Next Article in Journal
The Effect of Support and Reduction Methods on Catalyst Performance in the Selective Oxidation of 1,2-Propanediol
Previous Article in Journal
Unravel the Roles of the Acid Sites in Different Pore Channels of HZSM-5 Catalyst on Ethanol Conversion to Light Olefin
Previous Article in Special Issue
Recent Progress on the Production of Liquid Fuel 2,5-Dimethylfuran via Chemoselective Hydrogenolysis Biomass-Derived 5-Hydroxymethylfurfural
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 4
10.3390/catal15040303
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Hydroxyamide-Functionalized Azolium Anchored on Merrifield Resin for Enantioselective Ir-Catalyzed Reduction of Ketones with Silane

by
Satoshi Sakaguchi
*,
Masamune Koyabu
and
Kazuki Inui
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita 564-8680, Osaka, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(4), 303; https://doi.org/10.3390/catal15040303
Submission received: 26 February 2025 / Revised: 18 March 2025 / Accepted: 19 March 2025 / Published: 24 March 2025
(This article belongs to the Special Issue Heterogeneous Catalysis Towards a Sustainable Future)
Browse Figures
Versions Notes

Abstract

:
Polystyrene-supported chiral hydroxyamide-functionalized benzimidazolium chloride was synthesized by reacting Merrifield resin with a substituted-azole derived from (S)-leucinol. The combination of [IrCl(cod)]2 and the resulting polymer-supported N-heterocyclic carbene (NHC) ligand precursor catalyzed the enantioselective reduction of ketones using (EtO)2MeSiH under heterogeneous reaction conditions via a pre-mixing reaction procedure. Additionally, the solid-state resin could be easily recovered through simple filtration and the catalyst system’s reusability was evaluated.

Graphical Abstract

1. Introduction

Despite their efficiency and selectivity, most homogeneous transition metal catalysts have seen limited industrial use due to challenges with their recycling and the risk of metal contamination in the final products. In contrast, heterogeneous catalysis simplifies separation and recycling, making it more advantageous for industrial applications. The strong N-heterocyclic carbene (NHC)–metal bonds arising from the unique σ-donating and weak π-accepting properties of the carbene centers enhance the stability of NHC–metal catalysts by preventing their decomposition [1,2,3,4,5,6,7,8,9]. As a result, the NHC system is a highly attractive choice for heterogeneous catalysis applications [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].
Since the development of various organic-group-functionalized polymers, such as the widely known Merrifield resin, solid-phase synthesis has been recognized as a valuable tool. Merrifield resin contains chloromethyl groups, which react with substituted azoles to form polymer-supported azolium chloride. To date, several NHC–metal catalysts anchored on Merrifield resin have been developed and successfully used in catalytic transformations (Figure 1). In 2004, Lee demonstrated a polymer-supported NHC–Pd catalyst of the Suzuki cross-coupling reaction between phenylboronic acid and aryl halides [33,34]. The addition reaction of phenylboronic acid to aromatic aldehydes was achieved using an NHC–Rh catalyst under heterogeneous conditions [35]. Additionally, polymer-linked NHC–metal species have been applied to the three-component coupling reaction of alkynes, amines, and aldehydes, as well as to the hydroformination of alkenes and Heck-type reactions [36,37,38,39]. More recently, an alkyl-bridged bis(NHC)–Pd complex anchored on Merrifield resin was developed for the synthesis of alkynones through the reaction of alkynes, aryl iodides, and CO [40]. Meanwhile, studies have reported the coupling reaction of optically active azoles with Merrifield resin, yielding azolium salts with a chiral carbon center [41]. However, no reports have yet described catalytic asymmetric reactions using polymer-supported NHC–metal complexes derived from Merrifield resin.
Recently, we synthesized a new class of azolium compound L1 as a chiral NHC precursor (Figure 2). We developed two methods for Ir-catalyzed asymmetric ketone reduction using (EtO)2MeSiH and L1. First, a well-defined NHC–Ir complex was successfully synthesized via the formation of an intermediate NHC–Ag complex from the reaction of L1 with Ag2O (Figure 2a). The resulting IrCl(NHC)(cod) complex was fully characterized by NMR and an elemental analysis. Notably, IrCl(NHC)(cod) efficiently catalyzed the enantioselective hydrosilylation of ketones with (EtO)2MeSiH, yielding corresponding optically active alcohols (Figure 2a) [42,43,44]. Second, we demonstrated an operationally simple procedure using [Ir(cod)2]BF4 or [Ir(OMe)(cod)]2 as an Ir precursor (Figure 2b). The combination of the Ir precursor and L1 efficiently facilitated the silane reduction of ketones with high enantioselectivity. In this method, the Ir precursor, L1, and (EtO)2MeSiH were first combined in THF, after which the ketone was introduced into the resulting mixture, which was placed in MeOH containing a small amount of K2CO3. This pre-mixing reaction procedure proceeded at room temperature for 2 h, yielding a corresponding alcohol with up to 95% ee (Figure 2b) [45,46,47].
As shown in Figure 2b, we were surprised to find that replacing the benzyl substituent (L1) with a methyl substituent (L2) at the azolium ring substantially reduced both the product yield and enantioselectivity [47]. This observation, along with our continued interest in investigating the catalytic properties of the NHC–Ir species, motivated us to explore the Ir-catalyzed asymmetric reduction of ketones using the chiral azolium salt PS-L3 anchored on a polystyrene resin (Figure 2c). To achieve this, we synthesized the polymer-supported azolium salt PS-L3 through the coupling reaction of a chiral hydroxyamide-substituted benzimidazole 1 derived from (S)-leucinol with Merrifield resin 2. Azolium salt PS-L3 is insoluble in various organic solvents, including THF, Et2O, acetone, CH2Cl2, CHCl3, and MeOH. As a result, the catalytic asymmetric silane reduction of ketones can be performed under heterogeneous conditions using the [IrCl(cod)]2/PS-L3 catalytic system. Additionally, we present preliminary findings on the reusability of this solid-state catalyst system.

