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Article

Reactions of 1-Alkyl-3-phenylbenzimidazolium Salts with Ag2O: The Formation of a Ring-Opening Formamide Derivative and a Ag Complex with an N-heterocyclic Carbene Ligand

by
Satoshi Sakaguchi
*,
Takashi Higashino
,
Yudai Tasaki
,
Ryo Ichihara
and
Tatsuo Yajima
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(1), 18; https://doi.org/10.3390/inorganics13010018
Submission received: 20 December 2024 / Revised: 8 January 2025 / Accepted: 9 January 2025 / Published: 10 January 2025
(This article belongs to the Special Issue N-Heterocyclic Carbene Metal Complexes: Synthesis, Properties and Applications, 2nd Edition)
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Abstract

:
This study investigated the reactions of 1-alkyl-3-phenylbenzimidazolium salts with Ag2O. It was found that the selectivity of the reaction products was influenced by the N-alkyl substituent on the azolium ring. For example, treating 1-methyl-3-phenylbenzimidazolium iodide (2) with Ag2O for 24 h produced the ring-opening formamide derivative N-[2-(phenylamino)phenyl]-N-methylformamide (2b) in an 85% yield. In contrast, the reaction of 1-benzyl-3-phenylbenzimidazolium chloride (3) with Ag2O under the same conditions yielded the corresponding N-heterocyclic carbene (NHC)–Ag complex (1-benzyl-3-phenylbenzimidazol-2-ylidene) silver(I) chloride (3a) in an 86% yield. Furthermore, the corresponding monodentate NHC–Au complex 2c could be synthesized by allowing 2 to react with AuCl(SMe2) in the presence of Ag2O.

Graphical Abstract

1. Introduction

In recent years, N-heterocyclic carbenes (NHCs) have been widely used as ligands for metal catalysts because of their strong σ-donating ability [1,2,3,4,5]. Moreover, owing to the operational simplicity of their synthesis procedures, various N-substituents can be introduced into the side arms of NHCs. Consequently, these ligands are attracting rapidly increasing attention in the field of asymmetric catalysis [6,7,8,9]. To date, the most widely adopted method for the preparation of NHC species involves deprotonation at the C2 position of an azolium salt with a Ag base to form an NHC–Ag complex [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. This process can be accomplished using various Ag bases, such as Ag2O, AgOAc, and Ag2CO3. In a pioneering study, Lin et al. introduced a procedure using Ag2O to afford a Ag complex bearing 1,3-diethylbenzimidazol-2-ylidene [25,26,27,28]. Notably, NHC–Ag complexes play important roles as NHC transfer agents in the synthesis of numerous NHC–metal complexes (namely, the Ag2O method), granting them extraordinary versatility. Thus, several applications employing NHC–Ag complexes to synthesize NHC–M (M = Group 8, 9, 10, and 11 elements) complexes via transmetalation have been documented [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28].
Previously, we demonstrated the synthesis of a series of NHC–Pd(II) complexes derived from chiral hydroxyamide-functionalized benzimidazolium salts using the Ag2O method [29,30]. Importantly, these NHC–Pd(II) complexes catalyzed asymmetric oxidative Heck-type reactions and asymmetric allylic alkylation (AAA) reactions [31,32,33]. Among these, in the Pd-catalyzed AAA reactions, we screened libraries of chiral NHC ligand precursors using an in situ-generated NHC–Pd(II) catalyst derived from chiral benzimidazolium salts via the Ag2O method [33]. Consequently, we observed a reversal in enantioselectivity after changing the N-substituent from a methyl group to a phenyl group at the side arm of the chiral benzimidazolium ring. The intriguing observation that replacing the N-substituent on the chiral azolium ring with a aliphatic/aromatic group controlled the facial selection of the alkene substrate prompted us to study the reaction of a “simple” benzimidazolium salt with Ag2O.
In this study, we discovered that the reaction of 1-ethyl-3-phenylbenzimidazolium iodide (1) with Ag2O preferentially yielded the ring-opening formamide derivatives over the corresponding NHC–Ag complex. In the literature, Liu and Dai demonstrated that Ag2O in CH2Cl2 promoted the hydrolysis of unsymmetrical pyridine-bridged pincer-type benzimidazolium halides, affording the corresponding ring-opening formamide derivatives [34,35,36]. However, to the best of our knowledge, the ring-opening reaction of a “simple” benzimidazolium salt with Ag2O has not yet been reported. To address this, we focused on the ring-opening reaction of substituted benzimidazolium salts promoted by Ag2O.

