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Cite this: Chem. Commun., 2014, 50, 15251 Received 18th September 2014, Accepted 20th October 2014 DOI: 10.1039/c4cc07951e
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Co-catalytic oxidative coupling of primary amines to imines using an organic nanotube–gold nanohybrid† Dhanaji V. Jawale,a Edmond Gravel,*a Elise Villemin,a Nimesh Shah,b c b a Vale ´rie Geertsen, Irishi N. N. Namboothiri* and Eric Doris*
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A novel nanohybrid structure was synthesized by assembling gold nanoparticles on polymerized polydiacetylene nanotubes. Combination of the nanohybrid with gallacetophenone afforded an efficient cooperative co-catalytic system for the oxidative coupling of primary amines into imines. The system is highly efficient and sustainable as it operates in high yields using minimal amounts of the metal and the quinone, under ambient atmosphere, at room temperature, in water, and is easily recycled.
Imines are versatile intermediates for the synthesis of fine chemicals and are classically produced by condensation of carbonyl compounds with amines under dehydrating conditions. Other strategies include the oxidative coupling of amines with alcohols, the direct oxidation of secondary amines, and the in situ oxidation/condensation of primary amines.1 The latter process has recently emerged as a promising alternative to conventional methods,2 and some metalbased3–6 and metal-free7,8 approaches have been devised. Further developments have also led to the recent discovery of co-catalytic systems based on metalloenzyme mimics made of a redox-active co-factor and a metal centre.9 These cooperative biomimetic schemes are based on the use of a quinone-type co-catalyst together with Cu salts10 or Pt/Ir nanoclusters.11 Although these approaches are quite efficient for the conversion of primary (and secondary) amines to imines, some challenges still remain to be tackled such as, for example, the development of a bioinspired co-catalytic system that would operate at room temperature, under ambient air (no added O2), and that could be easily recycled. With these prerequisites in mind and as part of our long standing interest in heterogeneous gold-catalyzed reactions,12–14 we developed a novel nanohybrid made of gold nanoparticles
on organic nanotubes (AuONT). The latter was used as co-catalyst in the aerobic oxidative coupling of aliphatic and aromatic amines. Since gold nanoparticles have previously been shown to catalyze the oxidation of hydroquinones,13 we conceived that the metallic species could be applied to the catalytic oxidation of amines to imines in the presence of a redox-active quinone co-catalyst. This sequence mimics the process that takes place in metallo-enzymes: the quinone unit would catalyze the imine formation per se and gold particles would promote the in situ re-oxidation of the released hydroquinone, thus completing the catalytic cycle (Scheme 1). In addition, the use of a supra-molecular organic platform for the stabilizing and recycling of the noble metallic species is expected to lead to an overall improvement of the cooperative catalytic process. The AuONT assembly was prepared by a layer-by-layer approach. Preformed gold nanoparticles (AuNPs) were anchored on the surface of organic nanotubes (ONT) via the interfacing of a polycationic layer. First, the organic nanotube platform was produced from the assembly and photo-polymerization of diacetylene (DA) nitrilotriacetate (NTA) amphiphiles DANTA (Fig. 1). Based on the protonation state of the NTA polar head, different aggregates can be obtained such as spherical micelles (under alkaline conditions), or bilayer assemblies (at neutral/acidic pH).15 Here, a concentrated solution of DANTA in MeOH/EtOH 1 : 1 was added dropwise to cold water (pH 6.5). After stirring for a few minutes, the clear mixture became turbid, indicating the formation of supra-molecular nanostructures. DANTA self-assembled into bilayers with polyacid head-groups oriented towards the aqueous medium and diacetylene-containing tails densely packed into an inner hydrophobic domain.16
a
CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage, 91191 Gif-sur-Yvette, France. E-mail: [email protected], [email protected] b Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India. E-mail: [email protected] c CEA, IRAMIS, Nanosciences et Innovation pour les Mate´riaux, la Biome´decine et l’Energie, UMR3299, 91191 Gif-sur-Yvette, France † Electronic supplementary information (ESI) available: Experimental details and spectral data for all compounds. See DOI: 10.1039/c4cc07951e
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Scheme 1
Overview of the catalytic process.
