Accepted Manuscript Water Dispersed Gold Nanoparticles Catalyzed Aerobic Oxidative Cross-Dehydrogenative Coupling: An Efficient Synthesis of α-Ketoamides in Water Narasimha Swamy Thirukovela, Ramesh Balaboina, Ravinder Vadde, Chandra Sekhar Vasam PII: DOI: Reference:
S0040-4039(18)31043-8 https://doi.org/10.1016/j.tetlet.2018.08.042 TETL 50217
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Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
14 July 2018 17 August 2018 21 August 2018
Please cite this article as: Thirukovela, N.S., Balaboina, R., Vadde, R., Sekhar Vasam, C., Water Dispersed Gold Nanoparticles Catalyzed Aerobic Oxidative Cross-Dehydrogenative Coupling: An Efficient Synthesis of αKetoamides in Water, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.08.042
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Graphical Abstract
Water Dispersed Gold Nanoparticles Catalyzed Aerobic Oxidative CrossDehydrogenative Coupling: An Efficient Synthesis of α-Ketoamides in Water Narasimha Swamy Thirukovelaa, Ramesh Balaboinaa, Ravinder Vaddea *, Chandra Sekhar Vasamb *
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Tetrahedron Letters j o ur n al h om e p a g e : w w w . e l s e v i er . c o m
Water Dispersed Gold Nanoparticles Catalyzed Aerobic Oxidative CrossDehydrogenative Coupling: An Efficient Synthesis of α-Ketoamides in Water Narasimha Swamy Thirukovela,a Ramesh Balaboina,a Ravinder Vadde,*a Chandra Sekhar Vasam,*b aDepartment
of Chemistry, Kakatiya University, Warangal, Telangana State-506009, India, Tel :+91-9533945588, Email:
[email protected]; b Department of Pharmaceutical Chemistry, Telangana University, Nizamabad, Telangana State, India ;E-mail:
[email protected]
A R T IC LE IN F O
A B S TR A C T
Article history: Received Received in revised form Accepted Available online
An effective green synthesis of α-ketoamides was developed for the first time in water via gold nanoparticles (AuNPs) catalyzed aerobic oxidative cross-dehydrogenative coupling of secondary amines with phenylglyoxals with a broad substrate scope.
Keywords: α-Ketoamides Aerobic oxidative CDC Recyclable water dispersed AuNPs Phenylglyoxals Homogeneous catalysis
α-Ketoamides and their derivatives are valuable building blocks in natural products, drug research1-8 and organic synthesis.9-12 Hence, the optimal synthesis of αketoamides via both classical and catalytic methods has been receiving considerable attention. In classical methods, the condensation reaction between α-ketoacid derivatives with amines is generally used to obtain αketoamide.13 However, in recent years considerable efforts have been made towards catalyzed synthesis of α-ketoamides to implement green chemistry principles. 14 In literature, there are three aerobic oxidative crossdehydrogenative coupling (CDC) of amines and αcarbonyl aldehydes via homogeneous/heterogeneous catalysis have been reported (Scheme 1A-1C) to synthesize α-ketoamides.15-17 In general, aerobic oxidative CDC reactions are one of the referred methods to construct C-C and C-X bonds.18 Nevertheless, the probable disadvantages of reported catalytic aerobic oxidative CDCs those would limit the sustainable
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synthesis of α-ketoamides are : (i) use of toxic organic solvents15-17 (ii) use of excess of secondary amines to couple with phenylglyoxal17 and (iii) use of additives.15 Since the α-ketoamides are naturally occurring molecules, the use of water stable catalytic system in the oxidative CDC reaction to synthesize α-ketoamides would be more environmentally benign and highly desirable objective for sustainable organic synthesis.19 In line of the proposal, the use of coinage metal catalysts (Cu, Ag, Au) appears to be good choice in aqueous phase oxidative CDC because of their (i) high degree of tolerance toward air and moisture, (ii) relative lower toxicity than many of the other d-block metal catalysts and (iii) switchable stable oxidation numbers during catalytic cycle. Besides, metalophilic attractions between coinage metal centers also contribute to their catalytic activity as reported previously.
