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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 Ken-ichi Fujita, Ryuichi Tamura, Yuhi Tanaka, Masato Yoshida, Mitsuki Onoda, and Ryohei Yamaguchi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02560 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017
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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 Ken-ichi Fujita,* Ryuichi Tamura, Yuhi Tanaka, Masato Yoshida, Mitsuki Onoda, and Ryohei Yamaguchi* Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
ABSTRACT: A dicationic iridium complex bearing a bidentate ligand that comprises of N-heterocyclic carbene and αhydroxypyridine moieties has been designed and synthesized. The complex exhibited high catalytic performance in aqueous media for the dehydrogenative oxidation of secondary alcohols to ketones accompanying the evolution of hydrogen. Furthermore, dehydrogenative transformation of primary alcohols to the carboxylic acids in aqueous media was also catalyzed by the complex without using base. KEYWORDS: dehydrogenation, iridium catalyst, functional ligand, N-heterocyclic carbene, alcohol, ketone, carboxylic acid
One of the greatest challenges in modern chemistry is to perform efficient organic synthesis under environmentally friendly conditions with the aid of catalyst. In the past few decades, high performance catalysts have been designed for a variety of organic transformations. By using an appropriate catalyst, a number of reactions that would not usually proceed have been accomplished and ingenious reaction selectivities have been attained. These developments stimulated remarkable progress in the field of catalytic organic synthesis.1 Conversely, it is generally difficult to perform an organic synthesis in water which is the most environmentally benign solvent, regardless of whether a catalyst is used.2 In particular, in the case of reactions catalyzed by homogeneous transition metal complexes, only very few aqueous reaction systems have been developed because such complexes are either hardly soluble or unstable in water. We have previously demonstrated the design and synthesis of iridium catalysts bearing functional ligands and reported their utilization as catalysts for the dehydrogenative reactions of alcohols3 and amines.4 In the course of those studies, we succeeded in the synthesis of homogeneous transition metal catalysts that are readily soluble in water and stable over a long period. However, in many cases, the activity of such catalysts used in aqueous media is relatively low compared to those used in organic solvents.5,6 Therefore, it is necessary to redesign functional ligands and synthesize new catalysts that exhibit a high activity in aqueous media. In this study, we designed a new functional ligand that comprises N-heterocyclic carbene (NHC)7 and αhydroxypyridine moieties.8 A new dicationic iridium catalyst bearing the newly designed ligand was successfully synthesized. Furthermore, it was revealed that the new iridium catalyst showed high activity in the dehydrogenative reactions of secondary and primary alcohols in aqueous media to give ketones and carboxylic acids, respectively, accompanying the evolution of hydrogen.
To design a new catalyst that possesses a high capability for dehydrogenative reactions of organic molecules in aqueous media, three working hypotheses were considered: 1) the introduction of a strongly electron-donating ligand would be effective for dehydrogenation, which is based on our previous computational study,9 2) a chelating ligand that would form a strong bond with the transition metal would be important to avoid catalyst decomposition via dissociation of the ligand, and 3) an ionic catalyst would be advantageous for solubility in water. Based on these hypotheses, we designed a dicationic catalyst having a chelating bidentate ligand that comprises NHC and α-hydroxypyridine moieties because NHC is a very strong σ-donor and forms a strong metal-carbene bond.10 As shown in Scheme 1, starting from ligand precursor 1, iridium Scheme 1. Preparation of Water-Soluble Dicationic Iridium Complex 3 Bearing a Functional N-Heterocyclic Carbene Ligand
complex 2 was prepared by the carbene transfer method11 by using Ag2O followed by debenzylation under acidic conditions. Then, the reaction of 2 with two equivalents of AgOTf in water gave dicationic complex 3 with an aquo ligand in