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Fig. 1 (a) Structure of DANTA amphiphile; (b) structure of PDADMAC; (c) proposed mechanism for the polymerization of diacetylenes upon UV irradiation.
These bilayers slowly rearranged into twisted ribbons (Fig. 2a and b) and helices (Fig. 2c and d), and further torsion eventually led to well-defined nanotubular structures (Fig. 2e) of very narrow diameter distribution centred on ca. 45 nm (Fig. 2f). The organic nanotubes were subsequently photo-polymerized at 254 nm to reinforce cohesion of the supra-molecular assemblies. UV irradiation produced an ene–yne conjugated network (Fig. 1c) that connected the amphiphilic units together. At this point, stable ONT displaying carboxylate groups at their surface were obtained. In a second step, the nanotubes were coated with a cationic polymer, namely poly(diallyldimethylammonium chloride) (PDADMAC, Fig. 1b), which was immobilized on the anionic surface by electrostatic interactions. Preformed gold nanoparticles17 were finally anchored to the tubular platform by interaction with the polyammonium network to deliver the nanohybrid co-catalyst (Fig. 2g). Transmission electron microscopy (TEM) analysis indicated dense and homogeneous covering of the organic nanotube surface by AuNPs of regular shape and size (ca. 3 nm, Fig. 2h). Metal content of the nanohybrid aqueous suspension was assessed by ICP-MS which indicated a gold concentration of 0.5 mM. The Au0-oxidation state of gold was confirmed by X-ray photoelectron spectrometry (XPS). With the AuONT nanohybrid in hand, we next investigated its potential in the aerobic oxidative coupling of primary amines in the presence of a hydroquinone co-catalyst. Benzylamine (1a) was selected as a model substrate and the reaction conditions were set working with 3 mol% of the hydroquinone derivative and 0.1 mol% of the AuONT nanohybrid in water, under air, and at room temperature for 24 h (see ESI,† Table S1). Among the different quinone precursors that were tested as co-catalysts, dihydroxybenzenes like pyrocatechol (Table S1, entry 1, ESI†) or acetylhydroquinone (Table S1, entry 2, ESI†) only provided low conversions of 15 and 17%, respectively. The efficiency of the process could be improved with the use of pyrogallol derivatives such as gallic acid (38%, Table S1, entry 3, ESI†), propyl gallate (40%, Table S1, entry 4, ESI†), and finally gallacetophenone (20 ,3 0 ,4 0 -trihydroxyacetophenone, THAP) which promoted nearly full conversion of 1a into the corresponding
15252 | Chem. Commun., 2014, 50, 15251--15254
Fig. 2 Illustration (left) of the different shapes of DANTA bilayers and corresponding TEM pictures (right): twisted ribbons (a and b), helix (c and d), organic nanotube (e and f), and AuONT hybrid (g and h).
imine 2a (Table S1, entry 5, ESI†). Superiority of the THAP system can be rationalized by hydrogen-bond stabilization of the Schiff base transition state (Scheme S1, ESI†). Kinetic parameters were calculated by running the same reaction in the presence of 0.01 mol% of gold to give a turn-over number (TON) of 2900 and a turn-over frequency (TOF) of 121 h1, taking the total content of gold into account (see ESI†). When only gold surface atoms (those in contact with the medium and active for catalysis) were considered,18 TON and TOF reached values of 9667 and 402 h1, respectively. Gallacetophenone (THAP) was thus selected as co-catalyst, and the scope and limitations of the newly developed process were studied on a variety of amines (Table 1). In addition to 1a (entry 1), variously substituted benzylamines (1b–f) with either electrondonating or withdrawing groups, were tested (entries 2–6).