2. Results

Based on our previously reported procedure for the preparation of the NHC–Ag complex, we initially performed the reaction of 1 with Ag2O. Compound 1 (0.2 mmol) was allowed to react with Ag2O (0.1 mmol) in CH2Cl2 (10 mL) at room temperature. As the reaction progressed, the black Ag2O solid disappeared and a white precipitate was formed. After 24 h, the reaction produced bis(1-ethyl-3-phenylbenzimidazol-2-ylidene) silver(I) iodide (1a) and N-[2-(phenylamino)phenyl]-N-ethylformamide (1b) in 70% and 25% yields, respectively (Scheme 1, upper side). In the 13C-NMR spectrum of the NHC–Ag complex 1a, the carbene’s carbon resonance was observed at δ 194.7 ppm, which is a characteristic of carbene metal complexes. On the other hand, the molecular structure of 1b was established by single-crystal X-ray diffraction studies (Scheme 1a).
After the reaction of 1 with Ag2O for 24 h, we subjected the crude reaction mixture to 1H-NMR analysis in DMSO-d6 (Scheme 1b). On exposing the crude reaction mixture in the NMR tube to air at room temperature for several days, we observed a change in the product’s composition. The initially clear solution of the crude mixture in DMSO-d6 gradually transformed into a solution containing a white precipitate. The product yields monitored in the 1H-NMR analysis were as follows (Scheme 1b; see also Supplementary Materials): 1a (70% yield) and 1b (25% yield) after 1 day; 1a (47% yield) and 1b (47% yield) after 3 days; 1a (25% yield) and 1a (70% yield) after 5 days; 1a (5% yield) and 1b (90% yield) after 10 days. These results suggest the decomposition of 1a into 1b. The underlying reaction pathway will be discussed later.
Furthermore, we examined three azolium salts bearing N-aliphatic and N-aromatic side arms (Scheme 2). 1-Methyl-3-phenylbenzimidazolium iodide (2) was allowed to react with Ag2O in CH2Cl2 at room temperature for 3 h. This reaction preferentially afforded the corresponding ring-opening formamide derivative N-[2-(phenylamino)phenyl]-N-ethylformamide (2b) in a 59% yield over bis(1-methyl-3-phenylbenzimidazol-2-ylidene)silver(I) iodide (2a, 28% yield). Upon extending the reaction time to 24 h, 2b was obtained as the sole product (Scheme 2a; refer to Supplementary Materials for details).
To date, various documents have focused on the reaction of 2 with Ag2O in CH2Cl2 at room temperature, aimed at the synthesis of NHC–Ir complexes via transmetalation, which produces NHC–Ag complex 2a as an intermediate [37,38,39]. However, these publications do not provide the characterization data for 2a. In addition, they do not mention the formation of the ring-opening product 2b in the foregoing reaction. Conversely, Pozharskii et al. demonstrated that the reaction of 2 with KNH2 in liq. NH3 at −70 °C for 2 h (Chichibabin reaction conditions) afforded 2b in a 96% yield [40].
Next, the reaction of the N-benzyl-substituted benzimidazolium salt with Ag2O was examined. Treating 1-benzyl-3-phenylbenzimidazolium chloride (3) with Ag2O for 24 h yielded the corresponding NHC–Ag complex 3a in an 86% yield (Scheme 2b). Gök and Akkoç have reported a similar observation; in their study, the corresponding NHC–Ag complex was synthesized by reacting 1-phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazolium chloride with Ag2O in CH2Cl2 [19,41].
Furthermore, on the other hand, previous reports indicate that treating 1-methyl-3-phenylimidazolium iodide (4) with Ag2O affords the corresponding NHC–Ag complex 4a [24,42,43]. Therefore, we attempted to reproduce this reaction in our laboratory. On treating 4 with Ag2O in CH2Cl2, 4a was obtained as a major product in a 70% yield (Scheme 2c). The formation of 4a was confirmed by comparing its NMR spectra with those of a previously reported authentic sample. Thus, we concluded that the selectivity of the products in the reaction of azolium salt with Ag2O depended on both the N-alkyl substituent of the azolium ring (methyl vs. benzyl) and the azolium skeleton.

