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Open AccessArticle

Selective Hydration of Nitriles to Corresponding Amides in Air with Rh(I)-N-Heterocyclic Complex Catalysts

1
MTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group, P.O. Box 400, H-4002 Debrecen, Hungary
2
Doctoral School of Chemistry, University of Debrecen, H-4002 Debrecen, Hungary
3
Department of Physical Chemistry, University of Debrecen, P.O. Box 400, H-4002 Debrecen, Hungary
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(1), 125; https://doi.org/10.3390/catal10010125
Received: 23 December 2019 / Revised: 13 January 2020 / Accepted: 13 January 2020 / Published: 16 January 2020
(This article belongs to the Special Issue Catalysis in Heterocyclic and Organometallic Synthesis)

Abstract

A new synthetic method for obtaining [RhCl(cod)(NHC)] complexes (14) (cod = η4-1,5-cyclooctadiene, NHC = N-heterocyclic carbene: IMes, SIMes, IPr, and SIPr, respectively) is reported together with the catalytic properties of 14 in nitrile hydration. In addition to the characterization of 14 in solution by 13C NMR spectroscopy, the structures of complexes 3, and 4 have been established also in the solid state with single-crystal X-ray diffraction analysis. The Rh(I)-NHC complexes displayed excellent catalytic activity in hydration of aromatic nitriles (up to TOF = 276 h−1) in water/2-propanol (1/1 v/v) mixtures in air.
Keywords: hydration; metal catalysis; N-heterocyclic carbenes; nitriles; rhodium; synthesis of organometallics hydration; metal catalysis; N-heterocyclic carbenes; nitriles; rhodium; synthesis of organometallics

1. Introduction

From the viewpoint of the industrial and pharmacological applications, amides are important compounds in many fields and several ways are reported to obtain amides from nitriles [1,2]. Hydration of nitriles to amides is a 100% atom economic reaction, however the procedure is biased by selectivity issues. Traditionally, hydration of nitriles has been performed in the presence of strong inorganic acids (H2SO4) or bases (NaOH) under harsh conditions, that often results in over-hydrolysis and produces undesired carboxylic acids. To avoid this problem, in the last decades several remarkable catalytic systems have been developed to stop hydration at the amide stage, e.g., using enzymes as biocatalysts (nitrile hydratase, NHase) [3], nanocatalysts such as a Fe3O4 magnetic nanoparticles-supported Cu-NHC complex [4], or ruthenium hydroxide nanoparticles on magnetic silica [5], silver nanoparticles [6], and other heterogeneous [7,8,9,10] or homogenous catalysts. A broad spectrum of transition metal complexes based on rhodium [11,12], ruthenium [12,13,14,15], nickel [16], osmium [17], and gold [18] were employed as catalysts, and the field has been reviewed from various aspects [19,20,21,22,23,24,25,26,27,28,29].
Transition metal-free processes have been described, too, such as the CsOH/DMSO superbase system [30], NaOH as catalyst [31], or tBuOK under anhydrous conditions [32]. Nitrile hydratases catalyze the hydration of nitriles to the corresponding amides under softer conditions and have been successfully used, for example, for production of levetiracetam (Keppra®) for the treatment of epilepsy [3]. However, the application of most NHases is limited because of their substrate specificity, and the rapid decay of the catalytic activity at temperatures higher than 10–30 °C, not mentioning their high cost. Despite all the mentioned results, the development of efficient new catalysts is still required. Although the homogenous organometallic catalysts give the target amides with high selectivity and in high yield, many of the reported reactions were carried out at high temperature (> 150 °C) [33], or in several cases required specific reaction conditions such as microwave irradiation, inert atmosphere or long reaction times. For example, Oshiki et al. described a very efficient catalyst [33], cis-[Ru(acac)2(PPh2py)2] (acac = acetylacetonate, PPh2py = diphenyl-2-pyridylphosphine) for hydration of benzonitrile at 180 °C in 1,2-dimethoxyethane under argon with the highest turnover frequency reported to date for this reaction, i.e., TOF = 20,900 h−1 (TOF = mol amide × (mol catalyst × h)−1). However, this excellent activity was observed only at high temperature; the TOF value dropped to 222 h−1 upon reducing the temperature to 150 °C, and no product was observed at 80 °C.
The field of transition metal complex-catalyzed nitrile hydration is dominated by ruthenium-based catalysts [7,10,12,13,14,33,34] and only a few rhodium catalysts can be found in the literature for this transformation. Ajjou et al. reported that the water-soluble rhodium complex generated in situ from [RhCl(cod)]2 (cod = η4-1,5-cyclooctadiene) and P(m-C6H4SO3Na)3 (mtppts) very effectively catalyzed the hydration of nitriles under basic conditions. As an example, benzonitrile yielded the corresponding amide quantitatively in 24 h at 90 °C and at pH of ~11.7 [35]. Saito et al. described a Rh(I)-complex prepared in situ from [Rh(cod)(OMe)]2 and PCy3 (Cy = cyclohexyl), as a remarkable hydration catalyst for nitriles in 2-PrOH at 25 °C. The nitrile substrates included aromatic, aliphatic, and olefinic substituents, however, at this low temperature, 24–72 h reaction time was required to achieve quantitative yields [36]. Bera et al. have found that in the presence of a base, [Rh(cod)(κC2-PIN)Br] (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene) showed outstanding catalytic activity for hydration of organonitriles in 2-PrOH. A turnover frequency of 20,000 h−1 was possible to achieve for acrylonitrile and it was demonstrated that the naphthyridine group enhanced the hydration activity of the metal centre [37]. Recently, Cadierno et al. disclosed that [RhCl(cod){P(NMe2)3}] promoted very efficiently the selective hydration of an array of nitriles in water without the addition of a base or other additive [11].
In the last three decades, N-heterocyclic carbene (NHC) ligands and the transition metal complexes of them have attracted enormous interest in organometallic chemistry, as well as catalysis [38,39,40,41]. It is therefore surprising that Rh(I)-NHC complexes have not been employed for catalysis of nitrile hydration reactions in aqueous or partly aqueous systems. One reason for this relative lack of prominence may be in that generally, the Rh(I)-complexes—with a few exceptions—were found less reactive than the Ru(II)-based complex catalysts, and in the hydration of benzonitrile they were characterized with turnover frequencies TOF < 100 h−1. [RhCl(NHC)(cod)] complexes were first reported in 1974 by Lappert et al. [42], followed by pioneering contributions of Herrmann et al. [43]. Generally, [RhCl(cod)(NHC)] complexes are possible to be prepared by deprotonation of the imidazolium salts in the presence of [RhCl(cod)]2, and the reported procedures differ only in the nature of the deprotonation agent. The synthesis may involve the direct reaction of the free carbene (isolated or in situ generated) with [RhCl(cod)]2 [44]; reaction of an imidazolium halide salt with a [Rh(μ-OR)(cod)]2 alkoxide complex [45]; and transmetallation of [RhCl(cod)]2 with silver-NHC complexes [46]. In 2009, it was discovered that imidazolium-2-cyanides can transfer NHC ligands to rhodium complexes and this finding opened a new pathway of synthesis of [RhCl(NHC)(cod)] complexes, too [47]. Plenio et al. also reported the one-step synthesis of [RhCl(NHC)(cod)] complexes using K2CO3 as base in acetone at 60 °C [48]. tBuOK in THF could also be used at room temperature [49].
The first selective catalytic hydration of nitriles under anhydrous conditions in the presence of [RhCl(cod)(IMes)] as the catalyst was reported in 2009 by Lee et al. [50]. Hydration of nitriles was achieved with propionaldoxime as a water source; with 1 mol% catalyst, hydration of 4-methoxybenzonitrile yielded the respective amide with 87% conversion at 110 °C in 6 h. Later this group reported the selective hydration of nitriles into the respective amides on the catalytic action of Wilkinson’s catalyst with acetaldoxime as the water source; various functional groups were compatible with the reaction conditions [51].
To the best of our knowledge, there are no [RhCl(cod)(NHC)] type Rh(I)-catalysts reported until now for the selective hydration reaction of nitriles with water in aqueous or partly aqueous solvents. Therefore, we initiated a study of catalytic nitrile hydration with the use of the known [RhCl(cod)(NHC)] (14) complexes with the NHC ligands IMes, SIMes, IPr, and SIPr, respectively (Figure 1) [44,46,48,49,52,53,54,55,56]. In this article, we report on a simple, one-step synthetic procedure for obtaining these complexes using [RhX(cod)]2 (X = Cl, OH) as a metal precursor, the respective imidazolium/imidazolinium chlorides, and K2CO3 as the deprotonating agent, in toluene at 70 °C. Successful application of complexes 14 for the selective hydration of several aromatic and heteroaromatic nitriles to the corresponding amides is also described in detail below.