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Table 1
Entry
Communication
Formation of symmetrical iminesa
Amine 1
Imine 2
Yieldb (%)
1
95
2
98
3
98
4
97
5
95
6
98
7
87
8
33c
9
—
10e
NRd 90
a Conditions: 1 (0.1 mmol), AuONT (200 mL of a 0.5 mM suspension in H2O, 0.1 mol%), THAP (0.003 mmol, 3 mol%), H2O (1 mL), room temp., air, 24 h. b Yield of isolated product. c 1H NMR yield. d No reaction. e Reaction run in MeOH (1 mL).
These substrates were all converted into the corresponding symmetrical imines within 24 h, in excellent yields (95–98%). Moreover, under the same conditions, furfurylamine 1g readily reacted to afford imine 2g in 87% yield (entry 7). In contrast, a-substituted phenylethylamine 1h could only be partially converted (33%) after 24 h of reaction (entry 8), and secondary N-methylbenzylamine 1i failed to react (entry 9). To assess the reactivity of aliphatic amines with our catalytic system, cyclohexylmethylamine 1j was tested which underwent conversion into 2j in 90% yield (entry 10). It is to be noted that in the latter case, the aqueous solvent had to be replaced by methanol which had a beneficial effect on the isolated yield of product. The reactivity of the AuONT hybrid on the oxidation of benzylamine was compared to that of other co-catalysts, in association with THAP. The first control experiment was conducted using THAP alone (see ESI,† Table S2, entry 1) but no conversion was detected. Neither the combination of THAP with gold salts (HAuCl4, Table S2, entry 2, ESI†) nor non-supported gold nanoparticles (Table S2, entry 3, ESI†) provided satisfactory conversions, as the former was totally unreactive while the latter only led to 15% of the condensed imine, after 24 h. These results highlight
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the superior activity of the AuONT nanohybrid system (95% yield, Table S2, entry 4, ESI†). In addition, THAP in combination with the ONT/PDADMAC assembly (without AuNPs) also proved unsuccessful (Table S2, entry 5, ESI†). Recyclability of the heterogeneous catalyst was also assessed by performing five consecutive benzylamine oxidations. After each cycle, the AuONT co-catalyst was recovered by centrifugation and reused without any loss of catalytic activity, as constant yields of 95% were obtained throughout the sequential runs. In addition, TEM analysis of the recycled catalyst showed no sintering of the gold nanoparticles. A postulated mechanism for the co-catalytic AuONT/THAP-mediated oxidative dimerization is illustrated in Scheme 2. The first step involves the aerobic oxidation of THAP by the AuONT nanohybrid to afford the active ‘‘quinone’’ co-catalyst a. In the first catalytic cycle, the initial condensation of amine 1 leads to the formation of imine b with concomitant elimination of a molecule of water (X = O). Rearomatization through proton transfer then affords isomeric imine c which, upon addition of a second equivalent of 1, yields aminal intermediate d. Further evolution of the transient intermediate d, by elimination of acetylaminoresorcinol e, finally releases the expected imine 2. In the second catalytic cycle, the aerobic oxidation of e by AuONT produces iminoquinone a 0 (X = NH) which is likely also active in catalyzing imine formation. This mechanism involving the intermediacy of e is supported by the fact that ammonia could be detected in the course of the reaction using potassium tetraiodomercurate (Nessler’s reagent). In addition, aminoacetylresorcinol e could be isolated after completion of the reaction, whereas no remaining THAP was detected. The co-catalytic synthesis of unsymmetrical imines using the THAP/AuONT system was also studied by reacting benzylamine (1a) with less readily oxidized amines 3, under the above conditions (Table 2). The ratio of imine products was assessed by 1H-NMR analysis of the crude mixture after total consumption of starting 1a (ca. 24 h). The use of 1 equivalent of p-anisidine 3a afforded
Scheme 2 Proposed mechanism for the AuONT–gallacetophenone co-catalyzed formation of imines.