3. Discussion

Li and Ollevier conducted a systematic study on the hydrolysis of an NHC–Ag complex derived from an imidazolinium salt to produce a formamide [44]. They reported that the NHC–Ag bearing a saturated backbone derived from the imidazolinium salt was easily hydrolyzed. They proposed that imidazolidinol was produced as an intermediate and later underwent rearrangement to afford a formamide in the hydrolysis reaction. By contrast, we demonstrated that the benzimidazolium salt 2 (or 1) was hydrolyzed to formamide derivatives in the presence of Ag2O. Plausible pathways are shown in Scheme 3.
Although the reaction pathway for the reaction of 2 (or 1) with Ag2O remains unclear at this stage, the reaction might have proceeded via the formation of an azolium hydroxide and/or the NHC–Ag complex as an intermediate, as depicted in Scheme 3. First, the conversion of 2 into 1-methyl-3-phenylbenzimidazolium hydroxide (X) might have occurred through the action of Ag2O and water (Scheme 3a).
Subsequently, the nucleophilic addition of an internal hydroxide to the azolium ring in the intermediate X led to the formation of adduct Y. Finally, Y undergoes rearrangement to form formamide derivative 2b through intermediate Z. Notably, this anion-exchange (halide/hydroxide replacement) reaction is similar to the well-known Hofmann degradation (exhaustive methylation) for alkene synthesis (see Scheme 3a, lower side) [45].
On the contrary, the gradual decomposition of 1a to 1b observed in the reaction of 1 with Ag2O, as depicted in Scheme 1a, implies the possibility of the hydrolysis of the NHC–Ag 1a (or 2a) intermediate to form the formamide derivative 1b (or 2b) (Scheme 3b). In this case, compound 2a could have undergone direct hydrolyzation via the concerted addition of water and the elimination of AgI to generate imidazolidinol Y, which subsequently underwent a ring-opening reaction to form 2b (concerted pathway). Alternatively, 2a could have existed in equilibrium with free NHC and AgI. Here, note that free NHC formation from X via the intramolecular deprotonation of the azolium ring by hydroxide ions in X is also plausible. In this case, the subsequent incorporation of carbene into the O-H σ-bond of water would form Y (carbene formation/water insertion pathway). The latter pathway is similar to that proposed by Li and Ollevier [44].
To gain further insights into the reaction pathway, we investigated the reaction of 2 with tetramethylammonium hydroxide (Me4NOH). We hypothesized that the counter-anion exchange between the iodide in 2 and the hydroxide in Me4NOH would reversibly yield azolium hydroxide X. Indeed, treating 2 (0.2 mmol) with Me4NOH (0.2 mmol) in CH2Cl2 (10 mL) at room temperature produced the formamide derivative 2b in a 72% yield (Scheme 4a). This result suggests that halide/hydroxide replacement occurs in the reaction of 2 with Ag2O to form X. Subsequently, intermediate X is converted into the ring-opening formamide 2b, as depicted in Scheme 3a.
In addition, we hypothesized that the hydrolysis of Ag2O in the presence of a trace amount of water would generate hydroxide ions (Scheme 3a). To confirm this assumption, we allowed 2 to react with Ag2O in anhydride CH2Cl2. After sustaining this reaction for 24 h, the hydrolytic ring-opening product 2b was obtained in a 24% yield (Scheme 4b). These results suggest that the reaction of 1 with Ag2O proceeds though the formation of X as an intermediate, as depicted in Scheme 3a.
Finally, we investigated the synthesis of an NHC–metal complex from 2 using the Ag2O method. First, we employed the “conventional Ag2O technique”. After treating 2 with Ag2O in CH2Cl2 at room temperature, AuCl(SMe2) was added to the resulting mixture as a metal precursor. However, this reaction did not yield the desired NHC–Au complex but afforded the formamide derivative 2b as a major product. This is attributed to the preferential formation of the ring-opening product over the corresponding NHC–Ag complex, as described above. Therefore, we hypothesized that, in the presence of three reactants, such as 2, Ag2O, and AuCl(SMe2), the NHC ligand-transfer reaction between the NHC–Ag complex and Au complex precursor would proceed smoothly to afford the corresponding AuCl(NHC) complex. As expected, this “coexistence Ag2O technique” afforded the desired chloro(1-methyl-3-phenylbenzimidazol-2-ylidene)gold(I) (2c) in a 59% yield (Scheme 5). In a related study, Barnard et al. reported the synthesis of an NHC–Au complex from 1-methyl-3-phenylimidazolium iodide [24]. In addition, Togni and coworkers synthesized chloro(1-phenyl-3-trifluoromethylbenzimidazol-2-ylidene)gold(I) [46].
Complex 2c was characterized by 1H- and 13C-NMR spectroscopies and elemental analysis. In the 13C-NMR analysis of 2c, the characteristic carbene carbon resonance at δ 178 ppm was evident. The successful preparation of NHC–Au complex 2c implies that, once NHC–Ag complex 2a is generated from the reaction of 2 with Ag2O, transmetalation (NHC ligand-transfer reaction) proceeds smoothly, taking precedence over the hydrolysis reaction.