2. Results and Discussion

2.1. Synthesis and Characterization of the [RhCl(cod)(NHC)] Complexes 14

In this work, we explored the applicability of Rh(I)-N-heterocyclic complexes 14 (Figure 1) for catalysis of hydration of aromatic nitriles. We developed a synthetic method for obtaining these known compounds [44,46,48,49,52,53,54,55,56], which does not require the use of the isolated free carbenes or the use of the corresponding Ag(I)-NHC transmetallating agents.
In general, the synthesis of 14 (Scheme 1) involved stirring of the respective 1,3-diarylimidazolium or 1,3-diarylimidazolinium salt in toluene at 70 °C together with [RhCl(cod)]2 and K2CO3 as an efficient and mild base (A) [47] or with [Rh(OH)(cod)]2 (no base added; B). After removal of the toluene solvent the products were dissolved in CH2Cl2-ethyl acetate and purified by passing through a short silica column; complexes 14 were isolated in 58–88% yield.
The purity of the complexes was checked by 1H and 13C{1H} NMR spectroscopy. The 13C{1H} NMR spectra of all complexes displayed the diagnostic Rh(I)-C(carbene) doublet resonances at 183.2 and 185.5 ppm (1 and 3), and 212.4 and 214.9 ppm (2 and 4), respectively (further spectral details in the Materials and Methods Section).
Single-crystals of 4 could be obtained by crystallization from chloroform at room temperature. In addition, both 3 and 4 yielded single-crystals from benzene, however, these crystals contained solvating benzene molecules, too. (Further experimental details of the X-ray structure analysis can be found in Supplementary Materials). The crystals were subjected to X-ray diffraction measurements. The respective capped sticks representations are shown on Figure 2, Figure 3 and Figure 4, while the most important bond distances and bond angles are found in Table 1, Table 2 and Table 3.
The solid-state crystal structure of 3 has already been determined by single crystal X-ray diffraction and was resolved without solvent [54]. This gives a possibility to compare the structures of 3 and 4 (Table 1). The unit cell of [RhCl(cod)(SIPr)] (4), obtained from chloroform, does not contain solvent molecules, and, in contrast to [RhCl(cod)(IPr)] (3) [54] (P21/c), it crystallizes in the monoclinic P21/nspace group. There are two neutral Rh-complex molecules in the unit cells of both compounds. The lengths of the unit cell edges, and the unit cell angles show only slight differences. This is not surprising, since the sp2 or sp3 C-atoms in the IPr, and SIPr ligands, respectively, do not influence significantly the measures of the unit cell (the same is true for the two extra hydrogen atoms in SIPr). There are no significant differences in the Rh–Ccarbene and in the Rh–Cl bond lengths, either, however, the C2–C3 bond lengths in the [RhCl(cod)(SIPr)] (4) molecules are 1.501(7) Å, and 1.487(7) Å, respectively, which unambiguously refers to sp3 carbon atoms. Saturation of the heterocyclic ring does not alter significantly the Ccarbene–Rh–Cl angles, either. Interestingly, the data of the unit cells of 3 and 4 are almost identical to those of [IrCl(cod)(IPr)] [57]; the data are compared in Tables S2 and S3.
Crystallization of [RhCl(cod)(IPr)] and [RhCl(cod)(SIPr)] from benzene leads to incorporation of solvent molecules into the lattice, yielding crystals of [RhCl(cod)(IPr)]_benzene_3 and [RhCl(cod)(SIPr)]_benzene_4. Unfortunately, the benzene molecules are disordered. Both [RhCl(cod)(IPr)]_benzene_3 and [RhCl(cod)(SIPr)]_benzene_4 crystallize in the monoclinic CC (no. 9) space group, and the unit cells contain four different neutral Rh(I)-complexes together with eight benzene molecules. In the four molecules of [RhCl(cod)(IPr)]_benzene_3 in the unit cell, there are no significant differences in the Rh–Cl bond lengths, however, the Rh–Ccarbene distances are slightly lower (2.031–2.051 Å) than the average Rh–Ccarbene distances in similar complexes, 2.049 Å (CSD Version 5.40, 2019). The crystal structure of [RhCl(cod)(SIPr)]_benzene_4 is very similar to that of [RhCl(cod)(IPr)]_benzene_3. In this case, too, the average Rh–Ccarbene distances (2.028–2.046 Å) are somewhat shorter than those in the benzene-free crystals of [RhCl(cod)(SIPr)] (3). The C2–C3 distance in [RhCl(cod)(IPr)]_benzene_3 is 1.330–1.339 Å which refers to carbon atoms with sp2 hybridization, while the corresponding C2–C3 bond length in [RhCl(cod)(SIPr)]_benzene_4, i.e., 1.524–1.497 Å, agrees well with the presence of sp3-hybridized carbon atoms.