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Table 2
Entry
ChemComm
Formation of unsymmetrical iminesa
Amine 3
Imine 4
1
4/2ab 40 : 60c 55 : 45d
Support from the Indo-French Centre for the Promotion of Advanced Research (IFCPAR)/Centre Franco-Indien pour la ´e (CEFIPRA) is gratefully Promotion de la Recherche Avance acknowledged (Project no. 4705-1). The TEM-team platform (CEA, iBiTec-S) is acknowledged for help with TEM images. The ‘‘Service de Chimie Bioorganique et de Marquage’’ belongs to the Laboratory of Excellence in Research on Medication and Innovative Therapeutics (ANR-10-LABX-0033-LERMIT).
Notes and references 2
80 : 20
3
90 : 10
1 2 3 4 5 6
4
499 : 1
a
Conditions: 1a (0.1 mmol), 3 (0.2 mmol), AuONT (200 mL of a 0.5 mM suspension in MeOH, 0.1 mol%), THAP (3 mol%), MeOH (1 mL), room temp., air. b Determined by 1H-NMR after full consumption of 1a. c Reaction run with 1 equiv. of p-anisidine. d Reaction run with 3 equiv. of p-anisidine.
7 8 9 10 11 12
the classical imine 2a along with the expected heterocoupling product 4a in a 40 : 60 ratio. Increasing the amount of 3a to 3 equivalents permitted to improve the formation of 4a as it became the major product (4a/2a 55 : 45) (entry 1). a-Methylbenzylamine reacted more efficiently than p-anisidine since ca. 80% of the wanted product 4b was formed by using only 2 equivalents of 3b. The best results were obtained with the more nucleophilic cyclohexylamine 3c and 6-aminohexanol 3d. In these cases, cross-coupled imines 4c and 4d were the major products with ratios of 90 : 10 and 499 : 1, respectively. In summary, well-calibrated organic nanotubes were obtained from the self-assembly and polymerization of diacetyleneamphiphiles. These nanotubes served as support for gold nanoparticles that were anchored via a polyammonium layer. The nanohybrid was used as a heterogeneous co-catalyst for the oxidative coupling of primary amines in the presence of gallacetophenone. The system operates with minimal amounts of metal catalyst, under air, at room temperature, in green solvents,19 and can be readily recycled.
15254 | Chem. Commun., 2014, 50, 15251--15254
13 14 15 16 17 18
19
R. D. Patil and S. Adimurthy, Asian J. Org. Chem., 2013, 2, 726. M. Largeron, Eur. J. Org. Chem., 2013, 5225. E. Zhang, H. Tian, S. Xu, X. Yu and Q. Xu, Org. Lett., 2013, 15, 2704. R. D. Patil and S. Adimurthy, Adv. Synth. Catal., 2011, 353, 1695. X. Jin, Y. Liu, Q. Lu, D. Yang, J. Sun, S. Qin, J. Zhang, J. Shen, C. Chu and R. Liu, Org. Biomol. Chem., 2013, 11, 3776. N. Li, X. Lang, W. Ma, H. Ji, C. Chen and J. Zhao, Chem. Commun., 2013, 49, 5034. H. Huang, J. Huang, Y.-M. Liu, H.-Y. He, Y. Cao and K.-N. Fan, Green Chem., 2012, 14, 930. A. E. Wendlandt and S. S. Stahl, Org. Lett., 2012, 14, 2850. M. Largeron and M.-B. Fleury, Science, 2013, 339, 43. M. Largeron and M.-B. Fleury, Angew. Chem., Int. Ed., 2012, 51, 5409. H. Yuan, W.-J. Yoo, H. Miyamura and S. Kobayashi, J. Am. Chem. Soc., 2012, 134, 13970. `ge, H. Li, T. Gacoin and E. Doris, Angew. (a) J. John, E. Gravel, A. Hage Chem., Int. Ed., 2011, 50, 7533; (b) J. John, E. Gravel, I. N. N. Namboothiri and E. Doris, Nanotech. Rev., 2012, 1, 515; (c) R. Kumar, E. Gravel, `ge, H. Li, D. V. Jawale, D. Verma, I. N. N. Namboothiri and A. Hage `ge, E. Doris, Nanoscale, 2013, 5, 6491; (d) R. Kumar, E. Gravel, A. Hage H. Li, D. Verma, I. N. N. Namboothiri and E. Doris, ChemCatChem, 2013, 5, 3571; (e) D. V. Jawale, E. Gravel, V. Geertsen, H. Li, N. Shah, R. Kumar, J. John, I. N. N. Namboothiri and E. Doris, Tetrahedron, 2014, 70, 6140; ( f ) N. Shah, E. Gravel, D. V. Jawale, E. Doris and I. N. N. Namboothiri, ChemCatChem, 2014, 6, 2201; (g) N. Shah, E. Gravel, D. V. Jawale, E. Doris and I. N. N. Namboothiri, ChemCatChem, 2014, DOI: 10.1002/ cctc.201402782; (h) E. Gravel, D. Bernard, I. N. N. Namboothiri and E. Doris, Actual. Chim., 2014, in press. D. V. Jawale, E. Gravel, V. Geertsen, H. Li, N. Shah, I. N. N. Namboothiri and E. Doris, ChemCatChem, 2014, 6, 719. J. Mielby, S. Kegnæs and P. Fristrup, ChemCatChem, 2012, 4, 1037. O. Yarimaga, J. Jaworski, B. Yoon and J.-M. Kim, Chem. Commun., 2012, 48, 2469. T. Shimizu, M. Masuda and H. Minamikawa, Chem. Rev., 2005, 105, 1401. D. G. Duff, A. Baiker and P. P. Edwards, Langmuir, 1993, 9, 2301. ´ga-Mariadassou, Kinetics of Heterogeneous (a) M. Boudart and G. Dje Catalytic Reactions, Princeton University Press, Princeton, NJ, 1984, p. 26; (b) S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013, 15, 17. ¨hler, Green Chem., 2007, C. Capello, U. Fischer and K. Hungerbu 9, 927.