2
Tetrahedron Letters
Scheme 1. (A), (B) and (C) are previous aerobic oxidative CDC approaches; [15-17] (D) This work. As shown in scheme (1A-1C), previously the oxidative CDC of phenylglyoxals with secondary amines was investigated in organic solvents using homogeneous Cu(I)15 & Au(III)16 ion catalysts and heterogeneous Cu-MOF17 catalysts. Since the majority of known α-ketoamide products of this reaction are oils or gummy solids, we thought that it would be better to investigate the above CDC reaction in water/aqueous phase using homogeneous and heterogeneous coinage metal catalysts. At this juncture, we were intended to explore the catalytic potential of water dispersed gold nanoparticles (AuNPs) in the above CDC reaction to optimize the synthesis of α-ketoamides. It is interesting to note that previously variety of AuNPs including water-dispersed AuNPs were used in oxidative amide bond synthesis by alcohol-amine or aldehyde-amine coupling.20 The concept of AuNPs catalyzed amide synthesis via oxidative coupling also suits well to design AuNPs catalyzed ketoamide bond synthesis via oxidative CDC between α-ketoaldehydes and amines.
Cu(I) and Ag(I) salts were introduced as catalysts in the same oxidative CDC (Table 1, entries 2-6). It was noticed that the intended oxidative CDC was occurred and produced the desired α-ketoamide product 3a as yellow oil. Later, the product 3a was separated by extraction procedure using ethyl acetate. However, the yields of 3a were found to be very low in the range of 12-29% (Table 1, entries 2-6). In order to improve the yield of 3a in water, we next investigated the efficiency of Au(I) and Au(III) salts in the above oxidative CDC reaction (Table 1, entries 7-9). As shown table 1, this time a moderate yield (69%) of product 3a was obtained in the presence of NaAuCl4 as catalyst (Table 1, entry 9). On the other hand Au(I)Cl and Au(III)Br3 provided the yields of 3a in 44% and 51% respectively (Table 1, entry 7-8). Table 1. Optimization of the reaction conditions
Entry
Initially, we have investigated the aerobic oxidative CDC of phenylglyoxal monohydrate (1a) and morpholine (2a) under open air conditions in water medium as a model reaction using different coinage metal salts (Cu(I), Ag(I), Au(I) and Au(III)) as catalysts to optimize the reaction conditions. The details of the reaction conditions and the outcome of the reaction were depicted in table 1. It was observed that there was no oxidative CDC reaction between the substrates 1a and 2a in the absence of a catalyst (Table 1, entry 1). Next,
Temp(oC)
Time (h)
Yield(%)
1
-
60
24
NR
2
CuBr (3)
60
12
3
Cu(OTf)2 (3)
60
12 12
60
12
18
60
12
21 29
4 5
Herein we present the results of water-dispersed AuNPs (5 nm) catalyzed aerobic oxidative CDC coupling reaction between α-ketoaldehydes and secondary amines in the high yield synthesis of a range of α-ketoamides.
Catalyst(mol%)
Ag2O (3) AgBF4 (3)
14
6
Ag(OTf) (3)
60
12
7
AuCl (3)
60
6
44
8
AuBr3 (3)
60
6
51
9
Na.AuCl4.2H 2O (3)
60
5
69
10
AuNPs (1)
60
1.5
85
11
AuNPs (0.5)
60
2.5
79
12
AuNPs (1.5)
60
1.5
85
13
AuNPs (1)
RT
12
47
14
AuNPs (1)
50
4
69
AuNPs (1)
60
1.5
NRc
15 a
Unless otherwise stated, reaction was carried out with Phenylglyoxal monohydrate (1.0 mmol) and morpholine (1.1 mmol) in 5 mL of water under open air. b Isolated yields after coloumn chromatography. c Reactions was carried out under N2 atmosphere. NR = no reaction.
