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Selective Oxidation of Alcohols Catalyzed by a Hemicryptophane-Ruthenium Complex

Selective Oxidation of Alcohols Catalyzed by a Hemicryptophane-Ruthenium Complex Hemicryptophane, a covalent molecular capsule that is composed of a cyclotriveratrylene (CTV) host unit, a tris(2-aminoethyl)amine (tren) ligand, and three rigid phenyl spacers, is a useful ligand for preparing synthetic transition metal complexes containing well-defined cavities that mimic the active sites of metalloenzymes. A method for the selective oxidation of primary alcohols using 5 mol % hemicryptophane-ruthenium(II) dichloride as the catalyst and cerium(IV) ammonium nitrate (CAN) as the terminal oxidant is described. The corresponding aldehydes of both benzylic and aliphatic alcohols are afforded in good to excellent yields under mild conditions without significant over-oxidation to produce the corresponding carboxylic acids. Keywords Cyclotriveratrylene · Cerium(IV) ammonium nitrate (CAN) Yoshimasa Makita1*, Tomoyuki Fujita2, Tomofumi Danno2, Masashi Inoue2, Michio Ueshima2, Shin-ichi Fujiwara1, Akiya Ogawa2 Department of Chemistry, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata, Osaka 573-1121, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan © Versita Sp. z o.o. Received 30 June 2012 Accepted 25 July 2012 1. Introduction A considerable amount of scientific effort has been devoted to understanding and mimicking metalloenzymes, and it is well-established that the cavities around their active sites play important roles in their reactivity and selectivity [1-3]. Therefore, discoveries of new classes of synthetic transition metal complexes containing well-defined cavities that are analogous to the active sites of metalloenzymes have been of great interest for improving the reactivity and selectivity of bulk phase reactions [4-14]. Hemicryptophanes are covalent molecular capsules constructed from a cyclotriveratrylene (CTV) host unit and they are responsible for dissymmetry at the molecular cavity level [15-29]. We recently synthesized hemicryptophane 1 (Figure 1), which contains a tris(2-aminoethyl)amine (tren) ligand and three rigid phenyl spacers [30-33]. We also synthesized its zinc(II) complex, which contained the zinc(II) cation and the ligand exchange site within the cavity, and this complex efficiently catalyzed the hydrolysis of alkyl para-nitrophenyl carbonates [30,33]. Here, we report a highly active hemicryptophaneruthenium complex that catalyzes the selective oxidation of primary alcohols, including benzylic and aliphatic alcohols, to the corresponding aldehydes. Results of the comparative study of the catalytic activity of the ruthenium-hemicryptophane complex and that of ruthenium complexes of other simple trentype ligands are also disclosed. 2. Experimental Procedure Hemicryptophane 1 [30], tris[2-(benzylamino)ethyl]amine 3 [30], and [RuCl2(DMSO)4] [34] were synthesized according to literature methods. The other chemicals used in this study were reagent grade and used without further purification. 2.1 General method for the oxidation of primary alcohols A solution of ligand (5.0 mol) and RuCl2(DMSO)4 (2.6 mg, 5.0 mol) in acetonitrile (1.0 mL) and water (0.3 mL) was prepared at ambient temperature. Then, primary alcohol (100 mol) and Ce(NH4)2(NO3)6 (CAN, 219.3 mg, 400 mol) were added to the solution. The reaction mixture was stirred vigorously for 20 min at ambient temperature. The yields of the aldehyde or carboxylic acid products were analyzed by gas chromatography or 1H NMR experiments. Figure 1. Structure of hemicryptophane 1 and tren-based ligands 2­4. H NMR (CDCl3, 400 MHz): 2.08 (s, 3H), 2.85­3.07 (m, 12H), 3.48 (d, 3H, J = 6.0 Hz), 3.56 (s, 3H), 3.92 (br s, 6H), 4.69 (d, 3H, J = 6.0 Hz), 7.19 (s, 12H), 