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Cite this: Chem. Commun., 2014, 50, 15251 Received 18th September 2014, Accepted 20th October 2014 DOI: 10.1039/c4cc07951e
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Co-catalytic oxidative coupling of primary amines to imines using an organic nanotube–gold nanohybrid† Dhanaji V. Jawale,a Edmond Gravel,*a Elise Villemin,a Nimesh Shah,b c b a Vale ´rie Geertsen, Irishi N. N. Namboothiri* and Eric Doris*
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A novel nanohybrid structure was synthesized by assembling gold nanoparticles on polymerized polydiacetylene nanotubes. Combination of the nanohybrid with gallacetophenone afforded an efficient cooperative co-catalytic system for the oxidative coupling of primary amines into imines. The system is highly efficient and sustainable as it operates in high yields using minimal amounts of the metal and the quinone, under ambient atmosphere, at room temperature, in water, and is easily recycled.
Imines are versatile intermediates for the synthesis of fine chemicals and are classically produced by condensation of carbonyl compounds with amines under dehydrating conditions. Other strategies include the oxidative coupling of amines with alcohols, the direct oxidation of secondary amines, and the in situ oxidation/condensation of primary amines.1 The latter process has recently emerged as a promising alternative to conventional methods,2 and some metalbased3–6 and metal-free7,8 approaches have been devised. Further developments have also led to the recent discovery of co-catalytic systems based on metalloenzyme mimics made of a redox-active co-factor and a metal centre.9 These cooperative biomimetic schemes are based on the use of a quinone-type co-catalyst together with Cu salts10 or Pt/Ir nanoclusters.11 Although these approaches are quite efficient for the conversion of primary (and secondary) amines to imines, some challenges still remain to be tackled such as, for example, the development of a bioinspired co-catalytic system that would operate at room temperature, under ambient air (no added O2), and that could be easily recycled. With these prerequisites in mind and as part of our long standing interest in heterogeneous gold-catalyzed reactions,12–14 we developed a novel nanohybrid made of gold nanoparticles
on organic nanotubes (AuONT). The latter was used as co-catalyst in the aerobic oxidative coupling of aliphatic and aromatic amines. Since gold nanoparticles have previously been shown to catalyze the oxidation of hydroquinones,13 we conceived that the metallic species could be applied to the catalytic oxidation of amines to imines in the presence of a redox-active quinone co-catalyst. This sequence mimics the process that takes place in metallo-enzymes: the quinone unit would catalyze the imine formation per se and gold particles would promote the in situ re-oxidation of the released hydroquinone, thus completing the catalytic cycle (Scheme 1). In addition, the use of a supra-molecular organic platform for the stabilizing and recycling of the noble metallic species is expected to lead to an overall improvement of the cooperative catalytic process. The AuONT assembly was prepared by a layer-by-layer approach. Preformed gold nanoparticles (AuNPs) were anchored on the surface of organic nanotubes (ONT) via the interfacing of a polycationic layer. First, the organic nanotube platform was produced from the assembly and photo-polymerization of diacetylene (DA) nitrilotriacetate (NTA) amphiphiles DANTA (Fig. 1). Based on the protonation state of the NTA polar head, different aggregates can be obtained such as spherical micelles (under alkaline conditions), or bilayer assemblies (at neutral/acidic pH).15 Here, a concentrated solution of DANTA in MeOH/EtOH 1 : 1 was added dropwise to cold water (pH 6.5). After stirring for a few minutes, the clear mixture became turbid, indicating the formation of supra-molecular nanostructures. DANTA self-assembled into bilayers with polyacid head-groups oriented towards the aqueous medium and diacetylene-containing tails densely packed into an inner hydrophobic domain.16
a
CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage, 91191 Gif-sur-Yvette, France. E-mail: [email protected], [email protected] b Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India. E-mail: [email protected] c CEA, IRAMIS, Nanosciences et Innovation pour les Mate´riaux, la Biome´decine et l’Energie, UMR3299, 91191 Gif-sur-Yvette, France † Electronic supplementary information (ESI) available: Experimental details and spectral data for all compounds. See DOI: 10.1039/c4cc07951e
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Scheme 1
Overview of the catalytic process.
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Fig. 1 (a) Structure of DANTA amphiphile; (b) structure of PDADMAC; (c) proposed mechanism for the polymerization of diacetylenes upon UV irradiation.
These bilayers slowly rearranged into twisted ribbons (Fig. 2a and b) and helices (Fig. 2c and d), and further torsion eventually led to well-defined nanotubular structures (Fig. 2e) of very narrow diameter distribution centred on ca. 45 nm (Fig. 2f). The organic nanotubes were subsequently photo-polymerized at 254 nm to reinforce cohesion of the supra-molecular assemblies. UV irradiation produced an ene–yne conjugated network (Fig. 1c) that connected the amphiphilic units together. At this point, stable ONT displaying carboxylate groups at their surface were obtained. In a second step, the nanotubes were coated with a cationic polymer, namely poly(diallyldimethylammonium chloride) (PDADMAC, Fig. 1b), which was immobilized on the anionic surface by electrostatic interactions. Preformed gold nanoparticles17 were finally anchored to the tubular platform by interaction with the polyammonium network to deliver the nanohybrid co-catalyst (Fig. 2g). Transmission electron microscopy (TEM) analysis indicated dense and homogeneous covering of the organic nanotube surface by AuNPs of regular shape and size (ca. 3 nm, Fig. 2h). Metal content of the nanohybrid aqueous suspension was assessed by ICP-MS which indicated a gold concentration of 0.5 mM. The Au0-oxidation state of gold was confirmed by X-ray photoelectron spectrometry (XPS). With the AuONT nanohybrid in hand, we next investigated its potential in the aerobic oxidative coupling of primary amines in the presence of a hydroquinone co-catalyst. Benzylamine (1a) was selected as a model substrate and the reaction conditions were set working with 3 mol% of the hydroquinone derivative and 0.1 mol% of the AuONT nanohybrid in water, under air, and at room temperature for 24 h (see ESI,† Table S1). Among the different quinone precursors that were tested as co-catalysts, dihydroxybenzenes like pyrocatechol (Table S1, entry 1, ESI†) or acetylhydroquinone (Table S1, entry 2, ESI†) only provided low conversions of 15 and 17%, respectively. The efficiency of the process could be improved with the use of pyrogallol derivatives such as gallic acid (38%, Table S1, entry 3, ESI†), propyl gallate (40%, Table S1, entry 4, ESI†), and finally gallacetophenone (20 ,3 0 ,4 0 -trihydroxyacetophenone, THAP) which promoted nearly full conversion of 1a into the corresponding
15252 | Chem. Commun., 2014, 50, 15251--15254
Fig. 2 Illustration (left) of the different shapes of DANTA bilayers and corresponding TEM pictures (right): twisted ribbons (a and b), helix (c and d), organic nanotube (e and f), and AuONT hybrid (g and h).
