DSpace at VNU: Indium triflate in 1-isobutyl-3-methylimidazolium dihydrogen phosphate: an efficient and green catalytic system for Friedel-Crafts acylation

Tetrahedron Letters 56 (2015) 2187–2192 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet Indium triflate in 1-isobutyl-3-methylimidazolium dihydrogen phosphate: an efficient and green catalytic system for Friedel–Crafts acylation Phuong Hoang Tran a, Poul Erik Hansen b, Huy Manh Hoang a, Duy-Khiem Nguyen Chau a, Thach Ngoc Le a,⇑ a b Department of Organic Chemistry, Faculty of Chemistry, University of Science, Vietnam National University, Hochiminh City 70000, Viet Nam Department of Science, Systems and Models, Roskilde University, POB 260, Roskilde DK-4000, Denmark a r t i c l e i n f o Article history: Received 30 December 2014 Revised March 2015 Accepted 12 March 2015 Available online 18 March 2015 a b s t r a c t Indium triflate in the ionic liquid, 1-isobutyl-3-methylimidazolium dihydrogen phosphate ([i-BMIM]H2PO4), was found to show enhanced catalytic activity in the Friedel–Crafts acylation of various aromatic compounds with acid anhydrides The catalytic system was easily recovered and reused without a significant loss of activity Ó 2015 Elsevier Ltd All rights reserved Keywords: Indium triflate Aryl ketone Microwave irradiation Friedel–Crafts acylation Ionic liquids The Friedel–Crafts acylation is a fundamental reaction for the preparation of aromatic ketones, which are used among other things as precursors in the synthesis of pharmaceuticals and agrochemicals.1–7 Challenges still remain in Friedel–Crafts acylations as the process involves more than a stoichiometric amount of a metal chloride which is lost during the typical aqueous work-up.6 One way of avoiding such problems is to use metal triflate catalyzed Friedel–Crafts acylation as this method only uses a catalytic amount of the metal, and is stable in many organic solvents and also in aqueous medium, hence the catalyst can easily be recovered and reused without a significant loss of its activity.8–21 Recently, metal triflates in ionic liquids have been shown to be efficient and green catalytic systems for Friedel–Crafts acylations.6 The first report of a green Friedel–Crafts acylation reaction was described by Ross and Xiao.22 Copper triflate was an effective catalyst in the [BMIM]BF4 ionic liquid, but the substrate scope was unfortunately limited to highly activated examples In addition, the catalytic system Cu(OTf)2/[BMIM]BF4 showed low activity toward acid anhydrides, and gave only 10% conversion after one hour Gmouh et al reported Friedel–Crafts acylations using Bi(OTf)3/[EMIM]NTf2 with a catalyst loading as low as mol %.23 ⇑ Corresponding author E-mail address: lenthach@yahoo.com (T.N Le) http://dx.doi.org/10.1016/j.tetlet.2015.03.051 0040-4039/Ó 2015 Elsevier Ltd All rights reserved The catalytic system could be easily recycled without loss of activity However, the reaction was limited to benzoyl chloride There are also a few reports describing the use of metal triflates in ionic liquids for Friedel–Crafts acylations with substrates such as anisole24,25 and ferrocene.26,27 We have reported Bi(OTf)3/ [BMIM]PF6 as a good catalytic system for Friedel–Crafts acylations of highly activated substrates under both microwave irradiation and conventional heating.28 We have also improved the catalytic activity of bismuth triflate in triflate anion containing ionic liquids under microwave irradiation.29 However, the acylating reagent scope was restricted to benzoyl chloride In this Letter, we report a method for the Friedel–Crafts acylation of various aromatic compounds using four different acid anhydrides as acylating reagents Firstly, we investigated the catalytic activity of different metal triflates in the Friedel–Crafts acetylation of mesitylene under conventional heating in the absence of ionic liquids The results are presented in Table Among these metal triflates, the four metal triflates which gave the highest yields (Table 