7.28 (s, 3H), 7.54 (s, 3H); FAB-MS: calcd. for C51H54Cl1N4O6Ru1: 955.2793 ([M ­Cl]+); found: 955.2841 ([M ­Cl]+). RuCl21. * E-mail: makita@cc.osaka.dent.ac.jp Y. Makita et al. 3. Results and Discussion We previously reported the synthesis and structural characterization of hemicryptophane 1. The oxidation precatalyst complex (RuCl2)·1 was obtained from the reaction of 1 with one equivalent of RuCl2(DMSO)4 in acetonitrile. Formation of the (RuCl2)·1 complex was confirmed by 1H NMR and FABMS. Precatalyst (RuCl2)·1 was immediately formed during the reaction. Therefore, we employed (RuCl2)·1 in the following oxidation reactions without pre-preparing the catalyst complex. In preliminary studies, we examined the optimization of the oxidation of 1-octanol in the presence of RuCl2(DMSO)4, tris[2-(benzylamino)ethyl]amine 3, and cerium(IV) ammonium nitrate (Table 1). No reaction occurred when DMSO was used as the solvent (entry 1), but some of the 1-octanal product was obtained in toluene (entry 2). In chloroform, the oxidation reaction proceeded quantitatively to afford 1-octanal and 1-octanoic acid. In acetonitrile, 1-octanoic acid was the main product and was obtained in moderate yield. Notably, the time required for completion of the oxidation reactions was only 20 min. Furthermore, in the reaction without water as a co-solvent, no oxidation occurred. Thus, water is necessary as the co-solvent in this reaction, probably because one-electron oxidants such as CAN might serve to generate the cis-dioxo adduct. The formed ruthenium complexes were soluble in acetonitrile/water (1 mL: 0.3 mL). Next, we studied the reaction with different ligands under the optimized conditions. The oxidation of 1-octanol in the presence of ligands 2 or 4 proceeded in moderate yields (entries 5 and 6, respectively). However, the reaction employing hemicryptophane ligand 1 under the optimized conditions resulted in selective formation of 1-octanal in 90%. No over-oxidation to yield 1-octanoic acid was observed in the presence of excess water. To further compare the reactivity of hemicryptophane 1 and other simple tren ligands, we investigated the effect of reaction time on the distribution of reaction products. With ligand 3, 73% of the aldehyde and 27% of the unreacted alcohol were obtained Table 2. Ruthenium-catalyzed oxidation reactions.a Entry 1 2 3 4 5 6 after 3 min. The yield of the aldehyde decreased to 47% and the yield of the carboxylic acid increased to 37% after 4 min. When the reaction time was extended to 20 min, the amount of aldehyde decreased and the amount of carboxylic acid increased. On the other hand, with hemicryptophane 1, the aldehyde was obtained in a yield of 91% in 3 min. When the reaction time was extended to 60 min, no 1-octanoic acid was observed. These results show that the hemicryptophane ligand facilitated alcohol oxidation to yield the aldehyde and suppressed its over-oxidation to the carboxylic acid. Next, the oxidation of a variety of alcohols using (RuCl2)·1 was evaluated (Table 2). The oxidation of benzyl alcohol with ligand 3 took place quantitatively and benzoic acid was obtained in 48% yield. Conversely, with ligand 1, while oxidation of benzyl alcohol Table 1. Optimization of the oxidation of 1-octanol using ruthenium complexes. R uC l 2 (D M S O ) 4 (5 m ol% ), ligand (5 m ol% ), C A N (4 eqiv.) solven t, H 2 O , rt, 20 m in 1-octan ol 5 1 -octana l + 1 -o ctanoic acid 7 6 Entry Ligand Solvent Yield (%)a 6 7 0 0 48 82 78 75 0 0 Conversion (%)a Determined by GC. DMSO toluene chloroform MeCN MeCN MeCN MeCN MeCN 0 62 >99 >99 >99 >99 >99 >99 Substrate 1-octanol benzyl alcohol 4-methoxybenzyl alcohol 4-bromobenzyl alcohol 4-nitrobenzyl alcohol 2-dodecanol Ligand 3 1 3 1 3 1 3 1 3 1 3 1 Product 1-octanal benzaldehyde 4-methoxybenzaldehyde 4-bromobenzaldehyde 