2. Results and Discussion

2.1. Preparation of Polymer-Supported Azolium Salt PS-L3 Using Merrifield Resin

This study began with the synthesis of polymer-supported benzimidazolium chloride from commercially available Merrifield resin (1.3 mmol Cl/g, cross-linked with 1% DVB, 200–400 mesh). The hydroxyamide-functionalized azole 1 was derived from optically active (S)-leucinol [46]. A mixture of 1 and chloromethyl polystyrene resin 2 (1/2 = 10/1 molar ratio) in DMF was stirred at 70 °C for 5 days (Table 1). After the coupling reaction, the mixture was filtered through a membrane filter (pore size: 0.2 μm), and the resulting solid resin was washed with MeOH and dried, yielding the polystyrene-supported benzimidazolium chloride PS-L3. Its azolium loading was determined based on the nitrogen content obtained from an elemental analysis (Table 1). The nitrogen content introduced into the polymer was 3.55 wt%, indicating an azolium loading of 0.84 mmol/g (entry 1, run 1). The coupling reaction of 1 with 2 is reproducible, as confirmed by three independent trials (entry 1, runs 1–3), yielding consistent results in both the reaction and elemental analysis. The reaction conditions (70 °C for 5 days) follow those of a known procedure [41]. When 1 reacted with 2 at 70 °C for 1 day, a PS-L3 with a lower azolium content was obtained (entry 2).
Additionally, PS-L3 was characterized using IR spectroscopy (Figure 3). For comparison, the spectrum of Merrifield resin 2 is also included in Figure 3 to highlight the differences between PS-L3 and 2.
The characteristic methylene (CH2) group of the benzyl chloride moiety in Merrifield resin 2 was observed at 1265 cm−1 [39,40], but this band is absent in the spectra of PS-L3. The medium-intensity band at 1556 cm−1 in PS-L3 is attributed to the out-of-phase N–C–N stretching of the anchored benzimidazolium. A new absorption band at 1676 cm−1 appeared due to the C=O bond stretching vibration from the side arm of the azolium ring in PS-L3. The broad bands around 3300 cm−1 are associated with the hydroxy group, amide group, and tertiary nitrogen group, respectively. These spectral features confirm a successful substitution reaction between 1 and 2, resulting in the anchored NHC ligand precursors on the polystyrene resin.

2.2. Ir-Catalyzed Symmetric Silane Reduction of Ketone Using PS-L3

With the polymer-supported azolium salt PS-L3 prepared, we investigated its potential as a chiral ligand precursor in the Ir-catalyzed asymmetric silane reduction of ketones under heterogeneous conditions. As mentioned in the Introduction section, the catalytic reduction of ketones with (EtO)2MeSiH is efficiently promoted by the Ir(OMe)(cod)]2/L1 combined catalytic system under homogeneous conditions at room temperature (Figure 2b) [46]. Notably, we previously established the pre-mixing reaction procedure for this Ir-catalyzed silane reduction reaction in these earlier studies [46].
Table 2 summarizes the representative results of evaluating of Ir complex precursors and base additives with PS-L3 as the key chiral NHC ligand precursor. The reaction involved stirring catalytic amounts of [IrCl(cod)]2 and PS-L3, in the presence of Ag2O, in THF for 3 h. Then, (EtO)2MeSiH (4.5 equiv. with respect to the ketone) was added, and the resulting mixture was stirred at room temperature for 20 h, following the pre-mixing reaction procedure. Subsequently, a mixture of acetophenone (3), K2CO3, and MeOH was introduced, and the reduction reaction was performed at room temperature for 6 h. This catalytic process yielded (S)-1-phenyl-1-propanol ((S)-4) at a 68% yield with 74% ee (entry 1).
It is noteworthy that a dark green solid (likely a polymer-supported NHC–Ir species) appeared in the solution during the silane reduction reaction. At this point, it is important to highlight that the benzimidazolium ring in PS-L3, acting as the NHC ligand precursor, is covalently bonded to the polystyrene resin particle (via a chemical, not physical, bond). This strong bond prevents the NHC–Ir species, formed during the catalytic reaction, from dissociating from the resin. Furthermore, it was observed that no reaction occurred between 3 and (EtO)2MeSiH when using [IrCl(cod)]2 as the catalyst without PS-L3. These results strongly suggest that the solid phase contains the polymer-supported NHC–Ir species, and an asymmetric reduction reaction likely occurs between the liquid phase, which contains the reactants, and the surface of the solid-state catalyst. Therefore, the result shown in entry 1 represents the first example of a catalytic enantioselective reaction using a polymer-supported chiral NHC–Ir catalyst derived from Merrifield resin under heterogeneous conditions.
The use of a silver base in the pre-mixing reaction procedure is crucial. Without Ag2O, the reaction resulted in poor enantioselectivity (Table 2, entry 2). This base likely facilitates the deprotonation of the C2 position of the azolium ring in PS-L3, as shown in Figure 2a [42,43,44]. We then evaluated various base additives (entries 3–5). A promising result was obtained when treating [IrCl(cod)2] with PS-L3 in the presence of Ag2CO3, yielding (S)-4 at a 58% yield with 68% ee (entry 5). As mentioned earlier, PS-L3 was synthesized using (S)-leucinol as the starting material. The chiral NHC ligand precursor with the opposite configuration, PS-L4, was synthesized from (R)-leucinol. The catalytic reduction of 3 with (EtO)2MeSiH, using [IrCl(cod)2] and PS-L4, resulted in (R)-4 at a yield of 52% and 75% ee (entry 6).
Next, we investigated the use of [Ir(OMe)(cod)]2 as an Ir complex precursor. Previously, we successfully developed the [Ir(OMe)(cod)]2/L1 combined catalytic system under homogeneous conditions (Figure 2b) [45]. The OMe group in [Ir(OMe)(cod)]2 acts as an internal base, facilitating deprotonation at the C2 position of the azolium ring in L1, thereby forming a monodentate NHC–Ir complex [45]. However, unlike its reduction with [Ir(OMe)(cod)]2 under homogeneous conditions, the reduction of 3 with (EtO)2MeSiH under heterogeneous conditions using the [Ir(OMe)(cod)]2/PS-L3 catalytic system resulted in a lower product yield and stereoselectivity (Table 2, entry 7). The acetylacetonate (acac) moiety in Ir(acac)(cod) is also known to function as an internal base, facilitating the formation of NHC species through the deprotonation of the azolium ring [48,49,50]. However, the reduction of 3 with (EtO)2MeSiH using Ir(acac)(cod) as the Ir precursor resulted in the formation of (S)-4 with a poor yield (entry 8). Consequently, it can be concluded that the combination of [IrCl(cod)]2 with PS-L3 in the presence of an appropriate base, such as Ag2CO3, is the most effective system for the asymmetric silane reduction of 3.
Table 3 summarizes the results of the enantioselective silane reduction of several ketones catalyzed by the [IrCl(cod)]2/PS-L3 system. While optimization of the reaction conditions is still needed, these reactions provided corresponding optically active alcohols with moderate enantiomeric excess values (entries 1–8). Similarly to the asymmetric reduction of 3 with (EtO)2MeSiH, propiophenone (5) was reduced to (S)-1-phenyl-1-propanol ((S)-6) at a 47% yield with 73% ee (entry 1). Relatively good results were observed in the reductions of butyrophenone (7) and 4′-chloropropiophenone (17), which yielded (S)-1-phenyl-1-butanol ((S)-8) at a 68% yield with 66% ee and (S)-1-(4-chlorophenyl)propan-1-ol ((S)-18) at a 76% yield with 65% ee, respectively (entries 2 and 7). We previously demonstrated that ketones 7 and 17 could be reduced with satisfactory yields and stereoselectivities using the [Ir(OMe)(cod)]2/L1 catalytic system under homogeneous conditions [45]. These results will be discussed further. Valerophenone (7), cyclohexyl phenyl ketone (11), 4′-butylacetophenone (13), and 2′-acetonaphthone (19) were reduced with lower product yields, likely due to steric factors (entries 3–5 and 8). The reduction of 3′-methoxyacetophenone (15) with (EtO)2MeSiH was efficiently promoted by the [IrCl(cod)]2/PS-L3 catalytic system, yielding (S)-1-(3-methoxyphenyl)ethanol ((S)-16) with 65% ee (entry 6).