2.2. Hydration of Aromatic Nitriles Catalyzed by the [RhCl(cod)(NHC)] Complexes 14

Due to the importance of amides in the synthesis of important pharmaceuticals, there is a strong incentive to develop new transition metal catalysts which are able to facilitate the selective hydration reaction of aliphatic, as well as aromatic nitriles to corresponding amides (Scheme 2) with high activity under mild conditions (i.e., at temperatures below 100 °C, and preferably close to room temperature).
It was found that complexes 14 efficiently catalyzed the hydration of benzonitrile to benzamide in a water/2-propanol = 1/1 mixture in air and under mild conditions (≤80 °C). The choice of 2-propanol as the organic component of the solvent was based on its unique favourable effects on certain reactions, e.g., hydrogenation and transfer hydrogenation of ketones [58]. The reactions did not display an induction period (Figure S1) and they proved completely selective; no products other than benzamide were detected by GC-MS or 1H NMR spectroscopy. With these catalysts, fast hydration of benzonitrile was observed only in the presence of bases. The data in Table 4 show that in the lack of a base no reaction of benzonitrile was observed in 1.5 h, and even after 2 h the conversion reached only 3%. Conversely, with bases such as tBuOK, KOH, Na2CO3, and NaOH at a [base]/[Rh] = 1/1 ratio, the conversions in 1.5 h were in the 50–60% range and were not strongly dependent on the choice of the particular base. The use of NaOH resulted in the highest conversion, and therefore it was chosen for further studies. The possible catalytic effect of the bases in Table 4 were also checked in the hydration of benzonitrile in the absence of catalysts 14. Under the conditions used, conversion of benzonitrile to benzamide was < 1% with all four bases (only a trace of product could be detected by gas chromatography). These results show that the contribution of base-catalyzed hydration is negligible compared to the metal-complex catalyzed transformation.
The effects of various reaction parameters for the hydration of benzonitrile were studied in detail using complex 1 as the catalyst. The progress of the reactions could be conveniently monitored by gas chromatography. Representative results are summarized in Table 5.
The data in Table 5 show that [RhCl(cod)(IMes)] (1) is an active catalyst for benzonitrile hydration. The TOF values (up to 276 h−1) compare well with those of most transition metal catalysts although fall behind the highest activities [33]. With increasing temperatures, the yield of benzamide increased and reached a maximum (98%) at 80 °C. It is also evident from Table 5, that under the applied reaction conditions, 2 h is the optimum reaction time for the catalytic hydration of benzonitrile to benzamide. For the entries 1–10 of Table 5, the effect of the base (NaOH) alone (i.e., in the absence of the Rh(I)-complex catalyst) has been checked and the results are shown in parentheses in the Conversion (%) column of the Table, next to the values obtained with catalyst 1 + NaOH. Here, again, it can be concluded, that the base-catalyzed hydration increases the total benzamide yield only to a minor extent even at higher reaction temperatures and longer reaction times (entries 5 and 10). In order to determine the efficiency of the catalyst in the absence of NaOH, we had to increase the catalyst concentration to 5 mol% (entries 11–13). Even then, no reaction was observed at reflux conditions (approximately 81 °C, see Experimental) in 60 min, and only 26% conversion of benzonitrile was obtained after 180 min reaction time. In contrast, the reaction with 1 + NaOH led to 86% conversion already after 10 min (entry 14).
Table 5 also shows the effect of the water-soluble tertiary phosphines PTA (1,3,5-triaza-7-phosphaadamantane) and mtppms-Na (sodium diphenylphosphinobenzene-3-sulfonate or monosulfonated triphenylphosphine Na-salt). Compared to catalyst 1 (entry 11, 0% conversion in 60 min), both PTA and mtppms increased the reaction rate and at a [phosphine]/[Rh] ratio their effect is about the same (entries 18 and 21). In general, however, mtppms proved to be more effective. Nevertheless, with regard to the rate increase, both phosphines were much inferior to NaOH (entry 14).
The precedents in the literature show that with bmim (1-butyl-3-methyl-imidazole-2-ylidene) as the NHC ligand, PTA and mtppms form [Rh(cod)(bmim)(PTA)]Cl, and [Rh(cod)(bmim)(mtppms)] (a neutral zwitterionic complex), respectively [59]. In accordance with these earlier results, we expect that tertiary phosphines coordinate to the central Rh(I) ion in [RhCl(cod)(NHC)] complexes. However, the resulting complex species are coordinatively saturated and coordination of the nitrile substrate and/or H2O or OH to the metal ion in a Rh(I)-complex seems unlikely. In the case of [RuCl2(PTA)4]-catalyzed nitrile hydration, Frost suggested that the increased catalytic activity in the presence of a large excess of PTA was due to the pH shift into the alkaline region in concentrated PTA solutions caused by the protonation of PTA [34]. This may happen in our reactions with added PTA, too, however, it is certainly not the case with mtppms which is protonated only in concentrated aqueous acid solutions. Nevertheless, since the roles of PTA and mtppms were not clarified in detail, our observations on the effect of PTA and mtppms on the Rh(I)-complex catalyzed hydration of benzonitrile can be regarded only as an information of practical importance. Details of these phosphine effects were not scrutinized.