4-nitrobenzaldehyde 2-dodecanone Yield (%) 18b 90b 52c 91c 55c 94c 82c 94c 27c 88c 68c 26c Product 1-octanoic acid benzoic acid 4-methoxybenzoic acid 4-bromobenzoic acid 4-nitrobenzoic acid Yield (%)b 82b 0b 48c 9c 41c 6c 9c 6c 10c 12c Conditions: reactions were performed on a 0.10 mmol scale with 5 mol % RuCl2(DMSO)4, 5 mol % ligand, and 4 equiv. Ce(NH4)2(NO3)6 in 1:0.3 CH3CN/H2O. Determined by GC. c Determined by 1H NMR. was still quantitative, benzaldehyde was obtained in 91% yield with only a minor amount of benzoic acid (entry 2). The reactivity of 4-methoxybenzyl alcohol was the same as benzyl alcohol (entry 2). In contrast, 4-bromobenzaldehyde was selectively formed with both 1 and 3 (entry 4). The oxidation of 4-nitrobenzyl alcohol occurred efficiently with ligand 1, but oxidation with ligand 3 was less efficient (entry 5). However, the oxidation product of secondary alcohol 2-dodecanol was obtained in 68% yield with ligand 3, but was obtained in only 26% yield with ligand 1. These results indicate that the capped structure of the hemicryptophane ligand highly affects the reactivity and selectivity of the catalytically active ruthenium center. 4. Conclusions We have developed a method for the selective oxidation of primary alcohols to the corresponding aldehydes using a ruthenium(II)-hemicryptophane complex as a precatalyst. The hemicryptophane ligand is more effective than other simple tren-based ligands and the success of this process is attributed to the capped structure of the hemicryptophane framework. Continued efforts to advance methods for catalytic oxidation using the capped structures of hemicryptophanes will attempt to further exploit the applications of the artificial active centers of metalloenzyme models. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Supramolecular Catalysis de Gruyter

Selective Oxidation of Alcohols Catalyzed by a Hemicryptophane-Ruthenium Complex

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Publisher
de Gruyter
Copyright
Copyright © 2012 by the
ISSN
2084-7246
eISSN
2084-7246
DOI
10.2478/supcat-2012-0002
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Abstract

Hemicryptophane, a covalent molecular capsule that is composed of a cyclotriveratrylene (CTV) host unit, a tris(2-aminoethyl)amine (tren) ligand, and three rigid phenyl spacers, is a useful ligand for preparing synthetic transition metal complexes containing well-defined cavities that mimic the active sites of metalloenzymes. A method for the selective oxidation of primary alcohols using 5 mol % hemicryptophane-ruthenium(II) dichloride as the catalyst and cerium(IV) ammonium nitrate (CAN) as the terminal oxidant is described. The corresponding aldehydes of both benzylic and aliphatic alcohols are afforded in good to excellent yields under mild conditions without significant over-oxidation to produce the corresponding carboxylic acids. Keywords Cyclotriveratrylene · Cerium(IV) ammonium nitrate (CAN) Yoshimasa Makita1*, Tomoyuki Fujita2, Tomofumi Danno2, Masashi Inoue2, Michio Ueshima2, Shin-ichi Fujiwara1, Akiya Ogawa2 Department of Chemistry, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata, Osaka 573-1121, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan © Versita Sp. z o.o. Received 30 June 2012 Accepted 25 July 2012 1. Introduction A considerable amount of scientific effort has been devoted to understanding and mimicking metalloenzymes, and it is well-established that the cavities around their active sites play important roles in their reactivity and selectivity [1-3]. Therefore, discoveries of new classes of synthetic transition metal complexes containing well-defined cavities that are analogous to the active sites of metalloenzymes have been of great interest for improving the reactivity and selectivity of bulk phase reactions [4-14]. Hemicryptophanes are covalent molecular capsules constructed from a cyclotriveratrylene (CTV) host unit and they are responsible for dissymmetry at the molecular cavity level [15-29]. We recently synthesized hemicryptophane 1 (Figure 1), which contains a tris(2-aminoethyl)amine (tren) ligand and three rigid phenyl spacers [30-33]. We also synthesized its zinc(II) complex, which contained the zinc(II) cation and the ligand exchange site within the cavity, and this complex efficiently catalyzed the hydrolysis of alkyl para-nitrophenyl carbonates [30,33]. Here, we report a highly active hemicryptophaneruthenium complex that catalyzes the selective oxidation of primary alcohols, including benzylic and aliphatic alcohols, to the corresponding aldehydes. Results of the comparative study of the catalytic activity of the ruthenium-hemicryptophane complex and that of ruthenium complexes of other simple trentype ligands are also disclosed. 2. Experimental Procedure Hemicryptophane 1 [30], tris[2-(benzylamino)ethyl]amine 3 [30], and [RuCl2(DMSO)4] [34] were synthesized according to literature methods. The other chemicals used in this study were reagent grade and used without further purification. 2.1 General method for the oxidation of primary alcohols A solution of ligand (5.0 mol) and RuCl2(DMSO)4 (2.6 mg, 5.0 mol) in acetonitrile (1.0 mL) and water (0.3 mL) was prepared at ambient temperature. Then, primary alcohol (100 mol) and Ce(NH4)2(NO3)6 (CAN, 219.3 mg, 400 mol) were added to the solution. The reaction mixture was stirred vigorously for 20 min at ambient temperature. The yields of the aldehyde or carboxylic acid products were analyzed by gas chromatography or 1H NMR experiments. Figure 1. Structure of hemicryptophane 1 and tren-based ligands 2­4. H NMR (CDCl3, 400 MHz): 2.08 (s, 3H), 2.85­3.07 (m, 12H), 3.48 (d, 3H, J = 6.0 Hz), 3.56 (s, 3H), 3.92 (br s, 6H), 4.69 (d, 3H, J = 6.0 Hz), 7.19 (s, 12H), 7.28 (s, 3H), 7.54 (s, 3H); FAB-MS: calcd. for C51H54Cl1N4O6Ru1: 955.2793 ([M ­Cl]+); found: 955.2841 ([M ­Cl]+). RuCl21. * E-mail: makita@cc.osaka.dent.ac.jp Y. Makita et al. 3. Results and Discussion We previously reported the synthesis and structural characterization of hemicryptophane 1. The oxidation precatalyst complex (RuCl2)·1 was obtained from the reaction of 1 with one equivalent of RuCl2(DMSO)4 in acetonitrile. Formation of the (RuCl2)·1 complex was confirmed by 1H NMR and FABMS. Precatalyst (RuCl2)·1 was immediately formed during the reaction. Therefore, we employed (RuCl2)·1 in the following oxidation reactions without pre-preparing the catalyst complex. In preliminary studies, we examined the optimization of the oxidation of 1-octanol in the presence of RuCl2(DMSO)4, tris[2-(benzylamino)ethyl]amine 3, and cerium(IV) ammonium nitrate (Table 1). No reaction occurred when DMSO was used as the solvent (entry 1), but some of the 1-octanal product was obtained in toluene (entry 2). In chloroform, the oxidation reaction proceeded quantitatively to afford 1-octanal and 1-octanoic acid. In acetonitrile, 1-octanoic acid was the main product and was obtained in moderate yield. Notably, the time required for completion of the oxidation reactions was only 20 min. Furthermore, in the reaction without water as a co-solvent, no oxidation occurred. Thus, water is necessary as the co-solvent in this reaction, probably because one-electron oxidants such as CAN might serve to generate the cis-dioxo adduct. The formed ruthenium complexes were soluble in acetonitrile/water (1 mL: 0.3 mL). Next, we studied the reaction with different ligands under the optimized conditions. The oxidation of 1-octanol in the presence of ligands 2 or 4 proceeded in moderate yields (entries 5 and 6, respectively). However, the reaction employing hemicryptophane ligand 1 under the