2.3. Reusability of the Polymer-Supported Asymmetric Catalytic System

Finally, we investigated the reusability of the polymer-supported NHC–Ir catalyst system in asymmetric silane reductions. For this purpose, we selected two ketones, 7 and 17 (Table 4). In this study, we used commercially available Merrifield resin (200–400 mesh) with particle sizes ranging from 38 to 75 μm as the starting material for the synthesis of PS-L3. A membrane filter with a pore size of 0.2 μm was used for the reusability test. After performing the asymmetric silane reduction reactions of ketones 7 and 17 using the [IrCl(cod)]2/PS-L3 catalytic system, solid-state resins X and Y were recovered in 49 and 52 mg yields, respectively, through simple filtration using a membrane filter (entry 1, run 1 and entry 2, run 1).
The result showed that the combination of [IrCl(cod)]2 and X successfully catalyzed the reduction of ketone 7 using the pre-mixing reaction procedure, yielding (S)-8 with 77% ee (entry 1, run 2). Similarly, ketone 17 was reduced to (S)-18 in good yield with 61% ee when catalyzed by [IrCl(cod)]2 combined with Y (entry 2, run 2). In contrast, no reaction occurred when [IrCl(cod)]2 was absent (entry 1, run 2A). These results also suggest that the silane reduction is not promoted by a silver base alone. Although the exact reaction mechanism involved in this reusability test remains unclear, it is possible that an inert Ir(III) species in X (or Y) is replaced by the Ir(I) species in [IrCl(cod)]2, thereby regenerating the polymer-supported NHC–Ir(I) active catalyst. Further investigations into the reusability of this catalytic system are currently underway.

3. Materials and Methods

3.1. General

All chemical reagents and solvents were obtained from commercial suppliers. Chloromethylated polystyrene resin, cross-linked with 1% DVB (200–400 mesh, 1.3 mmol/g of Cl), was obtained from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Omnipore membrane filters (0.2 μm pore size, 80% porosity) were supplied by Merck Co., Inc. (Darmstadt, Germany). Column chromatography was performed using silica gel 60 purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). 1H-NMR spectra were recorded on a JEOL ECA400 spectrometer (400 MHz for 1H-NMR and 100 MHz for 13C-NMR). Chemical shifts in 1H-NMR were reported downfield from TMS (δ = 0 ppm). For 13C-NMR, chemical shifts were referenced to the solvent used as an internal standard. FT–IR spectra were recorded on a Shimadzu IRAffinity spectrometer. Elemental analyses were performed at Osaka University. Scanning probe microscopy (SPM) observations were performed at Shimadzu Corporation (Kyoto, Japan). Hydroxyamide-functionalized benzimidazole 1 was synthesized following our previously published procedure [47].
1-[2-((S)-1-hydroxy-4-methyl-2-pentanylamino)-2-oxoethyl]-3-benzylbenzimidazole (1): 1H-NMR (DMSO-d6) δ = 8.16–8.11 (m, 2H), 7.66 (d, J = 8.2 Hz, 1H), 7.43–7.41 (d, 1H), 7.25–7.17 (m, 2H), 4.93 (d, J = 16.0 Hz, 1H), 4.87 (d, J = 16.0 Hz, 1H), 4.74 (t, J = 6.4 Hz, 1H), 3.84–3.75 (m, 1H), 3.35–3.26 (m, 2H), 1.64–1.54 (m, 2H), 1.33–1.30 (m, 1H), 0.86 (d, J = 6.4 Hz, 3H), 0.79 (d, J = 6.4 Hz, 3H); 13C-NMR (DMSO-d6) δ =166.0, 144.9, 143.2, 134.2, 122.2, 121.5, 119.3, 110.1, 63.6, 49.1, 46.9, 24.2, 23.3, 21.8. NMR charts can be found in the Supplementary Materials.

3.2. Procedure for the Preparation of Polymer-Supported Azolium Salt PS-L3 Using Merrifield Resin

Hydroxyamide-substituted benzimidazole 1 (3.25 mmol, 894 mg) and Merrifield resin 2 (0.325 mmol, 250 mg) were added to DMF (8 mL) and heated at 70 °C for 5 days. The resulting mixture was then filtered through a membrane filter (pore size: 0.2 μm). The obtained solid-state resin was washed with DMF (10 mL × 3), CH2Cl2 (10 mL × 3), and MeOH (10 mL × 3) and then dried under reduced pressure, yielding 337 mg of PS-L3. Anal. Found: C, 79.94; H, 7.43; N, 3.55. IR: υmax (cm−1) 3387, 3059, 3024, 2922, 2850, 1944, 1676, 1556, 1490, 1450, 1367, 1269, 1182, 1068, 1028, 904, 821, 742, 696. The IR spectra of PS-L3 and the SPM observations are provided in the Supplementary Materials.