Table 6 presents the results of benzonitrile hydration with [RhCl(cod)(NHC)] complexes 14. It can be seen that in the presence of NaOH, high conversions (93 – >99%) could be obtained in reasonable reaction times (1–3 h) with all four catalysts (entries 2, 5, 8, 11). Conversely, in the absence of NaOH, each catalyst showed only low activity, and the highest conversion under such conditions was only 26% in 3 h (entry 1). It seems from the conversion data for the first hour of the reactions, that the evolution of the real catalytic species in the water/2-propanol mixed solvent from the precursor complexes 14 and NaOH needs noticeable time. It is fast with 1 and 2 (entries 2, 5), somewhat slower with 4 (entry 11) and significantly slower in the case of 3 (entry 8). Note, that even with catalyst 3, the conversion of benzonitrile to benzamide reached 93% in 3 h. Compared to NaOH, lower rates were achieved with PTA in the case of all four catalysts, similar to the observations discussed above in conjunction with Table 5.
[RhCl(cod)(IMes)] (1) proved suitable for hydration of benzonitriles with both electron donating and electron withdrawing substituents (Table 7). High conversions were achieved with as low as 1 mol% of catalyst. Para-chlorobenzonitrile showed more efficient conversion to p-chlorobenzamide than p-methylbenzonitrile which has an electron donating group in 4-position. Electron-withdrawing groups make the nitrile carbon more susceptible to nucleophilic attack by the activated water molecule or OH. These findings are in agreement with the previously reported observations [6,10].
The conversions of various pyridine-carbonitriles to the corresponding amides (picolinamide, nicotinamide, isonicotinamide) were explored with 5 mol% catalyst 1 and the results are summarized in Table 8. Remarkably, the reactions of 3- and 4-pyridinecarbonitrile proceeded efficiently even in the absence of NaOH; apparently the pyridine moiety provided the sufficient basicity. The coordinating ability of the pyridyl functionality of 2-pyridinecarbonitrile reduced the activity as a catalyst of the complex and the reaction resulted only in 9% picolinamide. However, heteroaromatic nitriles with the N heteroatom adjacent to the β or γ position of the CN group (3-pyridinecarbonitrile and 4-pyridinecarbonitrile) showed high reactivity. Addition of three equivalents of PTA increased the catalytic activity in all cases, and 3-pyridinecarbonitrile, too, was hydrated with > 99% conversion in only 1 h.
Finally, we studied the hydration of benzonitrile at 25 °C. It was found, that the use of 1 mol% catalyst 1 was sufficient to give a reasonable yield in 40 h (Table 9, entry 3). However, with a higher catalyst loading (2.5 mol%) 99% conversion was reached in 24 h (Table 9, entry 8). Lowering the concentration of 2-PrOH in the aqueous solvent mixture from 50% to 20% v/v, lead to a decrease in the conversion (entries 9, 10 vs. 1–3). The origin of this latter effect is presently unclear, since even at the lower 2-propanol concentration the reaction mixtures were homogeneous, and—formally—2-propanol is not involved in the hydration of benzonitrile.
The above results did not allow the suggestion of a detailed reaction mechanism. Nevertheless, the findings are in accord with the nucleophilic attack of a Rh(I)-coordinated hydroxide onto the nitrile carbon atom (Scheme 3), similar to the mechanism suggested in [17]. It is an important observation, that the hydration reactions proceed with high rate already with 1 equivalent of base per Rh(I). Since there is hardly any conversion of benzonitrile in the absence of a base, this points to an intermediate formation of a Rh(I)-OH hydroxo-complex. On the other hand, the complete selectivity of the reaction to benzamide shows that most probably the nitrile also coordinates to the Rh-based catalyst, thereby activating the nitrile carbon against a nucleophilic attack. It should also be mentioned, that at the moment the role of the cod ligand is unclear. It may stay coordinated to the rhodium throughout the catalytic cycle, but in the reductive milieu of basic 2-propanol it may also be hydrogenated and replaced by other ligands present in the solution.

3. Materials and Methods

3.1. Materials

All chemicals and reagents used in this work were purchased from Sigma-Aldrich, St. Louis, Missouri, USA; Molar Chemicals Kft., Halásztelek, Hungary and VWR International, West Chester, Pennsylvania, USA and were used as received without further purification. Analytical thin-layer chromatography (TLC) was performed on Merck Kieselgel 60 F254 plates (Merck, Darmstadt, Germany), TLC plates were visualised by UV fluorescence at 254 nm. Column chromatography was performed on silica gel (70–230 mesh, 63–200 µm, Sigma-Aldrich). The metal precursors [RhCl(cod)]2 [60], [Rh(OH)(cod)]2 [61], the ligands PTA [62], and mtppms-Na [63] were prepared by the methods described in the literature.