imine 2a (Table S1, entry 5, ESI†). Superiority of the THAP system can be rationalized by hydrogen-bond stabilization of the Schiff base transition state (Scheme S1, ESI†). Kinetic parameters were calculated by running the same reaction in the presence of 0.01 mol% of gold to give a turn-over number (TON) of 2900 and a turn-over frequency (TOF) of 121 h1, taking the total content of gold into account (see ESI†). When only gold surface atoms (those in contact with the medium and active for catalysis) were considered,18 TON and TOF reached values of 9667 and 402 h1, respectively. Gallacetophenone (THAP) was thus selected as co-catalyst, and the scope and limitations of the newly developed process were studied on a variety of amines (Table 1). In addition to 1a (entry 1), variously substituted benzylamines (1b–f) with either electrondonating or withdrawing groups, were tested (entries 2–6).
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Table 1
Entry
Communication
Formation of symmetrical iminesa
Amine 1
Imine 2
Yieldb (%)
1
95
2
98
3
98
4
97
5
95
6
98
7
87
8
33c
9
—
10e
NRd 90
a Conditions: 1 (0.1 mmol), AuONT (200 mL of a 0.5 mM suspension in H2O, 0.1 mol%), THAP (0.003 mmol, 3 mol%), H2O (1 mL), room temp., air, 24 h. b Yield of isolated product. c 1H NMR yield. d No reaction. e Reaction run in MeOH (1 mL).
These substrates were all converted into the corresponding symmetrical imines within 24 h, in excellent yields (95–98%). Moreover, under the same conditions, furfurylamine 1g readily reacted to afford imine 2g in 87% yield (entry 7). In contrast, a-substituted phenylethylamine 1h could only be partially converted (33%) after 24 h of reaction (entry 8), and secondary N-methylbenzylamine 1i failed to react (entry 9). To assess the reactivity of aliphatic amines with our catalytic system, cyclohexylmethylamine 1j was tested which underwent conversion into 2j in 90% yield (entry 10). It is to be noted that in the latter case, the aqueous solvent had to be replaced by methanol which had a beneficial effect on the isolated yield of product. The reactivity of the AuONT hybrid on the oxidation of benzylamine was compared to that of other co-catalysts, in association with THAP. The first control experiment was conducted using THAP alone (see ESI,† Table S2, entry 1) but no conversion was detected. Neither the combination of THAP with gold salts (HAuCl4, Table S2, entry 2, ESI†) nor non-supported gold nanoparticles (Table S2, entry 3, ESI†) provided satisfactory conversions, as the former was totally unreactive while the latter only led to 15% of the condensed imine, after 24 h. These results highlight
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the superior activity of the AuONT nanohybrid system (95% yield, Table S2, entry 4, ESI†). In addition, THAP in combination with the ONT/PDADMAC assembly (without AuNPs) also proved unsuccessful (Table S2, entry 5, ESI†). Recyclability of the heterogeneous catalyst was also assessed by performing five consecutive benzylamine oxidations. After each cycle, the AuONT co-catalyst was recovered by centrifugation and reused without any loss of catalytic activity, as constant yields of 95% were obtained throughout the sequential runs. In addition, TEM analysis of the recycled catalyst showed no sintering of the gold nanoparticles. A postulated mechanism for the co-catalytic AuONT/THAP-mediated oxidative dimerization is illustrated in Scheme 2. The first step involves the aerobic oxidation of THAP by the AuONT nanohybrid to afford the active ‘‘quinone’’ co-catalyst a. In the first catalytic