DSpace at VNU: Erbium trifluoromethanesulfonate catalyzed Friedel-Crafts acylation using aromatic carboxylic acids as acylating agents under monomode-microwave irradiation

Accepted Manuscript Erbium trifluoromethanesulfonate catalyzed Friedel–Crafts acylation using aromatic carboxylic acids as acylating agents under monomode-microwave irradiation Phuong Hoang Tran, Poul Erik Hansen, Hai Truong Nguyen, Thach Ngoc Le PII: DOI: Reference: S0040-4039(14)02092-9 http://dx.doi.org/10.1016/j.tetlet.2014.12.038 TETL 45562 To appear in: Tetrahedron Letters Received Date: Revised Date: Accepted Date: October 2014 30 November 2014 December 2014 Please cite this article as: Hoang Tran, P., Erik Hansen, P., Truong Nguyen, H., Ngoc Le, T., Erbium trifluoromethanesulfonate catalyzed Friedel–Crafts acylation using aromatic carboxylic acids as acylating agents under monomode-microwave irradiation, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet 2014.12.038 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Graphical Abstract Erbium trifluoromethanesulfonate catalyzed Friedel-Crafts acylation using aromatic carboxylic acids as acylating agents under monomode-microwave irradiation Leave this area blank for abstract info Phuong Hoang Tran, Poul Erik Hansen, Hai Truong Nguyen, Thach Ngoc Le Tetrahedron Letters journal homepage: www.elsevier.com Erbium trifluoromethanesulfonate catalyzed Friedel–Crafts acylation using aromatic carboxylic acids as acylating agents under monomodemicrowave irradiation Phuong Hoang Trana, Poul Erik Hansenb, Hai Truong Nguyena, Thach Ngoc Lea, * a Department of Organic Chemistry, Faculty of Chemistry, University of Science, Vietnam National University-Hochiminh City 70000, Vietnam b Department of Science, Systems and Models, Roskilde University, POB 260, Roskilde DK-4000, Denmark ARTICLE INFO ABSTRACT Article history: Received Received in revised form Accepted Available online Erbium trifluoromethanesulfonate is found to be a good catalyst for the Friedel–Crafts acylation of arenes containing electron-donating substituents using aromatic carboxylic acids as the acylating agents under microwave irradiation An effective, rapid and wastefree method allows the preparation a wide range of aryl ketones in good yields and in short reaction times with minimum amounts of waste Keywords: Erbium triflate Aryl ketone Microwave irradiation Friedel–Crafts acylation Aromatic carboxylic acid 2014 Elsevier Ltd All rights reserved The Friedel–Crafts acylation is an important route to prepare aromatic ketones, which are useful precursors in the synthesis of various pharmaceutical compounds.1 Traditionally, a greater than stoichiometric amount of Lewis acids such as AlCl3, BF3, FeCl3, TiCl4, SnCl2, etc., is required to mediate the reaction; the catalyst is often not recovered or is not reused.2 Since the traditional reaction is extremely sensitive to water, it must be carried out under a dry atmosphere, using anhydrous substrates and catalysts.3,4 In addition, typical procedures employ an acid chloride as the acylating reagent, which is usually prepared from a carboxylic acid and thionyl chloride, involving HCl and SO2 as by-products It would be better if carboxylic acids could be used in place of acid chlorides because the use of carboxylic acids as acylating agents produces water as the sole by-product Therefore, acylation using a carboxylic acid can shorten the synthetic route and is environmentally preferable Friedel-Crafts acylations using carboxylic acids have been studied extensively in the presence of catalysts such as: zeolites,5 Montmorillonite,6 alumina/TFAA,7 aluminum dodecatungstophosphate/TFAA,8 MeSO3H/graphite,9 P2O5/SiO2,10 Tf2O,11 TFAA,12 Pd(OAc)2/TFAA13 and rhodium.14 However, the use of a stoichiometric amount of additional TFAA or harsh reaction conditions are often required Metal triflates have received significant attention because of their unique reactivities.3 These triflates possess strong Lewis acidity and exhibit high tolerance towards water, and those that not require special handling are effective catalysts in Friedel– Crafts acylations.4,15 Additionally, metal triflates can be easily recovered and reused without loss of activity.3 While many aryl ketones