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96% yield. Complex 3 was highly soluble in water and stable in air for months. The structure of 3 was elucidated by spectroscopic data, elemental analysis, and single-crystal X-ray diffraction (see the supporting information). We next examined the dehydrogenative oxidation of 1phenylethanol to acetophenone in aqueous media in the presence of 0.50 mol% of various water-soluble iridium catalysts. The results are summarized in Table 1. Iridium catalyst 3, newly synthesized in this study, showed the highest activity, giving acetophenone in 46% yield with complete selectivity after 1 h reaction under reflux (entry 1). Catalyst 4, without the α-hydroxy group in the ligand, and catalyst 5, with only the NHC ligand, both exhibited very low catalytic activity (entries 2 and 3). With catalyst 6, which we have previously reported for the dehydrogenative oxidation of alcohols,5a 15% yield of acetophenone was obtained (entry 4), thus demonstrating the superiority of the new catalyst 3. Catalyst 7, which is also active for the dehydrogenative oxidation of alcohols in aqueous media in combination with NaOH,5c also showed lower activity than that of 3 (entry 5). Table 1. Comparison of Catalytic Activities of Complexes 3-7 for the Dehydrogenative Oxidation of 1-Phenylethanol under Reflux in Watera
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tones, although a relatively large amount of the catalyst 3 (1.0 or 2.0 mol%) was required (entries 13-15). Table 2. Dehydrogenative Oxidation of Various Secondary Alcohols under Reflux in Water Catalyzed by 3a catalyst 3
OH R
1
2
O
H2O (5 mL), reflux, 20 h
R
R
1
H2
+
R2
1.0 mmol
entry
alcohol
cat. 3 (mol%)
conv. (%)b
yield (%)b
OH R'
1 2 3 4 5 6 7 8 9 10 11
R' = H 4-OMe 2-Me 3-Me 4-Me 4-F 3-Cl 4-Cl 4-Br 4-CF3 4-CO2Me
0.25 0.10 1.0 0.25 0.25 1.0 0.25 0.50 1.0 1.0 0.25
99 99 95 98 97 100 92 98 99 87 96
99 97 (90) 85 (73) 98 (81) 97 (91) 86 (84) 89 (86) 98 (89) 99 (95) 87 (78) 96 (91)
1.0
99
51 (34)
2.0
97
92
OH
12
S
13
a The reaction was carried out with 1-phenylethanol (1.0 mmol) and the catalyst (0.50 mol%) under reflux in water (5 mL) for 1 h. bDetermined by GC. cNaOH (0.50 mol%) was also added.
To explore the scope of the new catalytic system employing 3, reactions of various secondary alcohols were conducted. The results are summarized in Table 2. When the reaction of 1-phenylethanol (1.0 mmol) was conducted in water (5 mL) in the presence of the catalyst 3 (0.25 mol%) under reflux for 20 h, acetophenone was obtained in 99% yield (entry 1). Reactions of 1-arylethanols bearing electron-donating and electronwithdrawing substituents in the aromatic ring proceeded smoothly to give the corresponding acetophenone derivatives in good to excellent yields (entries 2-12). Alkoxy, fluoro, chloro, bromo, trifluoromethyl, and methoxycarbonyl groups were tolerated in this catalytic system.12 Aliphatic and cyclic secondary alcohols were also converted to the corresponding ke-
OH
14
OH
1.0
98
98
15
OH
2.0
61
60
a The reaction was carried out with secondary alcohols (1.0 mmol) and catalyst 3 under reflux in water (5 mL) for 20 h. b Determined by GC (entries 1 and 13-15) and 1H NMR (entries 2-12). Isolated yields are indicated in parentheses.
Note that catalyst 3 could be reused according to the following simple procedure: After the dehydrogenation reaction shown in Scheme 2, 30 mL of hexane was added to extract the product and the remaining starting alcohol. The recovered aqueous phase containing the catalyst could be used for the second run of the catalytic dehydrogenation to give 93% yield of ketone. Reuse of the catalyst was achieved at least three times without the loss of activity. Scheme 2. Reuse of the Catalyst 3 in the Dehydrogenative Oxidation of 1-(4-Methylphenyl)ethanol in Water OH
O Ir cat. 3 (0.50 mol%)
+
H2O (5 mL), reflux, 20 h
H2
1.0 mmol
1
2
3
yield of ketone (%) 97
93
94
reuse
A possible mechanism for the present dehydrogenative oxidation of secondary alcohols catalyzed by 3 is shown in
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Scheme 3. The first step of the reaction would involve the elimination of trifluoromethanesulfonic acid and the aquo ligand from dicationic catalyst 3 to afford the monocationic unsaturated species A, which has an α-pyridonate moiety connected to an NHC ligand. Then, activation of the alcohol would occur via the concerted pathway through transition state B to give a ketone product and hydrido iridium species C. Finally, the protonolysis of the hydride on iridium by the hydroxy proton on the functional ligand would occur to release hydrogen accompanying the regeneration of the catalytically active unsaturated species A.