At this juncture, we were intended to explore the catalytic efficiency of water dispersed gold nanoparticles (AuNPs) by considering their special surface properties than the bulk gold. AuNPs can activate effectively the areal oxygen-Au surface binding,
3 which indeed is essential step to initiate the oxidative coupling reaction. We thought that this step would be further useful to design AuNPs catalyzed oxidative CDC reactions. In this circumstance, 85% of (3a) was obtained from oxidative CDC between 1a and 2a in the presence 1 mol% water dispersed PEG-stabilized AuNPs as catalyst at 60 oC within 1.5 hour only (Table 1, entry 10). The synthesis of PEG-stabilized AuNPs (~5 nm) dispersed in water21 is described in experimental (see ESI for details). Further we have also examined the effect of concentration of AuNPs and effect of temperature on above oxidative CDC (Table 1, entries 11-14). However, the activity of 1 mol% of AuNPs at 60 oC was found to best to optimize the synthesis of 3a. It is also interesting note that, under N2 inert atmosphere conditions (in the absence of air) only the hemi-aminal was observed, but there was no indication of forming the product 3a (Table 1, entry 15). This finding indicates the role of areal oxygen in the oxidative CDC between α-ketoaldehydes and amines to form α-ketoamides. Table 2.Substrate scope
a b
(ortho-, meta-, and para-position) of phenylglyoxal derivatives were smoothly converted into the desired products in good yields (3e-3w). The phenylglyoxals those bearing electron-withdrawing groups (3k-3u, 3w) were found suitable to give slightly higher yield of corresponding product than phenylglyoxal derivatives bearing electron-donating group (3e-3j, 3v). Later the substrate scope extended to use a variety of secondary amines. All the aliphatic secondary amines worked well in the oxidative CDC reaction and afforded the desired products in moderate to good yields. Among them, the reactions using pyrrolidine produced higher yields of corresponding α-ketoamides as compared to the other amines (3c, 3f, 3m, 3q, 3r and 3t). Acyclic amine such as diethylamine was also found to be appropriate for the oxidative CDC reaction to afford yields in 66%-73% range (3d-3e, 3j, 3p and 3s). All the synthesized products were characterized by 1H NMR, 13C NMR and mass spectrometry and the details are given in ESI. On the basis of the above results (Table 1, entry 1 & 15), and from the literature support, we proposed a plausible mechanism as shown in Scheme 3. Initially, iminium (A) was formed by the condensation of phenylglyoxal and secondary amine. Later water was added to iminium ion to form hemi-aminal (B).22 Now the hemi-aminal was activated by oxygen adsorbed/bound AuNPs (C) and forms surface activated AuOOH-hemiaminal intermediate (D) via dehydration process. This is because of the role of adsorbed oxygen or OH in facilitating β-H elimination on gold as suggested by a recent report,23 which demonstrates the thermodynamic favorability of direct H-transfer to adsorbed O2 or OH, forming OH or water. On the other hand, it is also possible that in the presence of excess of O2 on gold, all the liberated H2 immediately converts to water due to the high activation barrier for recombinative desorption of H2 on Au.24 Finally the β-H elimination of activated hemiaminal intermediate (D) could facilitate the formation of desired α-ketoamide product by removing water.
All products were characterized by 1H and 13 C NMR spectroscopy. Isolated yields after column chromatography.
With the optimized reaction conditions in hand, we further observed the scope of different phenylglyoxal derivatives and secondary amines. The results are summarized in table 2. It is found that the both electron deficient and electron-rich groups at different position
Scheme 3. A plausible reaction mechanism
4
Tetrahedron Letters
The reusability of the water dispersed AuNPs were also investigated for the aerobic oxidative CDC reaction of phenylglyoxal monohydrate (1a) and morpholine (2a) in pure water. After the formation of the desired 1morpholino-2-phenylethane-1,2-dione (3a), it was extracted into ethyl acetate and the gold nanoparticles present in the water were further used for the next catalytic run. As shown in chart 1, the AuNPs were efficiently catalyzed the formation of (3a) for five catalytic runs without any appreciable decrease in product yields.
Chart 1. Data showing the recyclability of AuNPs In conclusion, we have developed an environmentally benign catalytic methodology for the synthesis of biologically significant α-ketoamides via water dispersed AuNPs catalyzed oxidative CDC between α-ketoaldehydes and secondary-amines in water. The results of present work are comparable or relatively better than the previous works. Finally, the recyclability of AuNPs catalyst in the oxidative CDC was also established. The crucial role of areal O2 to initiate the oxidative CDC was also noticed. Acknowledgements The authors are thankful to the UGC-New Delhi (F.No: 17-80/2009(SA-I) and DST-India (DST/INT/SA/P15/2011 Indo-South Africa project) for financial support. A Supplementary data Experimental details, characterization data, TEM image of synthesized AuNPs, copies of 1H and 13 C NMR spectrum of products can be found, in the online version, at http://dx.doi.org/ References and Notes 1. (a) Tanaka, H.; Kuroda, A.; Marusawa, H.; Hatanaka, H.; Kino, T.; Goto, T.; Hashimoto,
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7 The results of this work are comparable are relatively better than the previous works. The crucial role of areal O2 to initiate the oxidative CDC was also noticed with the help of controlled experiments. At the end, we have also described the recyclability of above water dispersed Au nanoparticle catalytic system.
