechanism of CO2 activation and its catalytic reductive functionalisation leading to the formylation of a range of chemically inert primary amides under mild reaction conditions.

Mesoionic N‐Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

“…Immediately, the evolution of a light gas was observed from the reaction mixture, which was later characterized as dihydrogen from a 1 H NMR study (peak at δ =4.47 ppm) as well as by GC‐TCD analysis (Supporting Information, Figure S36–S38). Furthermore, the stoichiometric reaction of 4 a with PhSiH 3 at RT in the presence of catalyst 2 a shows the complete formation of N‐silyl amide 9 a , which was characterized by various NMR spectroscopic techniques (Figure S41–S43) [59] . This suggests the activation of amide to the N‐silylated amide (Figure 5c).…”

“…Furthermore, the stoichiometric reaction of 4 a with PhSiH 3 at RT in the presence of catalyst 2 a shows the complete formation of N-silyl amide 9 a, which was characterized by various NMR spectroscopic techniques (Figure S41-S43). [59] This suggests the activation of amide to the N-silylated amide (Figure 5c). Later, the N-silyl amide (9 a) was exposed to our standard reaction protocol in the presence of only 2 equiv of PhSiH 3 , (instead of 4 equiv used in the catalytic protocol) furnishing 74 % of the isolated product of 5 a (the catalytic yield of 5 a was 77 % which is consistent with this observation) confirming 9 a as the active substrate for this process (Figure 5d).…”

Transition Metal-Mimicking Relay Catalysis by a Low-Valent Phosphorus Compound

“…For this purpose, we considered utilizing 1,3-dimethyl-2-(phenylphosphanylidene)-2,3-dihydro-1H-imidazole (1) as our potential catalyst as it is already established as an active catalyst for CO 2 activation. 33 In this work, we demonstrate that 1 can act as a transition-metal mimicking catalyst via a relay process by regulating its reduced and oxidized states to synthesize γ-butyrolactones, activating both aldehydes and methyl acrylate, via benzoin formation. γ-Butyrolactones are a common structural motif in organic compounds, accounting for approximately 10% of all-natural products which include monocyclic, bicyclic, and tricyclic γbutyrolactones.…”

“…As part of our ongoing research on the catalytic activity of NHC-stabilized phosphinidene [NHC–P(Ph)], we investigated the role of NHC–P(Ph) ( 1 ) in activating benzaldehyde and methyl acrylate for the efficient synthesis of γ-butyrolactone. We initiated this study by conducting a benzoin condensation reaction using benzaldehyde with 1 as the catalyst.…”

“…31 In 2016, a singlet phosphinidene without NHC stabilization was isolated. 32 Despite these advancements, the enhanced nucleophilicity of NHC-stabilized phosphorus(I) compounds had not been explored as a metalfree catalyst until our recent work, 33 where we performed the catalytic reductive formylation of primary amides using CO 2 in the presence of silane and NHC-stabilized phosphinidene 1. The nucleophilic behavior of the phosphorus center of 1 has been instrumental in activating CO 2 through a noncovalent interaction with silane.…”