3.3. General Procedure for Catalytic Asymmetric Reduction of Ketone with (EtO)2MeSiH

A mixture of [IrCl(cod)]2 (0.015 mmol, 10 mg), PS-L3 (0.03 mmol, 42 mg), and Ag2CO3 (0.015 mmol, 4 mg) was stirred in THF (2 mL) at room temperature for 3 h. (EtO)2MeSiH (2.25 mmol, 302 mg) was then added, and the mixture was stirred at room temperature for an additional 20 h. Following this, ketone (0.50 mmol), K2CO3 (5 mg), and MeOH (2 mL) were added, and the mixture was stirred at room temperature for 6 h. Finally, the reaction mixture was filtered through a membrane filter (pore size: 0.2 μm). The solid-state resin was then washed repeatedly with MeOH, yielding 45 mg of recovered resin. Meanwhile, the filtrate was concentrated under reduced pressure using a rotary evaporator. The resulting residue was purified by column chromatography on silica gel using a hexanes/EtOAc (9/1) solvent system to obtain the corresponding alcohol product. Enantiomeric excess (ee) values were determined by chiral GC or chiral LC, following our previously reported procedure [45]. The results of the GC or LC analysis are provided in the Supplementary Materials.
1-Phenylethanol (4): 1H-NMR (CDCl3) δ = 7.40–7.25 (m, 5H), 4.90 (q, J = 6.4 Hz, 1H), 3.53 (br, 1H), 1.50 (d, J = 6.4 Hz, 3H); 13C-NMR (CDCl3) δ = 145.8, 128.5, 127.4, 125.4, 70.4, 25.1.
1-Phenyl-1-propanol (6): 1H-NMR (CDCl3) δ = 7.35–7.26 (m, 5H), 4.59 (t, J = 6.4 Hz, 1H), 3.53 (br, 1H), 1.84–1.71 (m, 2H), 0.92 (t, J = 6.4 Hz, 3H); 13C-NMR (CDCl3) δ = 144.6, 128.4, 127.5, 125.9, 76.0, 31.9, 10.1.
1-Phenyl-1-butanol (8): 1H-NMR (CDCl3) δ = 7.18–7.10 (m, 5H), 4.48 (t, J = 6.4 Hz, 1H), 1.64–1.58 (m, 1H), 1.55–1.46 (m, 1H), 1.32–1.21 (m, 1H), 1.19–1.08 (m, 2H), 0.77 (t, J = 7.2 Hz, 3H); 13C-NMR (CDCl3) δ = 144.9, 128.3, 127.3, 125.8, 74.3, 41.2, 18.9, 13.9.
1-Phenyl-1-pentanol (10): 1H-NMR (CDCl3) δ = 7.33–7.24 (m, 5H), 4.64–4.60 (m, 1H), 1.83–1.64 (m, 3H), 1.43–1.34 (m, 2H), 1.28–1.19 (m, 2H), 0.88 (t, J = 7.2 Hz, 3H); 13C-NMR (CDCl3) δ = 144.9, 128.4, 127.4, 125.9, 74.6, 38.8, 27.9, 22.6, 14.0.
Cyclohexyl(phenyl)methanol (12): 1H-NMR (CDCl3) δ = 7.35–7.26 (m, 5H), 4.36 (dd, J = 7.2 and 3.2 Hz, 1H), 1.98 (d, J = 12.8 Hz, 1H), 1.83–1.75 (m, 2H), 1.65–1.57 (m, 4H), 1.40–1.35 (m, 1H), 1.24–0.91 (m, 4H); 13C-NMR (CDCl3) δ = 143.6, 128.2, 127.4, 126.6, 79.4, 44.9, 29.3, 28.8, 26.4, 26.1, 26.0.
1-(4-Butylphenyl)ethanol (14): 1H-NMR (CDCl3) δ = 7.27 (d, J = 7.6 Hz, 2H), 7.16 (d, J = 7.6 Hz, 2H), 4.86 (q, J = 6.4 Hz, 1H), 2.60 (t, J = 7.6 Hz, 2H), 1.91 (br, 1H), 1.63–1.55 (m, 2H), 1.48 (d, J = 6.4 Hz, 3H), 1.40–1.31 (m, 2H), 0.92 (t, J = 7.6 Hz, 3H); 13C-NMR (CDCl3) δ = 143.0, 142.2, 128.5, 125.3, 70.2, 35.2, 33.6, 25.0, 22.3, 13.9.
1-(4-Methoxyphenyl)ethanol (16): 1H-NMR (CDCl3) δ = 7.31–7.26 (m, 2H), 6.89–6.86 (m, 2H), 4.84 (q, J = 6.4 Hz, 1H), 3.80 (s, 3H), 1.94 (br, 1H), 1.47 (d, J = 6.8 Hz, 3H); 13C-NMR (CDCl3) δ = 158.9, 138.0, 126.6, 113.8, 69.9, 55.2, 25.0.
1-(4-Chlorophenyl)propanol (18): 1H-NMR (CDCl3) δ = 7.32–7.25 (m, 4H), 4.57 (t, J = 6.4 Hz, 3H), 1.94 (br, 1H), 1.82–1.67 (m, 2H), 0.90 (t, J = 7.2 Hz, 3H); 13C-NMR (CDCl3) δ = 142.9, 133.0, 128.5, 127.3, 75.2, 31.9, 9.9.
1-(2-Naphthyl)ethanol (20): 1H-NMR (CDCl3) δ = 7.80–7.74 (m, 4H), 7.46–7.41 (m, 3H), 4.98 (q, J = 6.4 Hz, 1H), 1.87 (br, 1H), 1.53 (d, J = 6.4 Hz, 3H); 13C-NMR (CDCl3) δ = 143.2, 133.3, 132.8, 128.2, 127.9, 127.6, 126.1, 125.7, 123.8, 123.7, 70.4, 25.0.

3.4. Procedure for the Catalytic Asymmetric Reduction of 7 with the Recovered Solid-State Resin X

A mixture of [IrCl(cod)]2 (0.015 mmol, 10 mg), the recovered resin X (49 mg), and Ag2CO3 (0.03 mmol, 8 mg) was stirred in THF (2 mL) at room temperature. After 3 h of stirring, (EtO)2MeSiH (2.25 mmol, 302 mg) was added to the reaction vessel. The mixture was then stirred at room temperature for an additional 20 h. Following this, ketone 7 (0.50 mmol, 74 mg), K2CO3 (5 mg), and MeOH (2 mL) were introduced. Then, the resulting mixture was stirred at room temperature.