Scheme 4. Additional Experiments Supporting the Proposed Mechanism
Scheme 3. Possible Mechanism for the Dehydrogenative Oxidation of Secondary Alcohols Catalyzed by 3
To obtain information about the proposed mechanism, three experiments were conducted (Scheme 4). The first experiment addressed the formation of catalytically active species A (eq 1). When dicationic catalyst 3 was treated with one equivalent of sodium t-butoxide at room temperature for 1 h, a new monocationic complex 8, having an α-pyridonate connected to NHC ligand, which is closely related to species A in Scheme 3, was formed and isolated in 51% yield. The structure of 8 was elucidated by spectroscopic data, elemental analysis, and singlecrystal X-ray diffraction (see the supporting information). Then, the catalytic activity of 8 was examined (eq 2). As we expected, complex 8 exhibited high catalytic activity for the dehydrogenative oxidation of 1-phenylethanol in aqueous media with a loading of 0.25 mol% to give acetophenone in excellent yield (99%).13 Quantitative analysis of the evolved hydrogen gas was also conducted (eq 3). By running the dehydrogenative reaction on a larger scale (5 mmol scale), hydrogen was obtained in 89% yield, which was equimolar to the ketone product.14,15 The results of these three reactions strongly support the proposed mechanism shown in Scheme 3. After establishing a new and efficient catalytic system for the dehydrogenative oxidation of secondary alcohols in aqueous media catalyzed by 3, we turned our attention to the reaction of primary alcohols. We have previously reported the dehydrogenative oxidation of primary alcohols catalyzed by complex 6 in aqueous media, which resulted in the formation of aldehydes.5a However, interestingly, the reaction of benzyl alcohol under reflux in water for 20 h in the presence of catalyst 3 (2.0 mol%) gave benzoic acid in 80% yield, as shown in entry 1 of Table 3. Recently, much attention has been focused on the dehydrogenative transformation of primary alcohols
into carboxylic acids. Although several efficient catalytic systems for such a transformation have been published, those systems always require the addition of stoichiometric amounts of a strong base.16 Conversely, our present system does not require the addition of base and can be performed simply under reflux in water in the presence of only catalyst 3. To the best of our knowledge, this is the first example of the dehydrogenative (not employing any oxidant) transformation of primary alcohols into the corresponding carboxylic acid without using basic reagents. Table 3. Dehydrogenative Oxidation of Primary Benzylic Alcohols to Benzoic Acid Derivatives under Reflux in Water Catalyzed by 3a
a The reaction was carried out with primary alcohols (1.0 mmol) and catalyst 3 under reflux in water (5 mL) for 20 h. b Determined by 1H NMR. Isolated yields are indicated in parentheses. cWater (4.0 mL) and 1,4-dioxane (1.0 mL) were used as solvent.
In Table 3, results of the dehydrogenative oxidation of benzylic primary alcohols in aqueous media to give benzoic acid derivatives are summarized.17,18 In addition to the methyl
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group (entries 2 and 3), methoxy (entry 4), bromo (entry 5), and chloro (entry 6) substituents were tolerated in this catalytic system. The reaction of 2-naphthalenemethanol gave 2naphthoic acid in high yield (entry 7). In the case that the solubility of the substrates in water was extremely low, addition of 1,4-dioxane as a co-solvent improved the yields of carboxylic acids (entries 5-7). A possible reaction pathway for the dehydrogenative oxidation of primary alcohols to carboxylic acids in aqueous media is illustrated in Scheme 5. The first step of the reaction would be the dehydrogenation catalyzed by 3 to afford the aldehyde. This step would proceed by a similar mechanism to that shown in Scheme 3. Then, addition of water to the aldehyde would occur to give a gem-diol. Finally, dehydrogenation of the gemdiol catalyzed by 3 would proceed to give the carboxylic acid product. Scheme 5. Possible Reaction Pathway for the Dehydrogenative Oxidation of Primary Benzylic Alcohols under Reflux in Water Catalyzed by 3
Quantitative analysis of the evolved hydrogen gas in the dehydrogenative formation of the carboxylic acids from primary alcohols was also conducted. The reaction of 4-methylbenzyl alcohol on 2.5 mmol scale was conducted, as shown in eq 4. Hydrogen (2 equiv.) evolved in 78% yield, which was the corresponding amount expected because the carboxylic acid product was also obtained in 78% yield.