3.2. General Procedure for the Synthesis of [RhCl(cod)(NHC)] Complexes

Starting from [Rh(cod)X]2 (X = Cl, OH) and the appropriate 1,3-diaryl-imidazoli-um/imidazolinium salts the corresponding rhodium complexes 14, were prepared in 58–88% yield (Scheme 1).
Method A: To a solution of 0.88 mmol of the imidazolium/imidazolinium salt in 30 mL of toluene were added 0.44 mmol of [RhCl(cod)]2 and 8.88 mmol of K2CO3 in one portion. The mixture was stirred at 70 °C for 3–24 h (followed by TLC). After removal of the solvent the product was purified by passing through a short silica column in dichloromethane:ethyl acetate = 1:1 as solvent. The coloured fraction of the complex was collected, evaporated to dryness, and the yellow solid was vacuum-dried, characterized by 1H, 13C NMR, and HR ESI-MS.
Method B: To a solution of 0.88 mmol of the imidazolium/imidazolinium salt in 30 mL of toluene was added 0.44 of 0.44 mmol of [Rh(OH)(cod)]2 and the mixture was stirred at 70 °C for the required time (followed by TLC). After removal of the solvent the product was purified by passing through a short silica column in dichloromethane:ethyl acetate = 1:1 as solvent, and the complex was isolated and characterized as in Method A.
Chloro(η4-1,5-cyclooctadiene)(1,3-dimesitylimidazole-2-ylidene)rhodium(I), [RhCl(cod)(IMes)] (1). 302 mg (0.888 mmol) IMes HCl, 220 mg (0.444 mmol) [RhCl(cod)]2, 1228 mg (8.88 mmol) K2CO3, 3 h, yield of 1: A) 345 mg (0.627 mmol) 71%; B) 399 mg (0.725 mmol), 83%. 1H NMR (360 MHz, CD2Cl2) δ/ppm: 7.12–7.11 (m, 4H, HAr), 7.04 (s, 2H, NCH), 4.48 (br, 2H, Hcod), 3.38 (br, 2H, Hcod), 2.46–2.42 (m, 12H, Me), 2.18 (s, 6H, Me), 1.92–1.89 (m, 4H, Hcod), 1.63–1.59 (m, 4H, Hcod); 13C{1H} NMR (CD2Cl2), δ/ppm: 183.2 (d, 1JRh–C = 52.3 Hz); 138.6; 137.4; 136.5; 134.5; 129.3; 128.3; 123.7; 95.7 (d, 1JRh–C = 7.2 Hz); 68.1 (d, 1JRh-C = 14.3 Hz); 32.6; 28.3; 20.8; 19.5; 17.9. MS(ESI), positive mode, in MeOH, m/z for 1, [M]+ (C29H36N2Rh), calculated: 515.1928, found: 515.1928.
Chloro(ƞ4-1,5-cyclooctadiene)(1,3-dimesitylimidazolidin-2-ylidene)rhodium(I), [RhCl(cod)(SIMes)] (2). 304 mg (0.888 mmol) SIMes HCl, 220 mg (0.444 mmol) [RhCl(cod)]2, 1228 mg (8.88 mmol) K2CO3, 22 h, yield of 2: A) 350 mg (0.632 mmol), 71%, B) 428 mg (0.773 mmol), 88%. 1H NMR (360 MHz, CD2Cl2) δ/ppm: 7.09–7.06 (m, 4H, HAr), 4.43 (br, 2H, Hcod), 3.89 (br, 4H, NCH2), 3.46 (br, 2H, Hcod), 2.62 (s, 6H, Me), 2.41–2.38 (m, 12H, Me), 1.85–1.80 (m, 4H, Hcod), 1.64–1.55 (m, 4H, Hcod);13C{1H} NMR (90 MHz, CD2Cl2), δ/ppm: 212.4 (d, 1JRh–C = 48.4 Hz); 138.2; 137.7; 136.6; 135.4; 129.6; 128.5; 96.8 (d,1JRh–C= 6.4 Hz); 67.8 (d, 1JRh–C = 14.3 Hz); 51.47; 32.6; 28.1; 20.8; 19.7; 18.2. MS(ESI), positive mode, in MeOH, m/z for 2, [M]+ (C29H38N2Rh), calculated: 517.2085, found: 517.2085.
Chloro(ƞ4-1,5-cyclooctadiene)(1,3-bis(2,6-diisopropylphenylimidazol)-2-ylidene)rhodium(I), [Rh(Cl)(cod)(IPr)] (3). 378 mg (0.888 mmol) IPr HCl, 220 mg (0.444 mmol) [RhCl(cod)]2, 1228 mg (8.88 mmol) K2CO3, 21 h, yield of 3: A) 494 mg (0.777 mmol), 88%; B) 389 mg (0.612 mmol), 69%. 1H NMR (360 MHz, dmso-d6) δ/ppm: 7.62 (s, 2H, NCH), 7.54 (t, J = 7.7 Hz, 2H, HAr), 7.39 (br, 4H, HAr), 4.33 (br, 2H, Hcod), 3.51 (br, 2H, CH(CH3)2), 3.23 (s, 2H, Hcod), 2.35–2.31 (br, 2H, CH(CH3)2), 1.71–1.27 (m, 8H, Hcod +12H, CH(CH3)2), 1.06 (d, J = 6.8 Hz, 12H, CH(CH3)2);13C{1H} NMR (90 MHz, CDCl3), δ/ppm: 186.1 (d, 1JRh–C = 52.2 Hz); 147.9; 145.3; 136.4; 129.8; 124.6; 122.9; 96.4 (d, 1JRh–C = 7.2 Hz); 67.8 (d, 1JRh–C = 14.4 Hz); 32.7; 28.8; 28.3; 26.6; 22.8. MS(ESI), positive mode, in MeOH, m/z for 3, [M]+ (C35H48N2Rh), calculated: 599.2867, found: 599.2867.
Chloro(ƞ4-1,5-cyclooctadiene)(1,3-bis(2,6-diisopropylphenylimidazolidin)-2-ylidene)rhodium(I), [RhCl(cod)(SIPr)] (4). 380 mg (0.888 mmol) SIPr HCl, 220 mg (0.444 mmol) [RhCl(cod)]2, 1228 mg (8.88 mmol) K2CO3, 24 h, yield of 4: A) 400 mg (0.627 mmol) 70%, B) 386 mg (0.605 mmol) 68%.1H NMR (360 MHz, C6D6) δ/ppm: 7.31–7.24 (m, 4H, HAr), 7.15 (d, J = 6.8 Hz, 2H, HAr), 4.99 (br, 2H, Hcod), 4.43–3.36 (m, 4H, NCH2), 3.73–3.68 (m, 2H, Hcod), 3.43–3.39 (m, 2H, CH(CH3)2), 3.10–3.03 (m, 2H, CH(CH3)2), 1.82–1.70 (m, 10H, Hcod), 1.45–1.18 (m, 18H, CH(CH3)2), 1.05 (d, J = 6.8 Hz, 6H, CH(CH3)2); 13C{1H} NMR (90 MHz, CDCl2), δ/ppm: 214.9 (d, 1JRh–C = 47.7 Hz); 149.3; 146.4; 136.9; 128.8; 124.8; 123.3; 96.4 (d, 1JRh–C = 7.1 Hz); 67.8 (d, 1JRh–C = 13.9 Hz); 53.4; 32.4; 28.9; 28.6; 27.9; 26.6; 24.0; 22.7. MS(ESI), positive mode, in MeOH, m/z for 4, [M]+ (C35H50N2Rh), calculated: 601.3024, found: 601.3025.