4. Conclusions

We have successfully synthesized a novel chiral benzimidazolium salt PS-L3 anchored on polystyrene resin. This PS-L3 can serve as the NHC ligand in the Ir-catalyzed asymmetric silane reduction of ketones under heterogeneous conditions. Additionally, we were able to obtain preliminary results demonstrating the reusability of the polymer-supported NHC–Ir catalyst system.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15040303/s1, Figure S1: NMR and IR charts for 1; Figure S2: IR charts for 2 and PS-L3; Figure S3: SPM observations for 2 and PS-L3; Figure S4: Selected chiral GC and LC traces. References [42,43,44,45,46,47] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, S.S.; methodology, M.K. and K.I.; 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 this 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.

References

  1. Jayaraj, A.; Raveedran, A.V.; Latha, A.T.; Priyadarshini, D.; Swamy, P.C.A. Coordination Versatility of NHC-metal Topologies in Asymmetric Catalysis: Synthetic Insights and Recent Trends. Coord. Chem. Rev. 2023, 478, 214922. [Google Scholar] [CrossRef]
  2. Neshat, A.; Mastrorilli, P.; Mobarakeh, A.M. Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands. Molecules 2022, 27, 95. [Google Scholar] [CrossRef]
  3. Bellotti, P.; Koy, M.; Hopkinson, M.N.; Glorius, F. Recent advances in the chemistry and applications of N-heterocyclic carbenes. Nat. Rev. Chem. 2021, 5, 711–725. [Google Scholar] [CrossRef] [PubMed]
  4. Nahra, F.; Nelson, D.J.; Nolan, S.P. Design Concepts for N-Heterocyclic Carbene Ligands. Trends Chem. 2020, 2, 1096–1111. [Google Scholar] [CrossRef]
  5. Fliedel, C.; Labande, A.; Manoury, E.; Rinaldo, P. Chiral N-heterocyclic carbene ligands with additional chelating group(s) applied to homogeneous metal-mediated asymmetric catalysis. Coord. Chem. Rev. 2019, 394, 65–103. [Google Scholar] [CrossRef]
  6. Peris, E. Smart N-Heterocyclic Carbene Ligands in Catalysis. Chem. Rev. 2018, 118, 9988–10031. [Google Scholar] [CrossRef]
  7. Pape, F.; Teichert, J.F. Dealing at Arm’s Length: Catalysis with N-Heterocyclic Carbene Ligands Bearing Anionic Tethers. Eur. J. Org. Chem. 2017, 38, 4206–4229. [Google Scholar] [CrossRef]
  8. Hopkinson, M.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510, 485–496. [Google Scholar] [CrossRef]
  9. Wanga, F.; Liua, L.-J.; Wanga, W.; Li, S.; Shi, M. Chiral NHC–metal-based asymmetric catalysis. Coord. Chem. Rev. 2012, 256, 804–853. [Google Scholar] [CrossRef]
  10. Wang, W.; Cui, L.; Sun, P.; Shi, L.; Yue, C.; Li, F. Reusable N-Heterocyclic Carbene Complex Catalysts and Beyond: A Perspective on Recycling Strategies. Chem. Rev. 2018, 118, 9843–9929. [Google Scholar] [CrossRef]
  11. Koy, M.; Bellotti, P.; Das, M.; Glorius, F. N-Heterocyclic carbenes as tunable ligands for catalytic metal surfaces. Nat. Catal. 2021, 4, 352–363. [Google Scholar] [CrossRef]
  12. Pentela, N.; Gayathri, V.; Samanta, D. N-heterocyclic carbene bearing thermoresponsive poly(NIPAM) supported palladium (II) complex: Synthetic strategy and application. J. Organomet. Chem. 2020, 913, 121196. [Google Scholar] [CrossRef]
  13. Ye, R.; Zhukhovitskiy, A.; Kazantsev, R.; Fakra, S.; Wickemeyer, B.; Toste, F.; Somorjai, G. Supported Au Nanoparticles with N-Heterocyclic Carbene Ligands as Active and Stable Heterogeneous Catalysts for Lactonization. J. Am. Chem. Soc. 2018, 140, 4144–4149. [Google Scholar] [CrossRef] [PubMed]
  14. Price, G.; Hassan, A.; Chandrasoma, N.; Bogdan, A.; Djuric, S.; Organ, M. Pd-PEPPSI-IPent-SiO2: A Supported Catalyst for Challenging Negishi Coupling Reactions in Flow. Angew. Chem. Int. Ed. 2017, 56, 13347–13350. [Google Scholar] [CrossRef]
  15. Price, G.; Bogdan, A.; Aguirre, A.; Iwai, T.; Djuric, S.; Organ, M. Continuous flow Negishi cross-couplings employing silica-supported Pd-PEPPSI-IPr precatalyst. Catal. Sci. Technol. 2016, 6, 4733–4742. [Google Scholar] [CrossRef]
  16. Ernst, J.; Muratsugu, S.; Wang, F.; Tada, M.; Glorius, F. Tunable Heterogeneous Catalysis: N-Heterocyclic Carbenes as Ligands for Supported Heterogeneous Ru/K-Al2O3 Catalysts to Tune Reactivity and Selectivity. J. Am. Chem. Soc. 2016, 138, 10718–10721. [Google Scholar] [CrossRef]
  17. Fujita, K.; Fujii, A.; Sato, J.; Yasuda, H. Magnetically Recoverable N-Heterocyclic Carbene-Gold(I) Catalyst for Hydroamination of Terminal Alkynes. Synlett 2016, 27, 1941–1944. [Google Scholar] [CrossRef]
  18. Romanenko, I.; Norsic, S.; Veyre, L.; Sayah, R.; D’Agosto, F.; Raynaud, J.; Boisson, C.; Lacôte, E.; Thieuleux, C. Active and Recyclable Polyethylene-Supported Iridium-(N-Heterocyclic Carbene) Catalyst for Hydrogen/Deuterium Exchange Reactions. Adv. Synth. Catal. 2016, 358, 2317–2323. [Google Scholar] [CrossRef]
  19. Sun, Z.; Liu, Y.; Chen, J.; Huang, C.; Tu, T. Robust Iridium Coordination Polymers: Highly Selective, Efficient, and Recyclable Catalysts for Oxidative Conversion of Glycerol to Potassium Lactate with Dihydrogen Liberation. ACS Catal. 2015, 5, 6573–6578. [Google Scholar] [CrossRef]