4. Materials and Methods

4.1. General Notes

All chemical reagents and solvents were obtained from commercial sources. Super-dehydrated dichloromethane was purchased from the Fujifilm Wako Pure Chemicals Corporation (Tokyo, Japan). Column chromatography was performed with silica gel 60 purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). 1H-NMR spectra were recorded on a JEOL ECA400 (400 MHz for 1H-NMR and 100 MHz for 13C-NMR) spectrometer. Chemical shifts were reported downfield from TMS (δ = 0 ppm) for 1H-NMR. For 13C-NMR, chemical shifts were reported on the scale relative to the solvent used as an internal reference. Elemental analyses were performed at Osaka University. Benzimidazolium salts, 1–4, were synthesized according to the literature procedures.
1-Ethyl-3-phenylbenzimidazolium iodide (1) [47]: 1H-NMR (CDCl3): δ = 11.04 (s, 1H), 7.92–7.90 (m, 2H), 7.85 (d, J = 8.0 Hz, 1H), 7.76–7.62 (m, 6H), 4.95 (q, J = 7.2 Hz, 2H), 1.80 (t, J = 7.2 Hz, 3H); 1H-NMR (DMSO-d6): δ = 10.16 (s, 1H), 8.21 (d, J = 8.2 Hz, 1H), 7.86–7.68 (m, 8H), 4.61 (q, J = 7.2 Hz, 2H), 1.62 (t, J = 7.2 Hz, 3H); 13C-NMR (DMSO-d6): δ = 142.2, 133.1, 131.0, 131.0, 130.4, 130.3, 127.4, 126.8, 125.1, 113.9, 113.4, 42.4, 13.9. The NMR spectrum can be found in the Supplementary Materials.
1-Methyl-3-phenylbenzimidazolium iodide (2) [48,49,50,51]: 1H-NMR (DMSO-d6): δ = 10.13 (s, 1H), 8.15 (d, J = 8.0 Hz, 1H), 7.85–7.71 (m, 8H), 4.17 (s, 3H); 13C-NMR (DMSO-d6): δ = 143.1, 133.2, 131.9, 130.9, 130.5, 127.4, 126.9, 125.1, 113.9, 113.3, 33.5. One of the aryl group 13C NMR resonances was not observed. Mp.: 198.8–199.0 °C. The NMR spectrum can be found in the Supplementary Materials.
1-Benzyl-3-phenylbenzimidazolium chloride (3): 1H-NMR (DMSO-d6): δ = 10.88 (s, 1H), 8.06–7.67 (m, 11H), 7.43–7.34 (m, 3H), 5.95 (s, 2H). 13C-NMR (DMSO-d6): δ = 143.0, 133.8, 133.2, 131.3, 130.9, 130.5, 130.4, 128.9, 128.7, 128.6, 127.5, 127.0, 125.3, 114.3, 113.7, 50.1. Anal. Calc. for C20H17ClN2▪0.3H2O: C, 73.64; H, 5.44; N, 8.59%; Found: C, 73.68; H, 5.20; N, 8.60%. Mp.: 229.9–230.2 °C. The NMR spectrum can be found in the Supplementary Materials.
1-Methyl-3-phenylimidazolium iodide (4) [52]: 1H-NMR (CDCl3): δ = 10.33 (s, 1H), 7.78–7.72 (m, 4H), 7.59–7.52 (m, 3H), 4.26 (s, 3H); 13C-NMR (CDCl3): δ = 135.5, 134.2, 130.5, 130.3, 124.7, 122.1, 120.8, 37.6. The NMR spectrum can be found in the Supplementary Materials.