1, entries 11–14) were chosen to be tested for catalytic activity in the Brønsted acidic ionic liquid [iBMIM]H2PO4 This ionic liquid was synthesized with a Brønsted acidic counterion because Brønsted acids are known to be good catalysts for Friedel–Crafts acylations.30–41 Surprisingly, copper triflate, which showed the best catalytic activity under solvent-free 2188 P H Tran et al / Tetrahedron Letters 56 (2015) 2187–2192 Table Effect of different metal triflates on the Friedel–Crafts acetylation of mesitylene using acetic acid anhydride under conventional heating O O O equiv metal triflate (5 mol%) 120 oC, 90 O CH 3COOH Entry Metal triflate Conversiona (%) 10 11 12 13 14 Eu(OTf)3 Gd(OTf)3 Li(OTf) Er(OTf)3 Th(OTf)3 La(OTf)3 Ho(OTf)3 Nd(OTf)3 Dy(OTf)3 Y(OTf)3 Pr(OTf)3 Bi(OTf)3 In(OTf)3 Cu(OTf)2 11 12 16 17 29 43 48 49 50 60 68 74 76 (39) (61) (89) (15) a Conversions in parentheses are those obtained when the reactions were carried out in [i-BMIM]H2PO4 Only monoacetylated product was obtained conditions, afforded only a 15% yield when dissolved in the [i-BMIM]H2PO4 ionic liquid Unlike the reactions of other metal triflates, the acetylation of mesitylene catalyzed by copper triflate in [i-BMIM]H2PO4 generated immediately a dark-blue precipitate Previously, Ross and Xiao observed the formation of this precipitate and identified it as Cu(OAc)2ÁH2O by NMR spectroscopy.22 Interestingly, indium triflate improved the yield of the corresponding ketone when dissolved in [i-BMIM]H2PO4 (Table 1, entry 13) Indium triflate has been demonstrated to be a good catalyst for many organic reactions including alkylations,42,43 coupling reactions,44 Diels-Alder,45 and benzannulation reactions.46 Herein, we report the Friedel–Crafts acylations of aromatic compounds using In(OTf)3 in [i-BMIM]H2PO4 ionic liquid This is the first time [i-BMIM]H2PO4 has been synthesized and used as a catalytic system with In(OTf)3 in Friedel–Crafts acylations of aromatic compounds with acid anhydrides The general synthesis of ionic liquids involves an alkylation– metathesis procedure.47–50 In our synthesis, the first step generates a bromide-containing ionic liquid via the alkylation of 1-methylimidazole with isobutyl bromide The second step involves anion exchange in this precursor with KH2PO4 Both steps are carried out under solvent-free conditions at 80 °C in a few minutes [see Supporting information (SI), Section S2] Indium triflate in [i-BMIM]H2PO4 demonstrated a significant enhancement of the catalytic activity to afford better yields in comparison with only indium triflate as the catalyst Previously, the Friedel–Crafts acylation using acid anhydrides with challenging substrates such as alkylbenzenes gave low yields (48–53%) in long reaction times (overnight) in the presence of metal triflates and ionic liquids.22 It is noteworthy that our method has been applied extensively to slightly activated substrates The method gave good yields within only 0.5–3 h (conventional heating) and in 30 (microwave irradiation) (see below) The Friedel–Crafts acetylation of anisole was chosen as the model reaction and indium triflate (Table 1, entry 13) was selected as the catalyst in [i-BMIM]H2PO4 No corresponding ketone was obtained in the absence of indium triflate and the yield of the product decreased significantly on reducing the catalyst loading to mol % The conditions for the acetylation of anisole were optimized and the highest yield of 81% was obtained within only 30 at 100 °C (SI, Section S3) These conditions were then applied to other substrates and acylating reagents Interestingly, arenes containing slightly activated substituents such as alkylbenzenes and polycyclic benzenoid aromatic compounds such as naphthalene, fluorene, and anthracene were also acylated in good yields Under conventional heating, the Friedel–Crafts benzoylation of various substrates was investigated in the presence or absence of [i-BMIM]H2PO4 The range of aromatic compounds employed is presented in Table In all cases, the yields of