have been prepared by Friedel–Crafts acylation of aromatic compounds with acid chlorides and acid anhydrides catalyzed by metal triflates,16 the use of carboxylic acids as acylating agents has been reported in only a few papers Kobayashi and co-workers reported the C-acylation of phenol and naphthol derivatives using carboxylic acids as the acylating reagents in the presence of Hf(OTf)4, Zr(OTf)4 and Sc(OTf)3.17 Kawamura’s group demonstrated Lewis acid catalyzed Friedel–Crafts acylation reactions using carboxylic acids as the acylating agents.18 According to these reports, excess substrates or organic solvents and long reaction times were required Matsushita et al improved the Friedel–Crafts acylation of aromatic compounds with carboxylic acids by combining perfluoroalkanoic anhydride and bismuth or scandium triflate.19 Jin et al investigated the ——— Keywords: erbium triflate, aryl ketone, microwave irradiation, Friedel–Crafts acylation, aromatic carboxylic acid * Corresponding author E-mail: lenthach@yahoo.com Tetrahedron Friedel–Crafts acylation with carboxylic acids as acylating reagents using metal triflates under solvent-free conditions.20 Furthermore, Parvanak and co-worker reported Friedel-Crafts acylation with carboxylic acids using Al(OTf)3 supported on polystyrene.21 Although this is a green and efficent method for the preparation of aromatic ketones via a solventless Friedel– Crafts reaction, the long reaction time and presence of perfluoroalkanoic anhydride were disadvantageous Microwave irradiation provides a powerful tool for organic synthesis 22 Reactions under microwave irradiation usually offer high yields, good selectivity and short reaction times under solvent-free conditions.23 In our previous work, we investigated Friedel–Crafts benzoylation using benzoyl chloride as the acylating agent catalyzed by bismuth triflate under microwave irradiation.24 As part of our ongoing research on green organic synthesis, we report herein an efficient and rapid method for Friedel–Crafts acylation with aromatic carboxylic acids in the presence of metal triflates under microwave irradiation This is the first application of erbium triflate in Friedel–Crafts acylations with aromatic carboxylic acids as acylating agents The use of microwave irradiation resulted in short reaction times and improved yields of products This is also the first time that microwave irradiation has been applied in the Friedel–Crafts acylation using carboxylic acids catalyzed by a metal triflate The benzoylation of anisole using benzoic acid was selected as a model reaction with 10 mol% of Er(OTf)3 as the catalyst under microwave irradiation (220 oC, 30 min).25 The corresponding product was obtained as one regioisomer in 72% isolated yield However, all attempts to reduce the amount of catalyst led to lower yields and no reaction took place in the absence of a catalyst Twelve metal triflates were tested under microwave irradiation and the results are summarized in Table Er(OTf)3 was more effective than other metal triflates giving the highest yield (78%) However, gadolinium triflate was also efficient, while other rare-earth triflates proved to be less effective (46–60%) Surprisingly, bismuth triflate, a good catalyst for the Friedel– Crafts acylation with acyl chlorides and acid anhydrides, gave a poor yield in the present method LiOTf was the least active catalyst affording only a 3% yield The combination of erbium triflate and microwave irradiation under solvent-free conditions was found to shorten the reaction time in comparison with conventional heating.18 Prominent side-products in the reaction were methyl benzoate and phenyl benzoate generated from demethylation/esterification of anisole The failure of some of the rare earth metals is due to the formation of these products rather than 4-methoxybenzophenone (see Figure 1) With optimized conditions in hand, we next investigated the Friedel–Crafts acylation of anisole using aromatic carboxylic acids containing both electron-withdrawing and electron-donating substituents Anisole was chosen, as this substrate has been successfully used in a number of investigations.26 The results are summarized in Table p-Acylated products were obtained predominantly (95%) and 