4.2. General Procedure for Reaction of Benzimidazolium Salt with Ag2O to Give Ring-Opening Formamide Product

Benzimidazolium salt (0.2 mmol) and Ag2O (0.1 mmol) were stirred in CH2Cl2 (10 mL) at room temperature for 24 h. After passing through a paper filter, the filtrate was dried in a rotary evaporator. The ring-opening formamide product from the residue was purified by column chromatography on silica gel, using AcOEt as an eluent.
N-[2-(Phenylamino)phenyl]-N-ethylformamide (1b): Mixture of two rotamers (85:15); Major: 1H-NMR (CDCl3): δ = 8.18 (s, 1H), 7.35–6.89 (m, 9H), 5.74 (br, 1H), 3.73 (q, J = 7.6 Hz, 2H), 1.15 (t, J = 7.6 Hz, 3H); 13C-NMR (CDCl3): δ = 163.4, 141.6, 140.9, 130.1, 129.4, 129.2, 128.0, 122.2, 120.3, 119.4, 116.6, 39.5, 12.7. 1H-NMR (DMSO-d6): δ = 8.04 (s, 1H), 7.28–6.80 (m, 9H), 3.53 (q, J = 7.6 Hz, 2H), 0.94 (t, J = 7.6 Hz, 3H); 13C-NMR (DMSO-d6): δ = 162.5, 143.3, 140.2, 130.3, 130.2, 129.0, 128.6, 121.2, 120.1, 119.4, 117.4, 38.2, 12.4. Minor: 1H-NMR (CDCl3): δ = 8.37 (s, 1H), 7.43–6.88 (m, 9H), 5.82 (br, 1H), 3.71 (q, J = 7.6 Hz, 2H), 1.11 (t, J = 7.6 Hz, 3H); 13C-NMR (CDCl3): δ = 162.2, 143.3, 140.3, 128.5, 127.5, 122.2, 120.6, 120.2, 117.7, 45.4, 14.5. Two of the aryl group 13C NMR resonances were not observed. 1H-NMR (DMSO-d6): δ = 8.24 (s, 1H), 7.28–6.80 (m, 10H), 3.53 (q, J = 7.6 Hz, 2H), 0.98 (t, J = 7.6 Hz, 3H); 13C-NMR (DMSO-d6): δ = 163.2, 143.2, 140.0, 130.0, 129.0, 128.1, 127.9, 120.4, 120.3, 118.1, 118.0, 43.1, 14.3. Anal. Calc. for C15H16N2O: C, 74.97; H, 6.71; N, 11.66; Found, C, 75.03; H, 6.68; N, 11.66. Mp.: 119.5–119.8 °C. The NMR spectrum can be found in the Supplementary Materials. In addition, CCDC 2315032 contains the supplementary crystallographic data for 1b. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/ (accessed on 19 December 2024).
N-[2-(Phenylamino)phenyl]-N-methylformamide (2b) [40]: Mixture of two rotamers (85:15); Major: 1H-NMR (CDCl3): δ = 8.23 (s, 1H), 7.34–7.22 (m, 5H), 7.12–6.91 (m, 4H), 5.69 (br, 1H), 3.20 (s, 3H); 13C-NMR (CDCl3): δ = 163.7, 141.8, 140.4, 130.3, 129.3, 129.1, 128.6, 122.1, 120.7, 119.2, 117.2, 32.4. Minor: 1H-NMR (CDCl3): δ = 8.32 (s, 1H), 7.43–7.22 (m, 5H), 7.12–6.91 (m, 4H), 5.87 (br, 1H), 3.33 (s, 3H); 13C-NMR (CDCl3): δ = 162.3, 143.2, 139.6, 130.4, 129.2, 128.4, 126.7, 122.2, 120.3, 117.8, 37.4. One of the aryl group 13C NMR resonances was not observed. Anal. Calc. for C14H14N2O: C, 74.31; H, 6.24; N, 12.38; Found, C, 74.41; H, 6.08; N, 12.34. υC=O 1670 cm−1, υN-H 3313 cm−1. Mp.: 159.2–160.4 °C. The NMR spectrum can be found in the Supplementary Materials.