3.3. General Methods

1H and 13C{1H} NMR spectra were recorded on a Bruker Avance 360 MHz spectrometer (Bruker, Billerica, MA, USA) and were referenced to residual solvent peaks. Single crystal X-ray diffraction (SCXRD) measurements were performed using a Bruker D8 Venture diffractometer and the methods and software were described in [64,65,66,67,68,69,70]. Gas chromatographic measurements were done with the use of an Agilent Technologies 7890 A instrument (HP-5, 0.25 µm × 30 m × 0.32 mm, FID 300 °C (Agilent Technologies, Santa Clara, CA, USA); carrier gas: Nitrogen 1.9 mL/min). ESI-TOF-MS measurements were carried out on a Bruker maXis II MicroTOF-Q type Qq-TOF-MS instrument (Bruker Daltonik, Bremen, Germany) in positive ion mode. The mass spectra were calibrated internally using the exact masses of sodium formate clusters. The spectra were evaluated using the Compass Data Analysis 4.4 software from Bruker.
All catalytic reactions were carried out under air. The reaction temperatures were kept constant either by using a thermostated circulator (set e.g., to 80.0 ± 0.1 °C), or by running the reactions under reflux (lit. b.p. of 50% aqueous 2-propanol: 81.1 °C [71]). The products were identified by comparison of their retention time with known standard compounds.
a) Hydration of Benzonitrile without Product Isolation
100 µL (1.0 mmol) benzonitrile, 5.5 mg (0.01 mmol) [RhCl(cod)(IMes)] (1), 0.4 mg (0.01 mmol) NaOH, and 12.8 mg (0.1 mmol, 10 mol% of the substrate) naphthalene (internal standard) were dissolved in a mixture of 1.5 mL 2-propanol and 1.5 mL deionized water. This reaction mixture was placed into a temperature-controlled bath and stirred at 80 °C for 2 h. A 0.10 mL part of the resulting hot solution was extracted with 2 mL CH2Cl2, passed through a short MgSO4 column and subjected to gas chromatography. Conversion of benzonitrile: 98%.
b) Hydration of Benzonitrile with Product Isolation
200 µL (2.0 mmol) benzonitrile, 11 mg (0.02 mmol) [RhCl(cod)(IMes)] (1), and 0.8 mg (0.02 mmol) NaOH were stirred in a mixture of 1.5 mL 2-PrOH and 1.5 mL deionized water, at 80 °C. After 3 h reaction time, the resulting solution was evaporated to dryness on a rotary evaporator and the residue was chromatographed on a short silica gel column using ethyl acetate as the eluent. Yield of benzamide: 217.3 mg (89%).

4. Conclusions

We have realized one-step syntheses of the [RhCl(cod)(NHC)] complexes 1–4 without generation of the free carbene ligands or the silver-NHC complexes. The metal precursor [RhCl(cod)]2 and the respective imidazolium/imidazolinium salt was stirred overnight in toluene at 70 °C, and the desired complexes were produced in good to excellent yields. An efficient catalytic system for the selective hydration of nitriles to the corresponding amides in a water/2-propanol solvent, with tolerance of air and several functional groups, is also described. The suggested reaction mechanism considers the nucleophilic attack of a Rh(I)-coordinated OH onto the nitrile carbon atom activated by the N-coordination of nitrile group to the metal ion.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/1/125/s1. Table S1. Retention times of nitriles and amides used in this study; Table S2. Experimental conditions of X-ray diffraction measurements of Rh(I)-complexes; Table S3. Unit cell data of [RhCl(cod)(IPr)] [54] and [IrCl(cod)(IPr)] [57]; Figure S1. Time course of the conversion of benzonitrile with 1 mol% of catalyst 1; Figure S2 and S3. GC separation of benzonitrile/benzamide and 2-pyridinecarbonitrile/2-pyridin-carboxamide; Figures S4–S11. 1H and 13C{1H} NMR spectra of [RhCl(cod)(IMes)] (1), [RhCl(cod)(SIMes)] (2), [RhCl(cod)(IPr)] (3), [RhCl(cod)(SIPr)] (4); Figure S12 and S13. 1H and 13C{1H} NMR spectra of benzamide obtained by hydration of benzonitrile with catalyst 1; Figures S14 and S15. 1H and 13C{1H} NMR spectra of isonicotinamide obtained by hydration of 4-pyridinecarbonitrile with catalyst 1; Figures S16 and S17. HR ESI-MS spectra of benzamide and isonicotinamide; Figures S18–S21. HR ESI-MS spectra of catalysts 14; Figure S22. ORTEP view of [RhCl(cod)(SIPr)] (4) (50% ellipsoid level); Figure S23. ORTEP view of [RhCl(cod)(IPr)]_benzene_3 (50% ellipsoid level); Figure S24. ORTEP view of [RhCl(cod)(SIPr)]_benzene_4 (50% ellipsoid level).