cycle, the initial condensation of amine 1 leads to the formation of imine b with concomitant elimination of a molecule of water (X = O). Rearomatization through proton transfer then affords isomeric imine c which, upon addition of a second equivalent of 1, yields aminal intermediate d. Further evolution of the transient intermediate d, by elimination of acetylaminoresorcinol e, finally releases the expected imine 2. In the second catalytic cycle, the aerobic oxidation of e by AuONT produces iminoquinone a 0 (X = NH) which is likely also active in catalyzing imine formation. This mechanism involving the intermediacy of e is supported by the fact that ammonia could be detected in the course of the reaction using potassium tetraiodomercurate (Nessler’s reagent). In addition, aminoacetylresorcinol e could be isolated after completion of the reaction, whereas no remaining THAP was detected. The co-catalytic synthesis of unsymmetrical imines using the THAP/AuONT system was also studied by reacting benzylamine (1a) with less readily oxidized amines 3, under the above conditions (Table 2). The ratio of imine products was assessed by 1H-NMR analysis of the crude mixture after total consumption of starting 1a (ca. 24 h). The use of 1 equivalent of p-anisidine 3a afforded
Scheme 2 Proposed mechanism for the AuONT–gallacetophenone co-catalyzed formation of imines.
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Table 2
Entry
ChemComm
Formation of unsymmetrical iminesa
Amine 3
Imine 4
1
4/2ab 40 : 60c 55 : 45d
Support from the Indo-French Centre for the Promotion of Advanced Research (IFCPAR)/Centre Franco-Indien pour la ´e (CEFIPRA) is gratefully Promotion de la Recherche Avance acknowledged (Project no. 4705-1). The TEM-team platform (CEA, iBiTec-S) is acknowledged for help with TEM images. The ‘‘Service de Chimie Bioorganique et de Marquage’’ belongs to the Laboratory of Excellence in Research on Medication and Innovative Therapeutics (ANR-10-LABX-0033-LERMIT).
Notes and references 2
80 : 20
3
90 : 10
1 2 3 4 5 6
4
499 : 1
a
Conditions: 1a (0.1 mmol), 3 (0.2 mmol), AuONT (200 mL of a 0.5 mM suspension in MeOH, 0.1 mol%), THAP (3 mol%), MeOH (1 mL), room temp., air. b Determined by 1H-NMR after full consumption of 1a. c Reaction run with 1 equiv. of p-anisidine. d Reaction run with 3 equiv. of p-anisidine.
7 8 9 10 11 12
the classical imine 2a along with the expected heterocoupling product 4a in a 40 : 60 ratio. Increasing the amount of 3a to 3 equivalents permitted to improve the formation of 4a as it became the major product (4a/2a 55 : 45) (entry 1). a-Methylbenzylamine reacted more efficiently than p-anisidine since ca. 80% of the wanted product 4b was formed by using only 2 equivalents of 3b. The best results were obtained with the more nucleophilic cyclohexylamine 3c and 6-aminohexanol 3d. In these cases, cross-coupled imines 4c and 4d were the major products with ratios of 90 : 10 and 499 : 1, respectively. In summary, well-calibrated organic nanotubes were obtained from the self-assembly and polymerization of diacetyleneamphiphiles. These nanotubes served as support for gold nanoparticles that were anchored via a polyammonium layer. The nanohybrid was used as a heterogeneous co-catalyst for the oxidative coupling of primary amines in the presence of gallacetophenone. The system operates with minimal amounts of metal catalyst, under air, at room temperature, in green solvents,19 and can be readily recycled.
15254 | Chem. Commun., 2014, 50, 15251--15254
13 14 15 16 17 18
19
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