4.3. General Procedure for Reaction of Benzimidazolium Salt with Ag2O to Give NHC–Ag Complex Product

Benzimidazolium salt (0.2 mmol) and Ag2O (0.1 mmol) were stirred in CH2Cl2 (10 mL) at room temperature. After the reaction, a white precipitate was filtered with suction and washed with CH2Cl2 to form the desired silver complex.
Bis(1-ethyl-3-phenylbenzimidazol-2-ylidene) silver(I) iodide (1a): 1H-NMR (CDCl3): δ = 7.78–7.76 (m, 2H), 7.57–7.30 (m, 7H), 4.52 (q, J = 7.6 Hz, 2H), 1.48 (t, J = 7.6 Hz, 3H); 13C-NMR (CDCl3): δ = 194.7 (Ccarbene), 138.4, 134.4, 133.4, 129.5, 128.6, 126.1, 123.8, 123.7, 111.8, 111.0, 44.2, 15.6. Anal. Calc. for C30H28AgIN4▪2.5CH2Cl2▪0.5H2O: C, 43.34; H, 3.81; N, 6.22. Found, C, 43.28; H, 3.46; N, 6.52. The NMR spectrum can be found in the Supplementary Materials.
Bis(1-methyl-3-phenylbenzimidazol-2-ylidene) silver(I) iodide (2a): 1H-NMR (DMSO-d6): δ = 7.88 (d, J = 8.2 Hz, 1H), 7.73–7.70 (m, 2H), 7.66–7.60 (m, 3H), 7.56–7.45 (m, 3H), 4.08 (s, 3H). Due to the poor solubility of 2a, the measurement of 13C-NMR failed. Anal. Calc. for C28H24AgIN4▪3CH2Cl2▪H2O: C, 40.29; H, 3.49; N, 6.06. Found, C, 40.09; H, 3.28; N, 6.48. The NMR spectrum can be found in the Supplementary Materials.
(1-Benzyl-3-phenylbenzimidazol-2-ylidene) silver(I) iodide (3a): 1H-NMR (DMSO-d6): δ = 7.88–7.30 (m, 14H), 5.78 (s, 2H). 13C-NMR (DMSO-d6): δ = 137.6, 135.9, 133.9, 133.1, 130.1, 129.4, 128.9, 128.2, 127.6, 126.2, 124.9, 124.7, 112.8, 112.2, 52.2. Carbene 13C NMR resonance was not observed. Anal. Calc. for C20H16AgClN2: C, 56.17; H, 3.77; N, 6.55%; Found: C, 56.14; H, 3.76; N, 6.65%. Mp.: 201.6–201.8 °C. The NMR spectrum can be found in the Supplementary Materials.
(1-Methyl-3-phenylimidazol-2-ylidene) silver(I) iodide (4a) [24]: 1H-NMR (DMSO-d6): δ = 7.84 (d, J = 1.2 Hz, 1H), 7.67 (d, J = 2.0 Hz, 1H), 7.67–6.65 (m, 2H), 7.49–7.48 (m, 3H), 3.86 (s, 3H); 13C-NMR (DMSO-d6): δ = 179.9 (Ccarbene), 139.6, 129.6, 128.5, 124.0, 123.7, 122.1, 38.5. The NMR spectrum can be found in the Supplementary Materials.

4.4. Procedure for Preparation of NHC–Au Complex 2c

First, 2 (0.1 mmol, 34 mg), Ag2O (0.05 mmol. 13 mg), and AuCl(SMe2) (0.11 mmol, 32 mg) were stirred in CH2Cl2 (2 mL) at room temperature for 16 h. After passing through a paper filter, the filtrate was dried in a rotary evaporator to give an orange liquid. The NHC–Au complex 2c from the residue was purified by column chromatography on silica gel (hexane/EtOAc = 8/2) to afford 26 mg (0.59 mml) of 2c as a yellow solid.
(1-Methyl-3-phenylbenzimidazol-2-ylidene) gold(I) chloride (2c): 1H NMR (CDCl3): δ = 7.64–7.54 (m, 6H), 7.50 (dt, J = 1.2 and 8.0 Hz, 1H), 7.42 (dt, J = 1.2 and 8.0 Hz, 1H), 7.37 (d, J = 8.0 Hz, 1H), 4.15 (s, 3H); 13C NMR (CDCl3): δ = 178.3 (Ccarbene), 136.6, 134.0, 129.9, (129.9), 129.6, 126.6, 125.0, 124.9, 112.2, 111.2, 35.4; Anal. Calc. for C14H13AuClN2: C, 38.16; H, 2.74; N, 6.36%; Found: C, 37.77; H, 2.86; N, 6.16%. The NMR spectrum can be found in the Supplementary Materials.

5. Conclusions

We investigated the selective formation of ring-opening formamide derivatives and Ag complexes bearing an NHC ligand in the reaction of 1-alkyl-3-phenylbenzimidazolium salts with Ag2O. The selectivity of the products depended on both the N-alkyl substituent on the azolium ring (methyl vs. benzyl) and the azolium skeleton (benzimidazolium vs. imidazolium). Additional investigations of the reactions of various N-aryl substituted benzimidazolium salts with a Ag base, as well as the synthesis of various NHC–metal complexes using the “coexistence Ag2O technique”, are underway in our laboratory.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13010018/s1, Experimental procedure; spectral data for substrates and products; NMR charts for substrates and products; 1H-NMR of the crude reaction mixture after treating 1 with Ag2O for 10 days; 1H-NMR of the crude reaction mixture after treating 2 with Ag2O for 24 h; 1H-NMR of the crude reaction mixture after treating 2 with Ag2O for 3 h.

Author Contributions

Conceptualization, S.S.; methodology, T.H., Y.T. and R.I.; X-ray analysis, T.Y.; investigation, S.S.; data curation, S.S.; writing—original draft preparation, S.S.; writing—review and editing, S.S.; supervision, S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00018 sch001
Scheme 1. Reaction of 1 with Ag2O.
Scheme 1. Reaction of 1 with Ag2O.
Inorganics 13 00018 sch001
Inorganics 13 00018 sch002
Scheme 2. Reactions of the selected azolium salts with Ag2O.
Scheme 2. Reactions of the selected azolium salts with Ag2O.
Inorganics 13 00018 sch002
Inorganics 13 00018 sch003
Scheme 3. Proposed pathways for the reaction of 2 with Ag2O to form 2b.
Scheme 3. Proposed pathways for the reaction of 2 with Ag2O to form 2b.
Inorganics 13 00018 sch003
Inorganics 13 00018 sch004
Scheme 4. Mechanistic studies.
Scheme 4. Mechanistic studies.
Inorganics 13 00018 sch004
Inorganics 13 00018 sch005
Scheme 5. Synthesis of NHC–Au(I) complex 2c.
Scheme 5. Synthesis of NHC–Au(I) complex 2c.
Inorganics 13 00018 sch005
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MDPI and ACS Style

Sakaguchi, S.; Higashino, T.; Tasaki, Y.; Ichihara, R.; Yajima, T. Reactions of 1-Alkyl-3-phenylbenzimidazolium Salts with Ag2O: The Formation of a Ring-Opening Formamide Derivative and a Ag Complex with an N-heterocyclic Carbene Ligand. Inorganics 2025, 13, 18. https://doi.org/10.3390/inorganics13010018

AMA Style

Sakaguchi S, Higashino T, Tasaki Y, Ichihara R, Yajima T. Reactions of 1-Alkyl-3-phenylbenzimidazolium Salts with Ag2O: The Formation of a Ring-Opening Formamide Derivative and a Ag Complex with an N-heterocyclic Carbene Ligand. Inorganics. 2025; 13(1):18. https://doi.org/10.3390/inorganics13010018

Chicago/Turabian Style

Sakaguchi, Satoshi, Takashi Higashino, Yudai Tasaki, Ryo Ichihara, and Tatsuo Yajima. 2025. "Reactions of 1-Alkyl-3-phenylbenzimidazolium Salts with Ag2O: The Formation of a Ring-Opening Formamide Derivative and a Ag Complex with an N-heterocyclic Carbene Ligand" Inorganics 13, no. 1: 18. https://doi.org/10.3390/inorganics13010018

APA Style

Sakaguchi, S., Higashino, T., Tasaki, Y., Ichihara, R., & Yajima, T. (2025). Reactions of 1-Alkyl-3-phenylbenzimidazolium Salts with Ag2O: The Formation of a Ring-Opening Formamide Derivative and a Ag Complex with an N-heterocyclic Carbene Ligand. Inorganics, 13(1), 18. https://doi.org/10.3390/inorganics13010018

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