2-chlorobenzoic acid was the best acylating agent using the above-mentioned conditions On the other hand, the use of 4-methoxybenzoic acid, 2-phenylbenzoic and 2-naphthoic acid gave low yields in the acylation of anisole (Table 2, entries 2, and 9) 2-Nitrobenzoic acid, 3-nitrobenzoic acid and 2-methoxybenzoic acid all decomposed at 220 C over 30 minutes under microwave irradiation; little or no product was obtained at lower temperatures such as 150 C, 180 C and 200 C In all cases, the formation of a certain amount of side products was observed: phenyl and methyl benzoate and minor amounts of o-and p-methylanisole (95%) However, in cases where the para position is blocked (Table 3, entry 4), an o-benzoylated product was obtained For aromatic compounds containing two electron-donating groups such as 1,2-dimethoxybenzene (veratrole), 1,3dimethoxybenzene and 1,4-dimethoxybenzene, the corresponding benzoylated products were obtained in lower yields (Table 3, entries 2, and 4) because of the predominant demethylation side reaction Steric hindrance clearly played a role as very little osubstitution occurs for anisole However, this was not the explanation for veratrole (Table 3, entry 2) and it appears that the electron-withdrawing power also plays a role This is further supported by the fact that the reaction does not take place with halobenzenes Furthermore, p-nitrotoluene decomposes under microwave irradiation at high temperature Good yields of products were obtained when an excess of anisole or thioanisole was used as the starting material (mole ratio = 5:1) However, the use of excess starting material resulted in lower yields in the benzoylation of alkylbenzenes and dimethoxybenzenes Therefore, we investigated the mole ratio of starting material and benzoic acid Interestingly, the yield of the aromatic ketone was increased when the mole ratio of the starting material and benzoic acid was 1:2 The results are summarized in Table The substrate scope could be extended to alkylbenzenes which gave rise to the corresponding ketones with good regioselectivities Thioanisole was benzoylated in 95% yield with 94% regioselectivity at the p-position (Table 4, entry 1) Mono-, di- and tri-alkylbenzenes were also benzoylated in excellent yields and with good regioselectivities (Table 4, entries 2-9) Fluorene was benzoylated at the 2-position in 80% yield (Table 4, entry 10) It is noteworthy that naphthalene and anthracene (without electron-donating substituents) were also benzoylated in good yields (Table 4, entries 11 and 12) However, this method was not efficient for the Friedel–Crafts benzoylation of anisole and dimethoxybenzenes because of the demethylation of aryl methyl ethers when two equivalents of benzoic acid was used In the case of anisole, by-products such as phenol, methyl benzoate and phenyl benzoate were obtained as major products from the demethylation Previously, demethylation was observed when using Yb(OTf)3 in the presence of acyl chloride.27 This is the first time that demethylation has been observed with Er(OTf)3 in the presence of benzoic acid The carboxylic acid scope was also extended to ortho-substituted benzoic acids and 2-naphthoic acid in the Friedel–Crafts acylation of toluene and mesitylene (Table 5) o-Halobenzoic acids were reactive in the benzoylation of toluene and mesitylene o-Toluic acid also gave a good yield (Table 5, entries and 10) 2-Naphthoic acid afforded only a 55% yield in the case of toluene (Table 5, entry 6), but gave an 80% yield with mesitylene (Table 5, entry 12) This method was not effective for aliphatic carboxylic acids with chain lengths of to carbons Interestingly, the use of erbium triflate was successful in Friedel–Crafts acylations of activated aromatic compounds with aromatic carboxylic acids under microwave irradiation The recycling of erbium triflate was also studied and the catalyst could be easily recovered and reused in the benzoylation of anisole, toluene and mesitylene without any significant loss of the catalytic activity over three consecutive cycles (Scheme 1) In conclusion, a green method for the Friedel–Crafts acylation of aromatic compounds employing erbium triflate and aromatic carboxylic acids under microwave irradiation has