Author Contributions

Conceptualization, C.E.C. and F.J.; Methodology, G.P. (Gábor Papp) and A.U.; Synthesis and characterization of catalysts, C.E.C., N.J.D., G.P. (Gergely Papp), and S.D.; Catalysis experiments, C.E.C., N.J.D., G.P. (Gergely Papp), and S.D.; Discussion of experimental results, all authors; Writing—Original Draft Preparation, all authors; Writing—Review and Editing, C.E.C., F.J., S.D., and A.U. All authors have read and agreed to the published version of the manuscript

Funding

The research was supported by the EU and co-financed by the European Regional Development Fund (under the projects GINOP-2.3.2-15-2016-00008 and GINOP-2.3.3-15-2016-00004), and by the Thematic Excellence Programme of the Ministry for Innovation and Technology of Hungary (ED_18-1-2019-0028), within the framework of the Vehicle Industry thematic programme of the University of Debrecen. The financial support of the Hungarian National Research, Development and Innovation Office (FK-128333) is greatly acknowledged. S.D. is thankful to the Stipendium Hungaricum scholarship programme and Government of India for supporting his PhD study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The Rh(I)-NHC complexes 14 used in our study as catalysts for selective nitrile hydration.
Figure 1. The Rh(I)-NHC complexes 14 used in our study as catalysts for selective nitrile hydration.
Catalysts 10 00125 g001
Scheme 1. Synthesis of 1 from [RhCl(cod)]2 and [IMesH]Cl with K2CO3 as deprotonating agent.
Scheme 1. Synthesis of 1 from [RhCl(cod)]2 and [IMesH]Cl with K2CO3 as deprotonating agent.
Catalysts 10 00125 sch001
Figure 2. Capped sticks representation of the solid-state structure of [RhCl(cod)(SIPr)] (4) crystallized from CHCl3.
Figure 2. Capped sticks representation of the solid-state structure of [RhCl(cod)(SIPr)] (4) crystallized from CHCl3.
Catalysts 10 00125 g002
Figure 3. Capped sticks representation of the solid-state structure of [RhCl(cod)(IPr)](3) crystallized from benzene ([RhCl(cod)(IPr)]_benzene_3; benzene molecules are omitted for clarity.
Figure 3. Capped sticks representation of the solid-state structure of [RhCl(cod)(IPr)](3) crystallized from benzene ([RhCl(cod)(IPr)]_benzene_3; benzene molecules are omitted for clarity.
Catalysts 10 00125 g003
Figure 4. Capped sticks representation of the solid-state structure of [RhCl(cod)(SIPr)]_benzene_4 (benzene molecules are omitted for clarity).
Figure 4. Capped sticks representation of the solid-state structure of [RhCl(cod)(SIPr)]_benzene_4 (benzene molecules are omitted for clarity).
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Scheme 2. General scheme of the selective hydration of benzonitriles to benzamides.
Scheme 2. General scheme of the selective hydration of benzonitriles to benzamides.
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Scheme 3. Suggested mechanism of nitrile hydration catalyzed by [RhCl(cod)(NHC)] complexes.
Scheme 3. Suggested mechanism of nitrile hydration catalyzed by [RhCl(cod)(NHC)] complexes.
Catalysts 10 00125 sch003
Table 1. Comparison of the most important bond lengths (Å) angles (°) of [RhCl(cod)(IPr)] (3) [54] and [RhCl(cod)(SIPr)] (4) crystallized from CHCl3 (this work).
Table 1. Comparison of the most important bond lengths (Å) angles (°) of [RhCl(cod)(IPr)] (3) [54] and [RhCl(cod)(SIPr)] (4) crystallized from CHCl3 (this work).
[RhCl(cod)(IPr)] (3) [54][RhCl(cod)(SIPr)] (4)
Rh–Ccarbene2.056(4)2.052(1)2.043(3)2.053(1)
Rh–Cl2.3467(12)2.3713(12)2.3721(11)2.3466(10)
C2–C3--1.501(7)1.487(7)
C2=C31.328(6)1.324(5)--
Ccarbene–Rh–Cl85.61(11)88.26(11)86.92(9)84.32(10)
Table 2. The most important bond lengths (Å) angles (°) of the four individual molecules in the unit cells of the benzene solvate of 3, i.e., [RhCl(cod)(IPr)]_benzene_3.
Table 2. The most important bond lengths (Å) angles (°) of the four individual molecules in the unit cells of the benzene solvate of 3, i.e., [RhCl(cod)(IPr)]_benzene_3.
[RhCl(cod)(IPr)]_benzene_3
Rh–Ccarbene2.050(4)2.031(4)2.032(4)2.051(4)
Rh–Cl2.3752(11)2.3726(10)2.3706(10)2.3775(10)
C2=C31.330(6)1.338(6)1.339(6)1.333(6)
Ccarbene–Rh–Cl89.01(11)88.33(11)87.76(11)89.43(10)
Table 3. The most important bond lengths (Å) angles (°) of the four individual molecules in the unit cells of the benzene solvate of 4, i.e., [RhCl(cod)(SIPr)]_benzene_4.