optimized conditions resulted in selective formation of 1-octanal in 90%. No over-oxidation to yield 1-octanoic acid was observed in the presence of excess water. To further compare the reactivity of hemicryptophane 1 and other simple tren ligands, we investigated the effect of reaction time on the distribution of reaction products. With ligand 3, 73% of the aldehyde and 27% of the unreacted alcohol were obtained Table 2. Ruthenium-catalyzed oxidation reactions.a Entry 1 2 3 4 5 6 after 3 min. The yield of the aldehyde decreased to 47% and the yield of the carboxylic acid increased to 37% after 4 min. When the reaction time was extended to 20 min, the amount of aldehyde decreased and the amount of carboxylic acid increased. On the other hand, with hemicryptophane 1, the aldehyde was obtained in a yield of 91% in 3 min. When the reaction time was extended to 60 min, no 1-octanoic acid was observed. These results show that the hemicryptophane ligand facilitated alcohol oxidation to yield the aldehyde and suppressed its over-oxidation to the carboxylic acid. Next, the oxidation of a variety of alcohols using (RuCl2)·1 was evaluated (Table 2). The oxidation of benzyl alcohol with ligand 3 took place quantitatively and benzoic acid was obtained in 48% yield. Conversely, with ligand 1, while oxidation of benzyl alcohol Table 1. Optimization of the oxidation of 1-octanol using ruthenium complexes. R uC l 2 (D M S O ) 4 (5 m ol% ), ligand (5 m ol% ), C A N (4 eqiv.) solven t, H 2 O , rt, 20 m in 1-octan ol 5 1 -octana l + 1 -o ctanoic acid 7 6 Entry Ligand Solvent Yield (%)a 6 7 0 0 48 82 78 75 0 0 Conversion (%)a Determined by GC. DMSO toluene chloroform MeCN MeCN MeCN MeCN MeCN 0 62 >99 >99 >99 >99 >99 >99 Substrate 1-octanol benzyl alcohol 4-methoxybenzyl alcohol 4-bromobenzyl alcohol 4-nitrobenzyl alcohol 2-dodecanol Ligand 3 1 3 1 3 1 3 1 3 1 3 1 Product 1-octanal benzaldehyde 4-methoxybenzaldehyde 4-bromobenzaldehyde 4-nitrobenzaldehyde 2-dodecanone Yield (%) 18b 90b 52c 91c 55c 94c 82c 94c 27c 88c 68c 26c Product 1-octanoic acid benzoic acid 4-methoxybenzoic acid 4-bromobenzoic acid 4-nitrobenzoic acid Yield (%)b 82b 0b 48c 9c 41c 6c 9c 6c 10c 12c Conditions: reactions were performed on a 0.10 mmol scale with 5 mol % RuCl2(DMSO)4, 5 mol % ligand, and 4 equiv. Ce(NH4)2(NO3)6 in 1:0.3 CH3CN/H2O. Determined by GC. c Determined by 1H NMR. was still quantitative, benzaldehyde was obtained in 91% yield with only a minor amount of benzoic acid (entry 2). The reactivity of 4-methoxybenzyl alcohol was the same as benzyl alcohol (entry 2). In contrast, 4-bromobenzaldehyde was selectively formed with both 1 and 3 (entry 4). The oxidation of 4-nitrobenzyl alcohol occurred efficiently with ligand 1, but oxidation with ligand 3 was less efficient (entry 5). However, the oxidation product of secondary alcohol 2-dodecanol was obtained in 68% yield with ligand 3, but was obtained in only 26% yield with ligand 1. These results indicate that the capped structure of the hemicryptophane ligand highly affects the reactivity and selectivity of the catalytically active ruthenium center. 4. Conclusions We have developed a method for the selective oxidation of primary alcohols to the corresponding aldehydes using a ruthenium(II)-hemicryptophane complex as a precatalyst. The hemicryptophane ligand is more effective than other simple tren-based ligands and the success of this process is attributed to the capped structure of the hemicryptophane framework. Continued efforts to advance methods for catalytic oxidation using the capped structures of hemicryptophanes will attempt to further exploit the applications of the artificial active centers of metalloenzyme models.

Journal

Supramolecular Catalysis – de Gruyter

Published: Aug 21, 2012

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