  20. Wang, Z.; Yu, Y.; Zhang, Y.; Li, S.; Qian, H.; Lin, Z. A magnetically separable palladium catalyst containing a bulky N-heterocyclic carbene ligand for the Suzuki-Miyaura reaction. Green Chem. 2015, 17, 413–420. [Google Scholar] [CrossRef]
  21. Khairnar, B.; Bhanage, B. Amidation of Aryl Halides with Isocyanides Using a Polymer-Supported Palladium-N-Heterocyclic Carbene Complex as an Efficient, Phosphine-Free and Heterogeneous Recyclable Catalyst. Synthesis 2014, 46, 1236–1242. [Google Scholar] [CrossRef]
  22. Monge-Marcet, A.; Pleixats, R.; Cattoën, X.; Man, M. Catalytic applications of recyclable silica immobilized NHC-ruthenium complexes. Tetrahedron 2013, 69, 341–348. [Google Scholar] [CrossRef]
  23. Khedkar, M.; Khan, S.; Dhake, K.; Bhanage, B. Carbonylative Cyclization of o-Halobenzoic Acids for Synthesis of N-Substituted Phthalimides Using Polymer-Supported Palladium-N-Heterocyclic Carbene as an Efficient, Heterogeneous, and Reusable Catalyst. Synthesis 2012, 44, 2623–2629. [Google Scholar] [CrossRef]
  24. Qureshi, Z.; Revankar, S.; Khedkar, M.; Bhanage, B. Aminocarbonylation of aryl iodides with primary and secondary amines in aqueous medium using polymer supported palladium-N-heterocyclic carbene complex as an efficient and heterogeneous recyclable catalyst. Catal. Today 2012, 198, 148–153. [Google Scholar] [CrossRef]
  25. Bagal, D.; Watile, R.; Khedkar, M.; Dhake, K.; Bhanage, B. PS-Pd-NHC: An efficient and heterogeneous recyclable catalyst for direct reductive amination of carbonyl compounds with primary/secondary amines in aqueous medium. Catal. Sci. Tecnol. 2012, 2, 354–358. [Google Scholar] [CrossRef]
  26. Ranganath, K.; Schäfer, A.; Glorius, F. Comparison of Superparamagnetic Fe3O4-Supported N-Heterocyclic Carbene-Based Catalysts for Enantioselective Allylation. ChemCatChem 2011, 3, 1889–1891. [Google Scholar] [CrossRef]
  27. Yang, H.; Wang, Y.; Qin, Y.; Chong, Y.; Yang, Q.; Li, G.; Zhang, L.; Li, W. One-pot preparation of magnetic N-heterocyclic carbene-functionalized silica nanoparticles for the Suzuki-Miyaura coupling of aryl chlorides: Improved activity and facile catalyst recovery. Green Chem. 2011, 13, 1352–1361. [Google Scholar] [CrossRef]
  28. Yu, T.; Li, Y.; Yao, C.; Wu, H.; Liu, Y.; Wu, P. An Efficient and Recyclable Mesostructured Polymer-Supported N-Heterocyclic Carbene-Palladium Catalyst for Sonogashira Reactions. Chin. J. Catal. 2011, 32, 1712–1718. [Google Scholar] [CrossRef]
  29. Bagal, D.; Qureshi, Z.; Dhake, K.; Khan, S.; Bhanage, B. An efficient and heterogeneous recyclable palladium catalyst for chemoselective conjugate reduction of α,β-unsaturated carbonyls in aqueous medium. Green Chem. 2011, 13, 1490–1494. [Google Scholar] [CrossRef]
  30. Ranganath, K.; Kloesges, J.; Schäfer, A.; Glorius, F. Asymmetric Nanocatalysis: N-Heterocyclic Carbenes as Chiral Modifiers of Fe3O4/Pd nanoparticles. Angew. Chem. Int. Ed. 2010, 49, 7786–7789. [Google Scholar] [CrossRef]
  31. Worm-Leonhard, K.; Meldal, M. Green Catalysts: Solid-Phase Peptide Carbene Ligands in Aqueous Transition-Metal Catalysis. Eur. J. Org. Chem. 2008, 2008, 5244–5253. [Google Scholar] [CrossRef]
  32. Zhang, Y.; Zhao, L.; Patra, P.; Ying, J. Synthesis and catalytic applications of mesoporous polymer colloids in olefin hydrosilylation. Adv. Synth. Catal. 2008, 350, 662–666. [Google Scholar] [CrossRef]
  33. Jun, B.; Lee, Y. Preparation of polymer-supported palladium/N-heterocyclic carbene complex for Suzuki cross-coupling reactions. Tetrahedron Lett. 2004, 45, 1837–1840. [Google Scholar] [CrossRef]
  34. Lee, D.; Kim, J.; Jun, B.; Kang, H.; Park, J.; Lee, Y. Macroporous polystyrene-supported palladium catalyst containing a bulky N-heterocyclic carbene ligand for Suzuki reaction of aryl chlorides. Org. Lett. 2008, 10, 1609–1612. [Google Scholar] [CrossRef]
  35. Yan, C.; Zeng, X.; Zhang, W.; Luo, M. Polymer-supported N-heterocyclic carbene-rhodium complex catalyst for the addition of arylboronic acids to aldehydes. J. Organomet. Chem. 2006, 691, 3391–3396. [Google Scholar] [CrossRef]
  36. Li, P.; Wang, L.; Zhang, Y.; Wang, M. Highly efficient three-component (aldehyde-alkyne-amine) coupling reactions catalyzed by a reusable PS-supported NHC-Ag(I) under solvent-free reaction conditions. Tetrahedron Lett. 2008, 49, 6650–6654. [Google Scholar] [CrossRef]
  37. Gil, W.; Boczon, K.; Trzeciak, A.; Ziólkowski, J.; Garcia-Verdugo, E.; Luis, S.; Sans, V. Supported N-heterocyclic carbene rhodium complexes as highly selective hydroformylation catalysts. J. Mol. Catal. A Chem. 2009, 309, 131–136. [Google Scholar] [CrossRef]
  38. Kim, Y.; Shin, S.; Yoon, H.; Kim, J.; Cho, J.; Lee, Y. Polymer-supported N-heterocyclic carbene-iron (III) catalyst and its application to dehydration of fructose into 5-hydroxymethyl-2-furfural. Catal. Commun. 2013, 40, 18–22. [Google Scholar] [CrossRef]