Table 3. The most important bond lengths (Å) angles (°) of the four individual molecules in the unit cells of the benzene solvate of 4, i.e., [RhCl(cod)(SIPr)]_benzene_4.
[RhCl(cod)(SIPr)]_benzene_4
Rh–Ccarbene2.028(7)2.034(7)2.046(7)2.044(7)
Rh–Cl2.3800(17)2.3746(17)2.3797(18)2.3781(4)
C2–C31.515(11)1.505(11)1.497(11)1.524(10)
Ccarbene–Rh–Cl87.40(19)86.03(19)88.80(18)88.08(19)
Table 4. Effect of various bases on the hydration of benzonitrile catalyzed by [RhCl(cod)(IMes)] (1).
Table 4. Effect of various bases on the hydration of benzonitrile catalyzed by [RhCl(cod)(IMes)] (1).
EntryBaseConversion (%)TOF a (h−1)
1-0(3 b)0(4 b)
2tBuOK5269
3KOH5269
4K2CO35675
5NaOH5979
Conditions: 1 mmol benzonitrile, 0.5 mol% [RhCl(cod)(IMes)] (1), 0.005 mmol base, 1.5 mL 2-PrOH, 1.5 mL H2O, 80 °C, 1.5 h. a Turnover frequencies were calculated from the conversions at the indicated reaction times. b 2 h.
Table 5. The effect of various reaction parameters on the hydration of benzonitrile catalyzed by [RhCl(cod)(IMes)] (1).
Table 5. The effect of various reaction parameters on the hydration of benzonitrile catalyzed by [RhCl(cod)(IMes)] (1).
EntryCatalyst (mol%)Base aPhosphineT °Ct (min)Conversion (%) bTOF c (h−1)
11NaOH-4012048 (0)24
21NaOH-5012072 (1)36
31NaOH-6012082 (1)41
41NaOH-7012091 (3)45
51NaOH-8012098 (6)49
61NaOH-801046 (1)276
71NaOH-802063 (1)189
81NaOH-803074 (2)148
91NaOH-806086 (3)86
101NaOH-809094 (5)63
115--reflux6000
125--reflux120182
135--reflux180262
145NaOH-reflux1096115
155NaOH-reflux209758
165NaOH-reflux60>9920
175-0.05 mmol PTAreflux60173
185-0.15 mmol PTAreflux607014
195-0.25 mmol PTAreflux607816
205-0.05 mmol mtppmsreflux607515
215-0.15 mmol mtppmsreflux607615
225-0.25 mmol mtppmsreflux609419
Conditions: 1 mmol benzonitrile, 2-PrOH/H2O = 1:1 V = 3 mL. a [NaOH]/[Rh] = 1; b Conversions of base-catalyzed hydrations in parentheses (NaOH only). c Turnover frequencies were calculated from the conversions at the indicated reaction times.
Table 6. Hydration of benzonitrile with [RhCl(cod)(NHC)] catalyst 14.
Table 6. Hydration of benzonitrile with [RhCl(cod)(NHC)] catalyst 14.
EntryCatalyst Conversion (%)
1 h2 h3 h
1101826
21 + NaOH>99--
31 + PTA707177
420010
52 + NaOH99>99-
62 + PTA697888
73012
83 + NaOH668693
93 + PTA546164
1040012
114 + NaOH9498-
124 + PTA14753
Conditions: 1 mmol benzonitrile, 5 mol% [RhCl(cod)(NHC)], 1.5 mL 2-PrOH, 1.5 mL H2O, 0.05 mmol NaOH or 0.15 mmol PTA, reflux temperature.
Table 7. Hydration of various nitriles into amides with catalyst 1 with NaOH and catalysis of the same reaction with NaOH only.
Table 7. Hydration of various nitriles into amides with catalyst 1 with NaOH and catalysis of the same reaction with NaOH only.
EntryNitrilet(h)1 + NaOHNaOH
Conversion (%)TOF b(h−1)Conversion c (%)
1benzonitrile193933
2298496
34-chlorobenzonitrile188884
4294476
54-methylbenzonitrile170701
6284422
74-chlorophenyl-acetonitrile158580
8262312
Conditions:a 1 mol% [RhCl(cod)(IMes)] (1), 1 mmol nitrile, 0.01 mmol NaOH, 1 mL 2-PrOH, 1 mL H2O, 80 °C. c Same as in a, but without [RhCl(cod)(IMes)] (1). b Turnover frequencies were calculated from the conversions at the indicated reaction times.
Table 8. Hydration of substituted pyridinecarbonitriles with catalyst 1 in the absence of added base.
Table 8. Hydration of substituted pyridinecarbonitriles with catalyst 1 in the absence of added base.
EntrySubstrate PhosphineConversion (%)
1 h2 h3 h
12-pyridinecarbonitrile-689
22-pyridinecarbonitrile PTA81011
33-pyridinecarbonitrile-889696
43-pyridinecarbonitrile PTA> 99--
54-pyridinecarbonitrile-> 99--
64-pyridinecarbonitrile PTA> 99--
7 a4-pyridinecarbonitrile -90> 99-
Conditions: 1 mmol nitrile, 5 mol% [RhCl(cod)(IMes)] (1), 1.5 mL 2-PrOH, 1.5 mL H2O, 0.15 mmol PTA, reflux temperature. a 1 mol% [RhCl(cod)(IMes)] (1).
Table 9. Hydration of benzonitrile at 25 °C catalyzed by [RhCl(cod)(IMes)] (1) with equimolar amounts of NaOH a.
Table 9. Hydration of benzonitrile at 25 °C catalyzed by [RhCl(cod)(IMes)] (1) with equimolar amounts of NaOH a.
Entry1 (mol%) bt (h)Conversion (%)TOF (h−1)
1117734.3
2122793.6
3140942.4
4217842.5
5222851.9
62.517942.2
72.519962.0
82.524991.7
9 c117342.0
10 c134601.8
Conditions: 1 mmol benzonitrile, 1 mL 2-PrOH, 1 mL H2O, 25 °C; a [NaOH]/[Rh] = 1; b Relative to benzonitrile; c 20% v/v 2-PrOH.
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