the corresponding ketones were significantly improved when using the ionic liquid reaction medium The results of the benzoylation of aromatic compounds show that electron-rich (Table 2, entries 1–7) as well as some slightly activated arenes (Table 2, entries 11 and 14) were reactive Alkylbenzenes were benzoylated in moderate yields because of their low electrophilicity (Table 2, entries 8–10) Surprisingly, the benzoylation of mesitylene gave a 58% yield of a monobenzoylated product and 32% yield of a dibenzoylated by-product under conventional heating, whilst the yield of this by-product was only 10% under microwave irradiation (Table 2, entry 12) Aromatic rings without an electron-donating substituent, such as naphthalene and anthracene, were also reactive (Table 2, entries 13 and 15) In general, the benzoylation of several of the aromatic substrates proceeded smoothly to give the corresponding ketones in good to excellent yields within short reaction times (Table 2, entries 1–3 and 6–7) 1,3Dimethoxybenzene also gave a good yield of the expected product (Table 2, entry 4) However, in 1,4-dimethoxybenzene, in which the aromatic ring has a counteracting orientation of the two methoxy substituents, was only acylated in moderate yields (Table 2, entry 5) Reactions with aliphatic anhydrides under conventional heating and microwave irradiation are presented in Table The aliphatic anhydrides appeared to be less reactive than benzoic anhydride, therefore slightly activated substrates such as alkylbenzenes were not reactive, with the exception of mesitylene (Table 3, entry 6) Strongly activated substrates were acylated in good yields (Table 3, entries 1–4) Unfortunately, naphthalene and anthracene were found to be unreactive toward aliphatic anhydrides The Friedel–Crafts acylation was also studied under microwave irradiation Although microwave irradiation has been applied to a wide range of organic reactions, its use in metal triflate catalyzed 2189 P H Tran et al / Tetrahedron Letters 56 (2015) 2187–2192 Table Comparison of the benzoylation of aromatic compounds in [i-BMIM]H2PO4 and under solvent-free conditions O O O Entry Reaction conditions O + R In(OTf)3 /[i-BMIM]H 2PO4 Yield (%) (isomer distribution)a Product O R D, 100 °C, 30 MW, 80 °C, 30 Solvent-free [i-BMIM]H2PO4 72 (7:0:93)b 88 (6:0:94)b ndc 83 (4:0:96)b 78 (4:0:96)b 90 (0:0:100)b OMe O D, 140 °C, 120 MW, 100 °C, 15 c nd 92 (0:0:100)b 82 91 ndc 93 73 (6:94)d 82 (6:94)d 63 72 OEt O OMe D, 100 °C, 30 MW, 80 °C, 10 OMe O OMe D, 100 °C, 30 OMe O OMe D, 140 °C, 30 OMe O OMe 81 D, 140 °C, 30 MW, 80 °C, 10 93 c OMe nd 90 72 (8:0:92)b 86 (4:0:96)b OMe O D, 140 °C, 120 MW, 120 °C, 15 c nd 90 (0:0:100)b 52 (25:5:70)b 63 (25:4:71)b 55 (18:8:74)b 64 (16:5:79)b 56 (10:5:85)b 67 (13:5:82)b 69 (0:100)e 82 (7:93)e 74 ndc 58f 65g 52 (77:23)h 65 (74:26)h SMe O D, 180 °C, 120 Me O D, 160 °C, 180 Et O 10 D, 160 °C, 180 n-Pr O 11 Me D, 160 °C, 180 Me O 12 Me D, 160 °C, 180 MW, 100 °C, 15 Me Me O 13 D, 160 °C, 180 (continued on next page) 2190 P H Tran et al / Tetrahedron Letters 56 (2015) 2187–2192 Table (continued) Entry Reaction conditions Yield (%) (isomer distribution)a Product Solvent-free [i-BMIM]H2PO4 63 86 42 (68:32)i 70 (82:18)i O 14 D, 160 °C, 180 15 D, 160 °C, 180 O a b c d e f g h i Isolated yield of pure monobenzoylated product after column chromatography Isomer ratios in parentheses were determined by GC-FID o-/m-/p-Isomers nd: not determined 2,6-/2,4-Dimethoxybenzophenone isomers 2,6-/2,4-Dimethylbenzophenone isomers Dibenzoylated product was obtained in 32% yield Dibenzoylated product was obtained in 10% yield 1-/2-Benzoylated naphthalene isomers 9-/1-Benzoylated anthracene isomers Table Acylation using aliphatic acid anhydrides in [i-BMIM]H2PO4 O R O O O R + R' In(OTf) 3/[i-BMIM]H 2PO4 R R' R: CH3, C2 H 5, C3 H7 Entry Reaction conditionsa Product Yield (%) (Isomer distribution)b D MW 82 (4:96)b 83 (4:96)b 81 60 65 58 83 (5:95)b 90 (6:94)b 78 (3:97)b 65c (4:96)b 72d (3:97)b 60e (0:100)b 88 85 87 63f O H 3C OMe