  39. Mohammadi, E.; Movassagh, B. Synthesis of polystyrene-supported Pd(II)-NHC complex derived from theophylline as an efficient and reusable heterogeneous catalyst for the Heck-Matsuda cross-coupling reaction. J. Mol. Catal. A Chem. 2016, 418, 158–167. [Google Scholar] [CrossRef]
  40. Mansour, W.; Fettouhi, M.; Saleem, Q.; El Ali, B. Robust alkyl-bridged bis(N-heterocyclic carbene)palladium(II) complexes anchored on Merrifield’s resin as active catalysts for the selective synthesis of flavones and alkynones. Appl. Organomet. Chem. 2021, 35, e6195. [Google Scholar] [CrossRef]
  41. Yoshida, K.; Horiuchi, S.; Takeichi, T.; Shida, H.; Imamoto, T.; Yanagisawa, A. Bicyclic Imidazoles for Modular Synthesis of Chiral Imidazolium Salts. Org. Lett. 2010, 12, 1764–1767. [Google Scholar] [CrossRef] [PubMed]
  42. Manabe, Y.; Shinohara, K.; Nakamura, H.; Teramoto, H.; Sakaguchi, S. Chiral N-heterocyclic carbene iridium catalyst for the enantioselective hydrosilane reduction of ketones. J. Mol. Catal. A Chem. 2016, 421, 138–145. [Google Scholar] [CrossRef]
  43. Shinohara, K.; Kawabata, S.; Nakamura, H.; Manabe, Y.; Sakaguchi, S. Enantioselective Hydrosilylation of Ketones Catalyzed by a Readily Accessible N-Heterocyclic Carbene-Ir Complex at Room Temperature. Eur. J. Org. Chem. 2014, 2014, 5532–5539. [Google Scholar] [CrossRef]
  44. Kawabata, S.; Tokura, H.; Chiyojima, H.; Okamoto, M.; Sakaguchi, S. Asymmetric Hydrosilane Reduction of Ketones Catalyzed by an Iridium Complex Bearing a Hydroxyamide-Functionalized NHC Ligand. Adv. Synth. Catal. 2012, 354, 807–812. [Google Scholar] [CrossRef]
  45. Sakaguchi, S.; Nagao, C.; Ichihara, R.; Matsuo, S. Operationally Simple Enantioselective Silane Reduction of Ketones by the [Ir(OMe)(cod)]2/Azolium Catalytic System. Int. J. Org. Chem. 2024, 14, 1–19. [Google Scholar] [CrossRef]
  46. Matsuki, T.; Teramoto, H.; Ichihara, R.; Inui, K.; Sakaguchi, S. Asymmetric silane reduction of ketones and β-Keto esters catalyzed by a chiral azolium/iridium system in the presence of a base in methanol at room temperature. Results Chem. 2022, 4, 100364. [Google Scholar] [CrossRef]
  47. Teramoto, H.; Sakaguchi, S. Enantioselective catalytic hydrosilylation of propiophenone with a simple combination of a cationic iridium complex and a chiral azolium salt. J. Organomet. Chem. 2018, 875, 52–58. [Google Scholar] [CrossRef]
  48. Riener, K.; Bitzer, M.J.; Pöthig, A.; Raba, A.; Cokoja, M.; Herrmann, W.A.; Kühn, F.E. On the Concept of Hemilability: Insights into a Donor-Functionalized Iridium(I) NHC Motif and Its Impact on Reactivity. Inorg. Chem. 2014, 53, 12767–12777. [Google Scholar] [CrossRef]
  49. Jeletic, M.S.; Jan, M.T.; Ghiviriga, I.; Abboud, K.A.; Veige, A.S. New iridium and rhodium chiral di-N-heterocyclic carbene (NHC) complexes and their application in enantioselective catalysis. Dalton Trans. 2009, 15, 2764–2776. [Google Scholar] [CrossRef]
  50. Ortega-Lepe, I.; Rossin, A.; Sánchez, P.; Santos, L.L.; Rendón, N.; Álvarez, E.; López-Serrano, J.; Suárez, A. Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes. Inorg. Chem. 2021, 60, 18490–18502. [Google Scholar] [CrossRef]
Catalysts 15 00303 g001
Figure 1. Polymer-supported NHC–metal catalysts derived from Merrifield resin. References [33,34,35,36,37,38,39,40,41].
Figure 1. Polymer-supported NHC–metal catalysts derived from Merrifield resin. References [33,34,35,36,37,38,39,40,41].
Catalysts 15 00303 g001
Catalysts 15 00303 g002
Figure 2. Asymmetric silane reduction reaction of ketones catalyzed by an NHC–Ir complex. References [42,43,44,45,46,47].
Figure 2. Asymmetric silane reduction reaction of ketones catalyzed by an NHC–Ir complex. References [42,43,44,45,46,47].
Catalysts 15 00303 g002
Catalysts 15 00303 g003
Figure 3. IR spectra of Merrifield resin 2 (top) and PS-L3 (bottom).
Figure 3. IR spectra of Merrifield resin 2 (top) and PS-L3 (bottom).
Catalysts 15 00303 g003
Table 1. Preparation of PS-L3 and reproducibility of the experimental procedure.
Table 1. Preparation of PS-L3 and reproducibility of the experimental procedure.
Catalysts 15 00303 i001
Entry Elemental Analysis of PS-L3Azolium Content 1
1 2run 1C, 79.94; H, 7.43; N, 3.550.84 mmol/g
run 2C, 79.66; H, 7.39; N, 3.620.86 mmol/g
run 3C, 79.86; H, 7.58; N, 3.480.83 mmol/g
2 3run 1C, 85.08; H, 7.23; N, 1.220.29 mmol/g
1 Calculated using the following equation: N% × 1000/14.01 × 3. 2 Reaction conditions for each run: 1 (3.25 mmol, 894 mg); 2 (0.325 mmol, 250 mg); DMF (8 mL); 70 °C; 5 days. These reactions yielded 337 mg of PS-L3 per run. 3 Reaction conditions for each run: 1 (3.25 mmol, 894 mg); 2 (0.325 mmol, 250 mg); DMF (8 mL); 70 °C; 24 h. This reaction yielded 273 mg of PS-L3.
Table 2. Ir-catalyzed symmetric reduction of 3 with (EtO)2MeSiH under heterogeneous conditions.
Table 2. Ir-catalyzed symmetric reduction of 3 with (EtO)2MeSiH under heterogeneous conditions.
Catalysts 15 00303 i002
Entry 1Ir PrecursorAzoliumBaseProductYield [%] 2Ee [%] 3
1[IrCl(cod)]2PS-L3Ag2O(S)-46874
2[IrCl(cod)]2PS-L3none(S)-43113