In summary, we have designed and prepared the new dicationic iridium catalyst 3 bearing a chelating bidentate ligand that comprises NHC and α-hydroxypyridine moieties. Catalyst 3 exhibited high activity in aqueous media for the dehydrogenative oxidation of secondary alcohols to ketones accompanying the evolution of hydrogen. Furthermore, the dehydrogenative transformation of primary alcohols in aqueous media to give carboxylic acids was also accomplished by employing catalyst 3 without using base.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal. *****. Experimental details and characterization data (PDF) X-ray crystallographic data for 3 (CIF)
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X-ray crystallographic data for 8 (CIF)
AUTHOR INFORMATION Corresponding Author
[email protected] [email protected] ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number JP16H01018 in Precisely Designed Catalysts with Customized Scaffolding and Grant-in-Aid for Scientific Research (B) Grant Number 26288047.
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Vasquez, R. M.; Chambers, N.; Szulczewski, G. J.; Grotjahn, D. B.; Webster, C. E.; Papish, E. T. Organometallics 2017, 36, 1091-1106. (9) Zeng, G.; Sakaki, S.; Fujita, K.; Sano, H.; Yamaguchi, R. ACS Catal. 2014, 4, 1010-1020. (10) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 12901309. (b) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239-2246. (c) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247-2273. (d) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815-1828. (e) Díez-González, S. (Eds), N-Heterocyclic Carbenes, RSC Publishing, 2011. (f) Cazin, C. S. J. N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, Springer, 2011. (11) (a) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972-975. (b) Chianese, A., R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663-1667. (12) We have examined the reaction of a substrate having a pyridine ring [1-(pyridin-2-yl)ethanol]. But the yield of dehydrogenated product (2-acetylpyridine) was low (35%). (13) In order to carefully compare the catalytic activities of 3 and 8, we also carried out the reactions of 1-phenylethanol using a smaller amount (0.10 mol%) of catalyst. The yields of acetophenone were 68% (with cat. 3) and 78% (with cat. 8), respectively. These results indicate that the catalytic activity of 8 was higher than that of 3, supporting the proposed mechanism. (14) The volume of evolved hydrogen gas was measured using a gas burette. The molar amount of hydrogen was calculated using the ideal gas law. (15) Purity of the evolved hydrogen gas was confirmed by GC analysis. Details are shown in Figure S2 in the supporting information. (16) (a) Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Nat. Chem. 2013, 5, 122-125. (b) Choi, J.-H.; Heim, L. E.; Ahrens, M.; Prechtl, M. H. G. Dalton Trans. 2014, 43, 17248-17254. (c) Malineni, J.; Keul, H.; Möller, M. Dalton Trans. 2015, 44, 17409-17414. (d) Zhang, L.; Nguyen, D. H.; Raffa, G.; Trivelli, X.; Capet, F.; Desset, S.; Paul, S.; Dumeignil, F.; Gauvin, R. M. ChemSusChem 2016, 9, 1413-1423. (e) Wang, X.; Wang, C.; Liu, Y.; Xiao, J. Green Chem. 2016, 18, 4605-4610. (f) Sarbajna, A.; Dutta, I.; Daw, P.; Dinda, S.; Rahaman, S. M. W.; Sarkar, A.; Bera, J. K. ACS Catal. 2017, 7, 27862790. (17) We have also conducted the reaction of benzyl alcohol catalyzed by the complex 6 under the same reaction condition as Table 3. However, the yield of benzoic acid was 21%, showing the high catalytic performance of 3 for the dehydrogenative oxidation of primary benzylic alcohols to benzoic acid derivatives. (18) We have also conducted the reaction of aliphatic primary alcohols (1-hexanol and 1-octanol) under the same conditions shown in Table 3. However, the yields of carboxylic acids were very low (< 25%).
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