O D, 100 °C, 30 MW, 80 °C, 30 C H5 OMe O C H7 OMe O D, 140 °C, 120 MW, 100 °C, 15 H 3C OEt O D, 140 °C, 180 MW, 100 °C, 15 C H5 OEt O D, 140 °C, 120 MW, 100 °C, 15 C H7 OEt O D, 100 °C, 30 MW, 80 °C, 10 OMe H 3C OMe O D, 80 °C, 30 MW, 80 °C, 10 C H5 OMe OMe 2191 P H Tran et al / Tetrahedron Letters 56 (2015) 2187–2192 Table (continued) Entry Reaction conditionsa Product Yield (%) (Isomer distribution)b D MW 88 68g 80 80 86 62h 79i (8:92)d 70j (0:100)d 64 (0:100)b 52 (5:95)b 57k (0:100)b 55l (3:97)b 60m (0:100)b 76n (3:97)b 73 80 70o 47p 70q 50r O D, 100 °C, 30 MW, 80 °C, 10 OMe C H7 OMe O D, 100 °C, 30 MW, 80 °C, 10 OMe H 3C OMe OMe O D, 80 °C, 30 MW, 80 °C, 10 OMe C H5 OMe OMe O D, 100 °C, 30 MW, 80 °C, 10 OMe C H7 OMe OMe O H 3C SMe O D, 140 °C, 120 MW, 120 °C, 15 C H5 SMe O C H7 SMe O D, 120 °C, 90 MW, 100 °C, 15 Me H 3C Me O D, 120 °C, 30 MW, 100 °C, 15 Me Me C H5 Me O D, 120 °C, 90 MW, 100 °C, 15 Me Me C H7 Me Me a D: conventional heating, MW: microwave irradiation Isolated yield of pure monobenzoylated product after column chromatography Isomer ratios in parentheses were determined by GC-FID Diacylated product was also obtained as a by-product: c 20%, d 15%, e 25%, f 5%, g 10%, h 5%, i 10%, j 7%, k 32%, l 20%, m 15%, n 5% d 2,4,5-/2,3,6-Trimethoxybutyrophenone isomers o–r Di- and tri-acylated products were also obtained as by-products during the acylation of mesitylene: o 10% and 3%, p 5% and 5%, q 15% and 5%, r 5% and 5% b c–n Friedel–Crafts acylations has been reported in only a few Letters.27,51–53 Under microwave irradiation, mild conditions are sufficient For example, the yields for the acetylation of phenetol, the butyrylation of thioanisole and the acetylation of mesitylene were improved at low temperatures in short reaction times (Table 3, entries 2, 5, and 6) However, in some cases, propionylation and butyrylation of aromatic compounds under microwave irradiation afforded lower yields in comparison with conventional heating (Table 3) This can be explained by a decrease in microwave absorption by the acylating reagent with increasing carbon chain length In general, the Friedel–Crafts acylation usually stops cleanly after one reaction to give monoacylated products, whereas diacylated products were also obtained in the propionylation and butyrylation with acid anhydrides using our method (Table 3, entries 2–6) Also, the Friedel–Crafts propionylation and butyrylation of mesitylene generated triacylated products (Table 3, entry 6) Previously, in traditional Friedel–Crafts acylations of benzene derivatives, triacylated products were not obtained Although diacylated products were also reported in a few Letters, heteroaromatics or polybenzenes, as substrates, were required.54–59 The recyclability of the In(OTf)3/[i-BMIM]H2PO4 system was investigated in the Friedel–Crafts benzoylation of anisole under conventional heating The procedure for the recovery and reuse of the catalyst is simple and no significant loss in the catalytic activity was apparent after three consecutive runs (SI, Section S4) In conclusion, a new ionic liquid was synthesized and applied as a solvent in the Friedel–Crafts acylation of aromatic compounds with acid anhydrides The ionic liquid was found to enhance the catalytic activity of indium triflate under conventional heating 2192 P H Tran et al / Tetrahedron Letters 56 (2015) 2187–2192 and microwave irradiation The catalytic system can be easily recovered and reused in three consecutive cycles without any significant loss of the activity, which is promising for large scale applications This research has also provided a new method for the diand tri-acylations of aromatic substrates Further research to develop other catalytic systems for the Friedel–Crafts acylations of deactivated substrates are now in progress Acknowledgment We are grateful to the Vietnam National University—Hochiminh City (Grant No C2014-18-08) for financial support Supplementary 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catalytic system can be easily