3[IrCl(cod)]2PS-L3tBuOK 4(S)-42339
4[IrCl(cod)]2PS-L3AgOAc 4(S)-42444
5[IrCl(cod)]2PS-L3Ag2CO3(S)-45868
6[IrCl(cod)]2PS-L4 5Ag2CO3(R)-45275
7[IrOMe(cod)]2 6PS-L3none(S)-44938
8Ir(acac)(cod) 7PS-L3none(S)-42641
1 The catalytic reaction was performed as follows: A mixture of [IrCl(cod)]2 (0.015 mmol, 10 mg), PS-L3 (0.03 mmol, 42 mg), and base (0.015 mmol) within THF (2 mL) was stirred for 3 h. Then, (EtO)2MeSiH (2.25 mmol, 302 mg) was added, and the mixture was stirred for 20 h. Subsequently, 3 (0.5 mmol, 60 mg), K2CO3 (5 mg), and MeOH (2 mL) were added, and the reduction reaction was allowed to proceed for 6 h. 2 GC yield determined using the internal standard method. 3 Determined by GC using a chiral stationary phase. 4 Base (0.03 mmol) was used. 5 While PS-L3 was prepared from (S)-leucinol, PS-L4 was prepared from (R)-leucinol. 6 [Ir(OMe)(cod)]2 (0.015 mmol, 10 mg) was used. 7 Ir(acac)(cod) (0.03 mmol, 12 mg) was used.
Table 3. Ir-catalyzed symmetric silane reduction of various ketones under heterogeneous conditions.
Table 3. Ir-catalyzed symmetric silane reduction of various ketones under heterogeneous conditions.
Catalysts 15 00303 i003
Entry 1Substrate Product Yield [%] 2Ee [%]
1 3,4Catalysts 15 00303 i0045Catalysts 15 00303 i005(S)-64773
2 5Catalysts 15 00303 i0067Catalysts 15 00303 i007(S)-86866
3 5Catalysts 15 00303 i0089Catalysts 15 00303 i009(S)-103461
4 5Catalysts 15 00303 i01011Catalysts 15 00303 i011(S)-122371
5 3Catalysts 15 00303 i01213Catalysts 15 00303 i013(S)-144061
6 3Catalysts 15 00303 i01415Catalysts 15 00303 i015(S)-163365
7 5Catalysts 15 00303 i01617Catalysts 15 00303 i017(S)-187665
8 3Catalysts 15 00303 i01819Catalysts 15 00303 i019(S)-203750
1 The catalytic reaction was performed as follows: A mixture of [IrCl(cod)]2 (0.015 mmol, 10 mg), PS-L3 (0.03 mmol, 42 mg), and Ag2CO3 (0.015 mmol, 4 mg) within THF (2 mL) was stirred for 3 h. Then, (EtO)2MeSiH (2.25 mmol, 302 mg) was added, and the mixture was stirred for 20 h. Subsequently, ketone (0.5 mmol), K2CO3 (5 mg), and MeOH (2 mL) were added, and the reduction reaction was allowed to proceed for 6 h. 2 Isolated yield. 3 Ee was determined by GC using a chiral stationary phase. 4 Yield was determined by GC using the internal standard method. 5 Ee was determined by LC using a chiral stationary phase.
Table 4. The reusability of the polymer-supported asymmetric catalytic system.
Table 4. The reusability of the polymer-supported asymmetric catalytic system.
EntrySubstrate Catalytic SystemProductYield [%] 1Ee [%] 2
17run 1 3,4 [IrCl(cod)]2 combined with PS-L3 5(S)-86866
7run 2 6 [IrCl(cod)]2 combined with recovered resin X(S)-85277
7run 2A 7 recovered resin XNo reaction occurred.
217run 1 3,8 [IrCl(cod)]2 combined with PS-L3 5(S)-187665
17run 2 9 [IrCl(cod)]2 combined with recovered resin Y 6(S)-188061
1 Isolated yield. 2 Determined by LC on a chiral stationary phase. 3 The catalytic reaction was performed as follows: A mixture of [IrCl(cod)]2 (0.015 mmol, 10 mg), PS-L3 (0.03 mmol, 42 mg), and Ag2CO3 (0.015 mmol, 4 mg) within THF (2 mL) was stirred for 3 h, followed by the addition of (EtO)2MeSiH (2.25 mmol, 302 mg). After stirring for 20 h, ketone (0.5 mmol), K2CO3 (5 mg), and MeOH (2 mL) were added. The resulting mixture was then allowed to react for 6 h. 4 The same data are presented in Table 3, entry 2. 5 The solid-state resin X (or Y) was recovered as follows: After the catalytic reaction, the resulting mixture was filtered through a membrane filter (pore size: 0.2 μm). The recovered resin X (or Y) was then washed thoroughly with MeOH. 6 The catalytic reaction was performed with X (45 mg) instead of PS-L3. 7 The catalytic reaction was performed with X (45 mg) instead of PS-L3, without [IrCl(cod)]2. 8 The same data are presented in Table 3, entry 7. 9 The catalytic reaction was performed with Y (49 mg) instead of PS-L3.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Sakaguchi, S.; Koyabu, M.; Inui, K. Hydroxyamide-Functionalized Azolium Anchored on Merrifield Resin for Enantioselective Ir-Catalyzed Reduction of Ketones with Silane. Catalysts 2025, 15, 303. https://doi.org/10.3390/catal15040303

AMA Style

Sakaguchi S, Koyabu M, Inui K. Hydroxyamide-Functionalized Azolium Anchored on Merrifield Resin for Enantioselective Ir-Catalyzed Reduction of Ketones with Silane. Catalysts. 2025; 15(4):303. https://doi.org/10.3390/catal15040303

Chicago/Turabian Style

Sakaguchi, Satoshi, Masamune Koyabu, and Kazuki Inui. 2025. "Hydroxyamide-Functionalized Azolium Anchored on Merrifield Resin for Enantioselective Ir-Catalyzed Reduction of Ketones with Silane" Catalysts 15, no. 4: 303. https://doi.org/10.3390/catal15040303

APA Style

Sakaguchi, S., Koyabu, M., & Inui, K. (2025). Hydroxyamide-Functionalized Azolium Anchored on Merrifield Resin for Enantioselective Ir-Catalyzed Reduction of Ketones with Silane. Catalysts, 15(4), 303. https://doi.org/10.3390/catal15040303

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop