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Communication

Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands

by
Wan-Qiang Wang
1,2,
Hua Cheng
1,
Ye Yuan
2,
Yu-Qing He
1,
Hua-Jing Wang
2,
Zhi-Qin Wang
2,
Wei Sang
2,
Cheng Chen
2,* and
Francis Verpoort
2,3,4,*
1
Department of Chemical Engineering and Food Science, Hubei University of Arts and Science, 296 Longzhong Road, Xiangyang 441053, China
2
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
3
National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russia
4
Centre for Environmental and Energy Research, Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 406-840, Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(1), 10; https://doi.org/10.3390/catal10010010
Submission received: 21 November 2019 / Revised: 10 December 2019 / Accepted: 16 December 2019 / Published: 19 December 2019
(This article belongs to the Special Issue N‐Heterocyclic Carbenes and Their Complexes in Catalysis)
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Abstract

:
The transition-metal-catalyzed alcohol dehydrogenation to carboxylic acids has been identified as an atom-economical and attractive process. Among various catalytic systems, Ru-based systems have been the most accessed and investigated ones. With our growing interest in the discovery of new Ru catalysts comprising N-heterocyclic carbene (NHC) ligands for the dehydrogenative reactions of alcohols, we designed and prepared five NHC/Ru complexes ([Ru]-1–[Ru]-5) bearing different ancillary NHC ligands. Moreover, the effects of ancillary and additional ligands on the alcohol dehydrogenation with KOH were thoroughly explored, followed by the screening of other parameters. Accordingly, a highly active catalytic system, which is composed of [Ru]-5 combined with an additional NHC precursor L5, was discovered, affording a variety of acid products in a highly efficient manner. Gratifyingly, an extremely low Ru loading (125 ppm) and the maximum TOF value until now (4800) were obtained.

Graphical Abstract

1. Introduction

Direct conversion of primary alcohols into carboxylic acids is a representative organic transformation [1,2]. The traditional way to access carboxylic acids is the direct oxidation of primary alcohols [3]. However, this method usually requires toxic or strong oxidants and produces large amounts of by-products [4,5,6]. Even though oxygen is occasionally utilized as an oxidant [7,8,9,10,11,12,13,14,15,16,17], explosive mixtures may be formed and oxygen-sensitive substrates are not compatible. Therefore, green and sustainable alternatives for the production of various carboxylic acids are still in high demand. In recent years, a catalytic process in which various carboxylic acids could be afforded via alcohol dehydrogenation has been developed. Notably, this is a sustainable and atom-economical reaction, which releases dihydrogen gas as the sole byproduct. Originally, Grützmacher et al. pioneered this catalytic process by utilizing a hydrogen acceptor [18]. Later, they reported several other hydrogen acceptors for this transformation [19,20]. A major breakthrough in this area was accomplished by Milstein et al. who achieved this catalysis in an acceptorless manner [21]. With 0.2 mol % of a pincer-type Ru complex, several carboxylic acids were synthesized from the respective primary alcohols in an aqueous NaOH solution. Afterward, other research groups also reported efficient catalytic systems, including Ru- [22,23,24,25,26,27,28,29,30,31,32,33,34], Rh- [35], Pd- [36], Ir- [37,38,39], Ag- [40], Fe- [41], Mn- [41,42], and Zn-based [43] catalytic systems, for this dehydrogenation. In particular, Ru systems with catalyst loadings ranging from 250 ppm to 5.0 mol % have been most accessed.
In practice, ancillary ligands are pivotal for the catalytic activity and stability of organometallic complexes. The steric and electronic profiles also play a central role in tweaking their properties, as well as their catalytic performance [44]. N-Heterocyclic carbenes (NHCs) have evolved as ideal ligands for numerous metal catalysts, as their steric and electronic parameters could be easily modified to achieve the optimized properties required for efficient catalysis [45,46,47,48,49,50]. Hence, two monodentate NHC/Ru complexes were developed for this reaction. In 2014, Möller et al. reported a benzimidazole-based arene/Ru/NHC complex (as shown in Figure 1a), affording plenty of carboxylic acids applying 1 mol % of this Ru complex [23]. Later, an imidazole-derived NHC/Ru complex was also prepared, leading to satisfactory results with 1 mol % of the NHC/Ru complex combined with PCy3 as an additional ligand (Figure 1b) [26]. Despite these examples, the discovery of new and highly efficient NHC/Ru catalytic systems for this catalytic process is still of vital significance. Therefore, elucidating the steric and electronic effects of the ancillary NHC ligands is invaluable for the design and discovery of new efficient catalysts. Herein, we aimed to prepare several NHC/Ru complexes incorporating varied NHC ligands to explore the influence of steric/electronic changes to the ligands on the acceptorless conversion of alcohols into carboxylic acids (as shown in Figure 1c). On the other hand, inspired by our previous work in which the involvement of an additional NHC ligand considerably enhanced the catalytic potency for the dehydrogenative alcohol amidation to amides [51], we therefore evaluated the effects of an additional ligand on the catalytic performance (Figure 1c). Gratifyingly, an optimized catalytic system that demonstrated outstanding efficiency was discovered through extensive optimization. Notably, a catalyst loading of 125 ppm was applied for most substrates. Moreover, the maximum turnover number (TON) of 15,200 and the highest turnover frequency (TOF) of 4800 were attained. It is noteworthy that the catalyst loading (125 ppm) and maximum TOF value (4800) are superior over all reported catalytic systems (Tables S1 and S2, Supplementary Materials).

2. Results and Discussion

[Ru]-1, an NHC/Ru complex from our previous work [51], was synthesized first, and four other NHC/Ru complexes ([Ru]-2–[Ru]-5) bearing distinct backbone and wingtip substituents on the NHC ligands were prepared for comparative studies (Figure 2). The structures of [Ru]-3–[Ru]-5 (three new NHC/Ru complexes) were further identified with X-ray crystallography (Figure 3). The catalytic activity of the above five complexes was then assessed at different temperatures. A typical catalytic reaction using benzyl alcohol (1a) and KOH as reactants was selected, while 0.1 mol % of an NHC/Ru complex and a reaction time of 12 h were fixed (as shown in Figure 2). It appeared that all the complexes showed relatively high activity at 120 and 145 °C, but significantly lower activity at 100 and 165 °C. Presumably, the relatively low temperature of 100 °C was insufficient to efficiently promote this catalysis, and these complexes and the generated key catalytic intermediates may not be very stable at the high temperature of 165 °C. To probe the steric effects of the NHC ligands, we compared the activity of [Ru]-1–[Ru]-3 at various temperatures. It was observed that [Ru]-1 and [Ru]-2 displayed analogous activity, whereas a drop in activity was observed for [Ru]-3 at almost all temperatures. The results implied that a sterically encumbering iPr group in the N-terminus of [Ru]-3 was probably detrimental to the catalytic dehydrogenation. Furthermore, [Ru]-1, [Ru]-4, and [Ru]-5 were presented to explore the electronic effects of NHC ligands. In comparison with [Ru]-1, [Ru]-4 (with two methyl groups on the backbone of the benzimidazole framework) exhibited comparable or marginally reduced activity. Interestingly, [Ru]-5, which bears an electron-deficient NHC ligand, triggered remarkably improved activity compared to [Ru]-1 and [Ru]-4. It is also worth mentioning that [Ru]-5 could generate acid 2a in 85% yield at 120 °C, which was substantially higher than the other two complexes at the same temperature. All these facts allowed us to assume that the electron-deficient nature of [Ru]-5 could probably facilitate the catalytic cycle in a faster manner, and thus [Ru]-5 was selected as the optimal NHC/Ru complex, and toluene at 120 °C was retained for the following experiments.
After selecting the optimal NHC/Ru complex for this reaction, we explored the impact of an additional ligand (L5) on this catalysis (as illustrated in Figure 4). It was worth noting that a suspension of [Ru]-5, L5, KOH, and dry toluene were heated at 120 °C for 0.5 h. After 1a was added to the reaction mixture, heating was then continued for another 12 h. When the Ru loading was changed from 500 to 250, 125, and 100 ppm, the yield of 2a dramatically deteriorated from 52% to 15%. To our delight, introducing L5 into the catalytic system resulted in enhanced catalytic performance at all catalyst loadings, consistent with our previous observation for the NHC/Ru-catalyzed dehydrogenative alcohol amidation to amides [51]. As the ratio of L5/[Ru]-5 gradually increased, the yield of 2a increased up to a 4:1 ratio. Therefore, the optimized ratio of L5/[Ru]-5 was identified as 4:1. At relatively high Ru loadings (500 and 250 ppm), more than 90% of 2a could be achieved in the end. With regard to 125 and 100 ppm of [Ru]-5, maximum yield of 75% and 68% was obtained, respectively. From the perspectives of both catalytic loadings and the yield of 2a, 125 ppm (equal to 0.0125 mol %) of [Ru]-5 was finally chosen for further investigations.
Intending to further increase the yield of 2a, we continued to optimize other parameters (Table 1). As already discussed above, [Ru]-5 alone gave 18% of 2a, with the majority of 1a remaining (entry 1). Nevertheless, introducing an additional amount of L5 (the ratio of L5/[Ru]-5 was optimized as 4:1) imparted a markedly improved yield of 2a (entry 2). Afterwards, it was expected that pre-heating [Ru]-5, L5, and KOH in toluene at 120 °C for different periods could affect the efficiency of this catalysis. If everything was added simultaneously, only 18% of product 2a and 78% of unreacted 1a were detected (entry 3). However, the catalytic performance varied, with the pre-heating time spanning from 0.5 to 2.5 h (entries 2, 4–7), with 1.5 h as the optimum time (entry 5). Moreover, the volume of toluene was noticed to influence the reaction (entries 5, 8, 9). Either a larger or smaller amount of toluene prompted slightly lower yield of 2a and conversion (entries 8–9). It was found that a catalytic amount of L5 alone, or no catalyst, gave rise to only trace amounts of 2a, which demonstrated the essential role of Ru in this catalysis (entry 10–11). Finally, the Ru loading was decreased to 0.0025 mol % to obtain the maximum TON and TOF values, leading to a TOF of 4800 after 1 h and a TON of 15,200 after 36 h (entries 12–13). Therefore, the optimized conditions, recognized as 0.0125 mol % (equal to 125 ppm) of [Ru]-5, 0.05 mol % (equal to 500 ppm) of L5, 1.00 equiv. of 1a, and 1.20 equiv. of KOH in dry toluene at 120 °C (as listed in entry 5 of Table 1), were finalized.
Furthermore, we decided to thoroughly explore the substrate tolerance of our catalytic system (Figure 5). At the outset, several substituted benzyl alcohols were attempted. Under the optimized conditions, substrates containing electron-donating groups reacted smoothly with KOH to yield the respective acids (2b–2d) in excellent yield. In contrast, electron-deficient alcohols 1e–1g produced acids 2e–2g in moderate yield at a higher Ru loading (250 ppm of [Ru]-5 combined with 1000 ppm of L5). Similar to our previous observation [33], the desired product 2e and the reduced by-product 2a were detected for the reaction of 4-chlorobenzyl alcohol (1e) and KOH. Moreover, it seemed that the positions of the substituents on the phenyl group had a considerable impact on the product yield. 3-Methylbenzoic acid (2h) was assembled from 3-methylbenzyl alcohol (1h) in 80% yield, whereas 2-methylbenzyl alcohol (1i) generated product 2i in moderate yield even if a higher catalyst loading and longer reaction time were used. Additionally, substrates bearing a naphthyl group or a sulfur atom (1j and 1k) were also tolerated. With the exception of aromatic carboxylic acids, a handful of aliphatic carboxylic acids were prepared utilizing this NHC/Ru-catalyzed protocol, although a longer period was generally required to secure full conversion. Sterically non-hindered alcohols such as 1-hexanol (1l), cyclohexylmethanol (1m), and 3-phenylpropan-1-ol (1n) were efficiently converted to carboxylic acids 2l–2n in 88%–95% yield after 24 h. In addition, heterocycle-containing substrate 1o afforded acid 2o in 56% yield. In terms of hex-5-en-1-ol (1p) which comprises a terminal C=C bond, 74% of product 2p, 6% of isomerized by-product 2q, as well as 16% of reduced by-product 2l were obtained. Nevertheless, the alcohol containing an internal C=C bond was exclusively transformed into the desired acid (2q), without the observation of the isomerized and reduced byproducts. Finally, amino alcohols 1r–1s reacted well with KOH to furnish amino acids 2r–2s in a 65%–78% yield.
To probe the nature of the active species in this study, further investigations were carried out. Catalyst poisoning experiments were first conducted by the mercury test (as listed in Table S3, Supplementary Materials). It appeared that metallic mercury had almost no impact on the activity of our optimized catalytic system (entries 2–4 vs. entry 1), demonstrating the homogeneous nature of the real catalyst. Since the generation of Ru hydride species was reported to be crucial for the NHC/Ru-catalyzed dehydrogenative coupling of alcohols with amines or hydroxides [33,51,52,53,54,55,56], Ru hydride formation was monitored by two NMR reactions utilizing [Ru]-5 with and without L5, respectively (Figures S1 and S2, Supplementary Materials). These results implied that Ru-hydride species were involved in the reactions and probably paramount in the catalytic cycle. From the above contents and the previous reports about NHC/Ru-catalyzed alcohol dehydrogenation to acids [26,33], a plausible mechanism including various NHC/Ru species was presented (Figure 6). A refluxing mixture of [Ru]-5, L5, KOH, and toluene resulted in NHC/Ru species I, which could undergo ligand exchange to provide intermediate II. It is expected that the electron-deficient NHC ligand in [Ru]-5 promotes this step faster than other NHC ligands [54], consistent with our experimental results. After the formation of intermediate II, an analogous pathway was proposed with our previous publication [33].

3. Experimental Section

3.1. General Considerations

Most operations were done inside a glovebox, while most catalytic reactions were performed utilizing the standard Schlenk techniques. 1H NMR spectra were recorded on a Bruker Avance 500 spectrometer (Billerica, MA, USA) in CDCl3, DMSO-d6, or D2O with/without TMS as the internal reference, and 13C NMR spectra were recorded in CDCl3, DMSO-d6, or D2O on a Bruker Avance 500 (126 MHz) spectrometer (Billerica, MA, USA). Single-crystal samples were analyzed by a Bruker APEX-II CCD diffractometer (Billerica, MA, USA). The following abbreviations were used to designate multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, hept = heptet, m = multiplet, dd = doublet of doublets, tt = triplet of triplets, qq = quartet of quartets, and ddd = doublet of doublets of doublets. Melting points were taken on a Buchi M-560 melting point apparatus (Flawil, Switzerland) and were uncorrected. HRMS analysis was done with a Bruker Daltonics microTOF-QII instrument (Billerica, MA, USA). Generally, the chemicals were purchased from commercial suppliers including Aladdin (Shanghai, China), Energy Chemical (Shanghai, China) and Innochem (Beijing, China), and used directly for our experiments. NHC precursors L1 [57], L2 [54], L3 [57], L4 [58], and NHC/Ru complexes [Ru]-1 [51] and [Ru]-2 [59] were prepared according to literature procedures.

3.2. General Procedure for the Synthesis of L5

To a solution of 5,6-dichloro-1H-benzo[d]imidazole (1.12 g, 6.00 mmol) in dry DMF (5.0 mL) at 0 °C was added NaH (60% dispersion in mineral oil) (0.26 g, 6.60 mmol), and the suspension was warmed to room temperature and stirred for 2 h. Afterward, methyl iodide (0.41 mL, 6.60 mmol) was added, allowing the mixture to react for another 4 h. After the reaction was complete, ice-cold water was added dropwise to quench the reaction. The resulting mixture was extracted with ethyl acetate (3 × 30 mL). The organic layer was washed with water (3 × 50 mL), brine (50 mL), and dried over Na2SO4, followed by the evaporation of the solvent to afford a pale yellow solid. Afterwards, a mixture of the above pale yellow solid, ethyl iodide (1.91 mL, 24.00 mmol), and acetone (20 mL) was refluxed for 16 h. The reaction mixture was cooled to room temperature and concentrated to give crude L5, which was washed with diethyl ether to provide 1.61 g of pure L5 in 75% yield (in two steps).

3.3. General Procedure for the Synthesis of [Ru]-3–[Ru]-5

To a 25 mL sealed tube were added L5 (357.0 mg, 1.00 mmol), Ag2O (115.9 mg, 0.50 mmol), and dry dichloromethane (5.0 mL), and the mixture was refluxed in the dark for 12 h. Then [Ru(p-cymene)Cl2]2 (306.1 mg, 0.50 mmol) was added, allowing the resulting suspension to reflux for 6 h. Afterward, the mixture was filtered and the filtrate was then concentrated. A small amount of methanol was added to dissolve the residue, and an excess amount of diethyl ether was added to give a brown precipitate, which was then washed with diethyl ether a few times to obtain the analytically pure product [Ru]-5 as a brownish red powder. Yield: 62% (331.9 mg, 0.62 mmol).
Synthesis of [Ru]-3–[Ru]-4 was similar to that of [Ru]-5 mentioned above.

3.4. General Procedure for the NHC/Ru-Catalyzed Acceptorless Dehydrogenation of Alcohols to Acids

To a 25 mL Schlenk tube were added [Ru]-5 (1.4 mg, 0.0025 mmol), L5 (3.6 mg, 0.010 mmol), KOH (1.35 g, 24 mmol), and dry toluene (2.0 mL), and the mixture was refluxed under an open argon flow for 1.5 h. Then alcohol 1 was added to the flask, and the suspension was refluxed for 12 h. Afterwards, water (5 mL) was added, and the aqueous layer was washed with diethyl ether (3 × 30 mL) before acidification. The water layer was then acidified with 3 N HCl (3 mL) and extracted with ethyl acetate (3 × 30 mL). The organic phase was dried with Na2SO4, concentrated, and dried to obtain pure carboxylic acid 2. In addition, the detailed methods for obtaining the NMR and isolated yield of all the acids followed our previous publication [33].

3.5. X-ray Crystallography

Diethyl ether was slowly evaporated into a solution of an NHC/Ru complex in CDCl3, affording the resulting single-crystal sample suitable for analysis. Diffraction data were collected with CuKα radiation (λ = 1.54184 Å), and numerical absorption corrections were applied. The structures were solved by direct methods and refined on F2 with anisotropic thermal parameters for all non-hydrogen atoms. Protons were refined at the calculated positions by using a riding model. CCDC-1959447 (corresponding to [Ru]-3), CCDC-1959445 (corresponding to [Ru]-4), and CCDC-1959448 (corresponding to [Ru]-5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

4. Conclusions

In summary, we designed and synthesized five benzimidazole-derived NHC/Ru complexes, incorporating distinct substituents on the NHC ligands. The effects of ancillary NHC ligands and an additional ligand were also extensively studied, followed by the screening of other reaction parameters. As a result, a combination of [Ru]-5 with an electron-poor ancillary NHC ligand and L5 as an additional ligand was verified to be highly efficient for the alcohol dehydrogenation to carboxylic acids. Under the optimized conditions, numerous carboxylic acids were efficiently synthesized from the corresponding aromatic or aliphatic alcohols. Notably, a low Ru loading of 125 ppm was enough to achieve satisfactory results. Furthermore, TONs up to 15,200 and TOFs up to 4800 were also achieved. It is worth mentioning that this work demonstrates the lowest catalyst loading as well as the highest TOF value ever reported for this catalytic process.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/1/10/s1, Table S1: Comparison of this catalytic system with the reported Ru systems. Table S2: Comparison of this catalytic system with other metal-based systems. Table S3: Catalyst poisoning by the mercury test. a Figure S1: NMR reactions utilizing [Ru]-5 without an additional ligand. Figure S2: NMR reactions utilizing [Ru]-5 with L5 as an additional ligand. [Ru system]: [Ru]-5 (1.0 equiv.), L5 (4.0 equiv.), KOH (12 equiv.) and toluene-d8 (0.6 mL) were heated at reflux for 1.5 h. 1H NMR, 13C NMR, HRMS data as well as spectra of NHC precursor L5, NHC/Ru complexes [Ru]-3–[Ru]-5 and carboxylic acids 2a-2s. The single-crystal data of [Ru]-3–[Ru]-5.

Author Contributions

W.-Q.W., C.C., and F.V. discussed and designed the project. W.-Q.W., H.C., Y.-Q.H., and H.-J.W. performed the experiments. C.C. wrote the draft of the manuscript, then Y.Y., Z.-Q.W., W.S., and F.V. revised the manuscript. C.C. and F.V. together finalized the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (Grant No. 21502062), the Scientific Research Program Guiding Project of Hubei Provincial Department of Education (B2019135), the Teacher’s Scientific Research Ability Cultivation Fund, Hubei University of Arts and Science (Natural Science, Grant No. 2018kypy001), and the Open Foundation of Discipline of Hubei University of Arts and Science (Grant No. XK2019038). F.V. acknowledges the support from the Tomsk Polytechnic University Competitiveness Enhancement Program grant (VIU69-2019).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Ryland, B.L.; Stahl, S.S. Practical aerobic oxidations of alcohols and amines with homogeneous copper/TEMPO and related catalyst systems. Angew. Chem. Int. Ed. 2014, 53, 8824–8838. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  2. Wu, L.; Moteki, T.; Gokhale, A.A.; Flaherty, D.W.; Toste, F.D. Production of fuels and chemicals from biomass: Condensation reactions and beyond. Chem 2016, 1, 32–58. [Google Scholar] [CrossRef] [Green Version]
  3. Tojo, G.; Fernández, M. Oxidation of Primary Alcohols to Carboxylic Acids; Springer: Berlin/Heidelberg, Germany, 2007. [Google Scholar]
  4. Mazitschek, R.; Muelbaier, M.; Giannis, A. IBX-mediated oxidation of primary alcohols and aldehydes to form carboxylic acids. Angew. Chem. Int. Ed. 2002, 41, 4059–4061. [Google Scholar] [CrossRef]
  5. Tohma, H.; Kita, Y. Hypervalent iodine reagents for the oxidation of alcohols and their application to complex molecule synthesis. Adv. Synth. Catal. 2004, 346, 111–124. [Google Scholar] [CrossRef]
  6. Patel, S.; Mishra, B.K. Chromium (VI) oxidants having quaternary ammonium ions: Studies on synthetic applications and oxidation kinetics. Tetrahedron 2007, 63, 4367–4406. [Google Scholar] [CrossRef]
  7. ten Brink, G.J.; Arends, I.W.; Sheldon, R.A. Green, catalytic oxidation of alcohols in water. Science 2000, 287, 1636–1639. [Google Scholar] [CrossRef]
  8. Yamada, Y.M.A.; Arakawa, T.; Hocke, H.; Uozumi, Y. A nanoplatinum catalyst for aerobic oxidation of alcohols in water. Angew. Chem. Int. Ed. 2007, 46, 704–706. [Google Scholar] [CrossRef]
  9. Zope, B.N.; Hibbitts, D.D.; Neurock, M.; Davis, R.J. Reactivity of the gold/water interface during selective oxidation catalysis. Science 2010, 330, 74–78. [Google Scholar] [CrossRef] [Green Version]
  10. Wang, J.; Liu, C.; Yuan, J.; Lei, A. Transition-metal-free aerobic oxidation of primary alcohols to carboxylic acids. New J. Chem. 2013, 37, 1700–1703. [Google Scholar] [CrossRef]
  11. Han, L.; Xing, P.; Jiang, B. Selective aerobic oxidation of alcohols to aldehydes, carboxylic Acids, and imines catalyzed by a Ag-NHC complex. Org. Lett. 2014, 16, 3428–3431. [Google Scholar] [CrossRef]
  12. Jiang, X.; Zhang, J.; Ma, S. Iron catalysis for room-temperature aerobic oxidation of alcohols to carboxylic acids. J. Am. Chem. Soc. 2016, 138, 8344–8347. [Google Scholar] [CrossRef] [PubMed]
  13. Kim, S.M.; Shin, H.Y.; Kim, D.W.; Yang, J.W. Metal-free chemoselective oxidative dehomologation or direct oxidation of alcohols: Implication for biomass conversion. ChemSusChem 2016, 9, 241–245. [Google Scholar] [CrossRef]
  14. Ahmed, M.S.; Mannel, D.S.; Root, T.W.; Stahl, S.S. Aerobic oxidation of diverse primary alcohols to carboxylic acids with a heterogeneous Pd–Bi–Te/C (PBT/C) catalyst. Org. Process Res. Dev. 2017, 21, 1388–1393. [Google Scholar] [CrossRef]
  15. Hazra, S.; Deb, M.; Elias, A.J. Iodine catalyzed oxidation of alcohols and aldehydes to carboxylic acids in water: A metal-free route to the synthesis of furandicarboxylic acid and terephthalic acid. Green Chem. 2017, 19, 5548–5552. [Google Scholar] [CrossRef]
  16. Liu, K.J.; Jiang, S.; Lu, L.H.; Tang, L.L.; Tang, S.S.; Tang, H.S.; Tang, Z.; He, W.M.; Xu, X. Bis (methoxypropyl) ether-promoted oxidation of aromatic alcohols into aromatic carboxylic acids and aromatic ketones with O2 under metal- and base-free conditions. Green Chem. 2018, 20, 3038–3043. [Google Scholar] [CrossRef]
  17. Zhang, L.; Luo, X.; Li, Y. A new approach for the aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid without using transition metal catalysts. J. Energy Chem. 2018, 27, 243–249. [Google Scholar] [CrossRef] [Green Version]
  18. Zweifel, T.; Naubron, J.V.; Grützmacher, H. Catalyzed dehydrogenative coupling of primary alcohols with water, methanol, or amines. Angew. Chem. Int. Ed. 2009, 48, 559–563. [Google Scholar] [CrossRef]
  19. Trincado, M.; Grützmacher, H.; Vizza, F.; Bianchini, C. Domino rhodium/palladium-catalyzed dehydrogenation reactions of alcohols to acids by hydrogen transfer to inactivated alkenes. Chem. Eur. J. 2010, 16, 2751–2757. [Google Scholar] [CrossRef]
  20. Gianetti, T.L.; Annen, S.P.; Santiso-Quinones, G.; Reiher, M.; Driess, M.; Grützmacher, H. Nitrous oxide as a hydrogen acceptor for the dehydrogenative coupling of alcohols. Angew. Chem. Int. Ed. 2016, 55, 1854–1858. [Google Scholar] [CrossRef]
  21. Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Catalytic transformation of alcohols to carboxylic acid salts and H-2 using water as the oxygen atom source. Nat. Chem. 2013, 5, 122–125. [Google Scholar] [CrossRef]
  22. Choi, J.H.; Heim, L.E.; Ahrens, M.; Prechtl, M.H.G. Selective conversion of alcohols in water to carboxylic acids by in situ generated ruthenium trans dihydrido carbonyl PNP complexes. Dalton Trans. 2014, 43, 17248–17254. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Malineni, J.; Keul, H.; Möller, M. A green and sustainable phosphine-free NHC-ruthenium catalyst for selective oxidation of alcohols to carboxylic acids in water. Dalton Trans. 2015, 44, 17409–17414. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  24. Li, Y.; Nielsen, M.; Li, B.; Dixneuf, P.H.; Junge, H.; Beller, M. Ruthenium-catalyzed hydrogen generation from glycerol and selective synthesis of lactic acid. Green Chem. 2015, 17, 193–198. [Google Scholar] [CrossRef]
  25. Zhang, L.; Nguyen, D.H.; Raffa, G.; Trivelli, X.; Capet, F.; Desset, S.; Paul, S.; Dumeignil, F.; Gauvin, R.M. Catalytic conversion of alcohols into carboxylic acid salts in water: Scope, recycling, and mechanistic insights. ChemSusChem 2016, 9, 1413–1423. [Google Scholar] [CrossRef] [PubMed]
  26. Santilli, C.; Makarov, I.S.; Fristrup, P.; Madsen, R. Dehydrogenative synthesis of carboxylic acids from primary alcohols and hydroxide catalyzed by a ruthenium N-heterocyclic carbene complex. J. Org. Chem. 2016, 81, 9931–9938. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  27. Hu, P.; Ben-David, Y.; Milstein, D. General synthesis of amino acid salts from amino alcohols and basic water liberating H-2. J. Am. Chem. Soc. 2016, 138, 6143–6146. [Google Scholar] [CrossRef] [PubMed]
  28. Dahl, E.W.; Louis-Goff, T.; Szymczak, N.K. Second sphere ligand modifications enable a recyclable catalyst for oxidant-free alcohol oxidation to carboxylates. Chem. Commun. 2017, 53, 2287–2289. [Google Scholar] [CrossRef]
  29. Dai, Z.; Luo, Q.; Meng, X.; Li, R.; Zhang, J.; Peng, T. Ru (II) complexes bearing 2, 6-bis (benzimidazole-2-yl) pyridine ligands: A new class of catalysts for efficient dehydrogenation of primary alcohols to carboxylic acids and H2 in the alcohol/CsOH system. J. Organomet. Chem. 2017, 830, 11–18. [Google Scholar] [CrossRef]
  30. Sarbajna, A.; Dutta, I.; Daw, P.; Dinda, S.; Rahaman, S.M.W.; Sarkar, A.; Bera, J.K. Catalytic conversion of alcohols to carboxylic acid salts and hydrogen with alkaline water. ACS Catal. 2017, 7, 2786–2790. [Google Scholar] [CrossRef]
  31. Singh, A.; Singh, S.K.; Saini, A.K.; Mobin, S.M.; Mathur, P. Facile oxidation of alcohols to carboxylic acids in basic water medium by employing ruthenium picolinate cluster as an efficient catalyst. Appl. Organomet. Chem. 2018, 32, e4574. [Google Scholar] [CrossRef]
  32. Gong, D.; Hu, B.; Chen, D. Bidentate Ru (ii)-NC complexes as catalysts for the dehydrogenative reaction from primary alcohols to carboxylic acids. Dalton Trans. 2019, 48, 8826–8834. [Google Scholar] [CrossRef] [PubMed]
  33. Wang, Z.Q.; Tang, X.S.; Yang, Z.Q.; Yu, B.Y.; Wang, H.J.; Sang, W.; Yuan, Y.; Chen, C.; Verpoort, F. Highly active bidentate N-heterocyclic carbene/ruthenium complexes performing dehydrogenative coupling of alcohols and hydroxides in open air. Chem. Commun. 2019, 55, 8591–8594. [Google Scholar] [CrossRef] [PubMed]
  34. Liu, H.M.; Jian, L.; Li, C.; Zhang, C.C.; Fu, H.Y.; Zheng, X.L.; Chen, H.; Li, R.X. Dehydrogenation of Alcohols to Carboxylic Acid Catalyzed by in Situ-Generated Facial Ruthenium-CPP Complex. J. Org. Chem. 2019, 84, 9151–9160. [Google Scholar] [CrossRef] [PubMed]
  35. Sawama, Y.; Morita, K.; Yamada, T.; Nagata, S.; Yabe, Y.; Monguchi, Y.; Sajiki, H. Rhodium-on-carbon catalyzed hydrogen scavenger- and oxidant-free dehydrogenation of alcohols in aqueous media. Green Chem. 2014, 16, 3439–3443. [Google Scholar] [CrossRef]
  36. Sawama, Y.; Morita, K.; Asai, S.; Kozawa, M.; Tadokoro, S.; Nakajima, J.; Monguchi, Y.; Sajiki, H. Palladium on carbon-catalyzed aqueous transformation of primary alcohols to carboxylic acids based on dehydrogenation under mildly reduced pressure. Adv. Synth. Catal. 2015, 357, 1205–1210. [Google Scholar] [CrossRef]
  37. Fujita, K.I.; Tamura, R.; Tanaka, Y.; Yoshida, M.; Onoda, M.; Yamaguchi, R. Dehydrogenative oxidation of alcohols in aqueous media catalyzed by a water-soluble dicationic iridium complex bearing a functional N-heterocyclic carbene ligand without using base. ACS Catal. 2017, 7, 7226–7230. [Google Scholar] [CrossRef]
  38. Cherepakhin, V.; Williams, T.J. Iridium catalysts for acceptorless dehydrogenation of alcohols to carboxylic acids: Scope and mechanism. ACS Catal. 2018, 8, 3754–3763. [Google Scholar] [CrossRef]
  39. Fujita, K.I. Development and application of new iridium catalysts for efficient dehydrogenative reactions of organic molecules. Bull. Chem. Soc. Jpn. 2019, 92, 344–351. [Google Scholar] [CrossRef] [Green Version]
  40. Ghalehshahi, H.G.; Madsen, R. Silver-catalyzed dehydrogenative synthesis of carboxylic acids from primary alcohols. Chem. Eur. J. 2017, 23, 11920–11926. [Google Scholar] [CrossRef]
  41. Nguyen, D.H.; Morin, Y.; Zhang, L.; Trivelli, X.; Capet, F.; Paul, S.; Desset, S.; Dumeignil, F.; Gauvin, R.M. Oxidative transformations of biosourced alcohols catalyzed by earth-abundant transition metals. ChemCatChem 2017, 9, 2652–2660. [Google Scholar] [CrossRef]
  42. Shao, Z.; Wang, Y.; Liu, Y.; Wang, Q.; Fu, X.; Liu, Q. A general and efficient Mn-catalyzed acceptorless dehydrogenative coupling of alcohols with hydroxides into carboxylates. Org. Chem. Front. 2018, 5, 1248–1256. [Google Scholar] [CrossRef]
  43. Monda, F.; Madsen, R. Zinc oxide-catalyzed dehydrogenation of primary alcohols into carboxylic acids. Chem. Eur. J. 2018, 24, 17832–17837. [Google Scholar] [CrossRef] [PubMed]
  44. Bauri, S.; Donthireddy, S.N.R.; Illam, P.M.; Rit, A. Effect of ancillary ligand in cyclometalated Ru (II)-NHC-catalyzed transfer hydrogenation of unsaturated compounds. Inorg. Chem. 2018, 57, 14582–14593. [Google Scholar] [CrossRef] [PubMed]
  45. Huynh, H.V.; Han, Y.; Jothibasu, R.; Yang, J.A. 13C NMR spectroscopic determination of ligand donor strengths using N-heterocyclic carbene complexes of palladium (II). Organometallics 2009, 28, 5395–5404. [Google Scholar] [CrossRef]
  46. Chen, C.; Kim, M.H.; Hong, S.H. N-heterocyclic carbene-based ruthenium-catalyzed direct amidation of aldehydes with amines. Org. Chem. Front. 2015, 2, 241–247. [Google Scholar] [CrossRef]
  47. Kaufhold, S.; Petermann, L.; Staehle, R.; Rau, S. Transition metal complexes with N-heterocyclic carbene ligands: From organometallic hydrogenation reactions toward water splitting. Coord. Chem. Rev. 2015, 304, 73–87. [Google Scholar] [CrossRef]
  48. Xie, X.; Huynh, H.V. Cyclometallated ruthenium (ii) complexes with ditopic thienyl NHC-ligands: Syntheses and alkyne annulations. Org. Chem. Front. 2015, 2, 1598–1603. [Google Scholar] [CrossRef]
  49. Peris, E. Smart N-heterocyclic carbene ligands in catalysis. Chem. Rev. 2017, 118, 9988–10031. [Google Scholar] [CrossRef]
  50. Huynh, H.V. Electronic properties of N-heterocyclic carbenes and their experimental determination. Chem. Rev. 2018, 118, 9457–9492. [Google Scholar] [CrossRef]
  51. Wang, W.Q.; Yuan, Y.; Miao, Y.; Yu, B.Y.; Wang, H.J.; Wang, Z.Q.; Sang, W.; Chen, C.; Verpoort, F. Well-defined N-heterocyclic carbene/ruthenium complexes for the alcohol amidation with amines: The dual role of cesium carbonate and improved activities applying an added ligand. Appl. Organomet. Chem. 2019. [Google Scholar] [CrossRef]
  52. Zhang, Y.; Chen, C.; Ghosh, S.C.; Li, Y.; Hong, S.H. Well-defined N-heterocyclic carbene based ruthenium catalysts for direct amide synthesis from alcohols and amines. Organometallics 2010, 29, 1374–1378. [Google Scholar] [CrossRef]
  53. Kim, K.; Kang, B.; Hong, S.H. N-Heterocyclic carbene-based well-defined ruthenium hydride complexes for direct amide synthesis from alcohols and amines under base-free conditions. Tetrahedron 2015, 71, 4565–4569. [Google Scholar] [CrossRef]
  54. Cheng, H.; Xiong, M.Q.; Cheng, C.X.; Wang, H.J.; Lu, Q.; Liu, H.F.; Yao, F.B.; Chen, C.; Verpoort, F. Insitu generated ruthenium catalyst systems bearing diverse N-heterocyclic carbene precursors for atom-economic amide synthesis from alcohols and amines. Chem. Asian J. 2018, 13, 440–448. [Google Scholar] [CrossRef] [PubMed]
  55. Chen, C.; Miao, Y.; De Winter, K.; Wang, H.J.; Demeyere, P.; Yuan, Y.; Verpoort, F. Ruthenium-based catalytic systems incorporating a labile cyclooctadiene ligand with N-heterocyclic carbene precursors for the atom-economic alcohol amidation using amines. Molecules 2018, 23, 2413. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  56. Wu, X.J.; Wang, H.J.; Yang, Z.Q.; Tang, X.S.; Yuan, Y.; Su, W.; Chen, C.; Verpoort, F. Efficient and phosphine-free bidentate N-heterocyclic carbene/ruthenium catalytic systems for the dehydrogenative amidation of alcohols and amines. Org. Chem. Front. 2019, 6, 563–570. [Google Scholar] [CrossRef]
  57. Astakhov, A.V.; Khazipov, O.V.; Degtyareva, E.S.; Khrustalev, V.N.; Chernyshev, V.M.; Ananikov, V.P. Facile hydrolysis of nickel (II) complexes with N-heterocyclic carbene ligands. Organometallics 2015, 34, 5759–5766. [Google Scholar] [CrossRef]
  58. Cheng, H.; Xiong, M.Q.; Zhang, N.; Wang, H.J.; Miao, Y.; Su, W.; Yuan, Y.; Chen, C.; Verpoort, F. Efficient N-heterocyclic carbene/ruthenium catalytic systems for the alcohol amidation with amines: Involvement of poly-carbene complexes? ChemCatChem 2018, 10, 4338–4345. [Google Scholar] [CrossRef]
  59. Oehninger, L.; Stefanopoulou, M.; Alborzinia, H.; Schur, J.; Ludewig, S.; Namikawa, K.; Muñoz-Castro, A.; Köster, R.W.; Baumann, K.; Wölfl, S.; et al. Evaluation of arene ruthenium (ii) N-heterocyclic carbene complexes as organometallics interacting with thiol and selenol containing biomolecules. Dalton Trans. 2013, 42, 1657–1666. [Google Scholar] [CrossRef]
Catalysts 10 00010 g001 550
Figure 1. The design strategy (a) Möller’s work; (b) Madsen’s work; (c) this work. Note: NHC = N-heterocyclic carbene, TON = turnover number, TOF = turnover frequency.
Figure 1. The design strategy (a) Möller’s work; (b) Madsen’s work; (c) this work. Note: NHC = N-heterocyclic carbene, TON = turnover number, TOF = turnover frequency.
Catalysts 10 00010 g001
Catalysts 10 00010 g002a 550Catalysts 10 00010 g002b 550
Figure 2. Effects of ancillary ligands on the catalytic activity of [Ru]-1–[Ru]-5 under different temperatures. Note: The solvents used for the experiments were toluene (for 100 °C), toluene (for 120 °C), m-xylene (for 145 °C), and mesitylene (for 165 °C).
Figure 2. Effects of ancillary ligands on the catalytic activity of [Ru]-1–[Ru]-5 under different temperatures. Note: The solvents used for the experiments were toluene (for 100 °C), toluene (for 120 °C), m-xylene (for 145 °C), and mesitylene (for 165 °C).
Catalysts 10 00010 g002aCatalysts 10 00010 g002b
Catalysts 10 00010 g003 550
Figure 3. The molecular structures of complexes [Ru]-3–[Ru]-5, showing the atom labeling scheme of carbon atoms C1 and the heteroatoms. The hydrogen atoms and solvent molecules were omitted for clarity, and the ellipsoids of atoms were shown at probability levels of 30% for [Ru]-3–[Ru]-4 and 20% for [Ru]-5.
Figure 3. The molecular structures of complexes [Ru]-3–[Ru]-5, showing the atom labeling scheme of carbon atoms C1 and the heteroatoms. The hydrogen atoms and solvent molecules were omitted for clarity, and the ellipsoids of atoms were shown at probability levels of 30% for [Ru]-3–[Ru]-4 and 20% for [Ru]-5.
Catalysts 10 00010 g003
Catalysts 10 00010 g004 550
Figure 4. Effects of an additional ligand (L5) on the catalytic activity at different catalyst loadings.
Figure 4. Effects of an additional ligand (L5) on the catalytic activity at different catalyst loadings.
Catalysts 10 00010 g004
Catalysts 10 00010 g005 550
Figure 5. Synthesis of carboxylic acids from various alcohols and KOH catalyzed by [Ru]-5 combined with L5.
Figure 5. Synthesis of carboxylic acids from various alcohols and KOH catalyzed by [Ru]-5 combined with L5.
Catalysts 10 00010 g005
Catalysts 10 00010 g006 550
Figure 6. The plausible mechanism for this [Ru]-5/L5-catalyzed acid synthesis from alcohols and KOH.
Figure 6. The plausible mechanism for this [Ru]-5/L5-catalyzed acid synthesis from alcohols and KOH.
Catalysts 10 00010 g006
Table
Table 1. Optimization of other reaction parameters a.
Table 1. Optimization of other reaction parameters a.
Catalysts 10 00010 i001
EntryxymnYield (%) bTONs for 2aTOFs for 2a
2aUnreacted 1a
10.012500.51218801440120
20.01250.050.51275236000500
30.01250.0501218781440120
40.01250.051.01286116880573
50.01250.051.5129437520627
60.01250.052.0129257360613
70.01250.052.5128987120593
8 c0.01250.051.51285136800567
9 d0.01250.051.5129187280607
1000.051.512trace91--
11001.512trace92--
120.00250.011.51128748004800
130.00250.011.536385915,200422
a(1) A suspension of [Ru]-5 (x mol %), L5 (y mol %), KOH (1.20 equiv.), and dry toluene (2.0 mL) was stirred at 120 °C for m hours; (2) 1a (1.00 equiv.) was added and the mixture was stirred at 120 °C for n hours; (3) 3N HCl aqueous solution was added. b NMR yield (average of two consistent runs) using 1,3,5-trimethoxybenzene as an internal standard. c Dry toluene (1.0 mL). d Dry toluene (4.0 mL). Note: TONs = turnover numbers, TOFs = turnover frequencies.

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Wang, W.-Q.; Cheng, H.; Yuan, Y.; He, Y.-Q.; Wang, H.-J.; Wang, Z.-Q.; Sang, W.; Chen, C.; Verpoort, F. Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands. Catalysts 2020, 10, 10. https://doi.org/10.3390/catal10010010

AMA Style

Wang W-Q, Cheng H, Yuan Y, He Y-Q, Wang H-J, Wang Z-Q, Sang W, Chen C, Verpoort F. Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands. Catalysts. 2020; 10(1):10. https://doi.org/10.3390/catal10010010

Chicago/Turabian Style

Wang, Wan-Qiang, Hua Cheng, Ye Yuan, Yu-Qing He, Hua-Jing Wang, Zhi-Qin Wang, Wei Sang, Cheng Chen, and Francis Verpoort. 2020. "Highly Efficient N-Heterocyclic Carbene/Ruthenium Catalytic Systems for the Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Effects of Ancillary and Additional Ligands" Catalysts 10, no. 1: 10. https://doi.org/10.3390/catal10010010

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