been developed The method gives products in good yields over short reaction times without the use of metal halides or acid chlorides Additionally, the easy recycling of erbium triflate is convenient to apply in large-scale synthesis The microwave irradiation assisted Friedel–Crafts acylations are accomplished in short times (20-30 min) and have widened the substrate scope to alkylbenzenes, naphthalene and anthracene Acknowledgments We are grateful to the Vietnam National University - Hochiminh City (Grant No C2014-18-08) and the Hochiminh Department of Science and Technology (Grant No 355/2013/HĐ-SKHCN) Toshiba is also thanked for a PhD Student Award We thank Prof Fritz Duus (Roskilde University, Denmark), Dr Hien Quang Do (Caltech, USA) and Khiem Duy Nguyen Chau MSc (University of Minnesota Duluth, USA) for their help References and notes 10 11 12 13 14 15 16 17 18 19 (a) Whitehead, A J.; Ward, R A.; Jones, M F Tetrahedron Lett 2007, 48, 911-913; (b) Shuju, G.; Jing, L.; Ting, L.; Dayong, S.; Lijun, H Chin J Oceanol Limnol 2011, 29, 68-74; (c) Mizuno, M.; Inagaki, A.; Yamashita, M.; Soma, N.; Maeda, Y.; Nakatani, H Tetrahedron 2006, 62, 4065-4070; (d) Rajitha, C.; Dubey, P K.; Sunku, V.; Piedrafita, F J.; Veeramaneni, V R.; Pal, M Eur J Med Chem 2011, 46, 4887-4896; (e) Yadav, G D.; George, G Microporous Mesoporous Mater 2006, 96, 36-43 Olah, G A Friedel-Crafts Chemistry; John Wiley and Sons: New York, 1973 (a) Kobayashi, S.; Sugiura, M.; Kitagawa, H Chem Rev 2002, 102, 2227-2302; (b) Nguyen, V T A.; Duus, F.; Le, T N Asian J Org Chem 2014, 3, 963-968 Sartori, G.; Maggi, R Advances in Friedel-Crafts Acylation Reactions: Catalytic and Green Processes; Taylor & Francis: Boca Raton, 2010 (a) Singh, A P.; Pandey, A K J Mol Catal A: Chem 1997, 123, 141-147; (b) De Castro, C.; Primo, J.; Corma, A J Mol Catal A: Chem 1998, 134, 215-222; (c) Lezcano-González, I.; Vidal-Moya, J A.; Boronat, M.; Blasco, T.; Corma, A Angew Chem 2013, 125, 5242-5245; (d) Bai, G.; Han, J.; Zhang, H.; Liu, C.; Lan, X.; Tian, F.; Zhao, Z.; Jin, H RSC Adv 2014, 4, 27116-27121; (e) Yamashita, H.; Mitsukura, Y.; Kobashi, H J Mol Catal A: Chem 2010, 327, 80-86 Chiche, B.; Finiels, A.; Gauthier, C.; Geneste, P J Mol Catal 1987, 42, 229-235 Ranu, B C.; Ghosh, K.; Jana, U J Org Chem 1996, 61, 9546-9547 Firouzabadi, H.; Iranpoor, N.; Nowrouzi, F Tetrahedron Lett 2003, 44, 5343-5345 Savari, M H.; Sharghi, H Synthesis 2004, 2165-2168 Zarei, A.; Hajipour, A R.; Khazdooz, L Tetrahedron Lett 2008, 49, 6715-6719 Khodaei, M M.; Alizadeh, A.; Nazari, E Tetrahedron Lett 2007, 48, 4199-4202 Kim, S H.; Lim, J W.; Yu, J.; Kim, J N Bull Korean Chem Soc 2013, 34, 2604-2608 Lu, J.; Zhang, H.; Chen, X.; Liu, H.; Jiang, Y.; Fu, H Adv Synth Catal 2013, 355, 529-536 Fukuyama, T.; Maetani, S.; Miyagawa, K.; Ryu, I Org Lett 2014, 16, 3216-3219 (a) Kobayashi, S.; Iwamoto, S Tetrahedron Lett 1998, 39, 4697-4970; (b) Kobayashi, S.; Manabe, K Pure Appl Chem 2000, 72, 1373-1380; (c) Koshima, H.; Kabota, M., Synth Commun 2003, 33, 3983-3988; (d) Tran, P H.; Duus, F.; Le, T N Tetrahedron Lett 2012, 53, 222-224 (a) Hachiya, I.; Moriwaki, M.; Kobayashi, S Tetrahedron Lett 1995, 36, 409-412; (b) Desmurs, J R.; Labrouillère, M.; Roux, C L.; Gaspard, H.; Laporterie, A.; Dubac, J Tetrahedron Lett 1997, 38, 8871-8874; (c) Leonard, N M.; Wielan, L C.; Mohan, R S Tetrahedron 2002, 58, 8373-8397; (d) Prakash, G K S.; Yan, P.; Török, B.; Bucsi, I.; Tanaka, M.; Olah, G A Catal Lett 2003, 85, 1-6; (e) Pârvulescu, A N.; Gagea, B C.; Pârvulescu, V I.; Vos, D D.; Jacobs, P A Appl Catal A 2006, 306, 159-164; (f) Ghosh, R.; Maiti, S J Mol Catal 2007, 264, 1-8 Kobayashi, S.; Moriwaki, M.; Hachiya, I Tetrahedron Lett 1996, 37, 4183-4186 (a) Kawamura, M.; Cui, D M; Hayashi, T.; Shimada, S Tetrahedron Lett 2003, 44, 7715-7717; (b) Kawamura, M.; Cui, D M; Hayashi, T.; Shimada, S Tetrahedron 2006, 62, 9201-9209 Matsushita, Y.; Sugamoto, K.; Matsui, T Tetrahedron Lett 2004, 45, 4723-4727 4 Tetrahedron 20 Jin, C.; Li, J.; Su, W J Chem Res 2009, 607-611 21 Parvanak-Boroujeni, K.; Parvanak, K J Serb Chem Soc 2011, 76, 155-163 22 Loupy, A., Microwaves in Organic Synthesis; Wiley-VCH: Weinheim, 2006 23 (a) Gronnow, M J.; Macquarrie, D J.; Clark, J H; Ravenscroft, P J Mol Catal A 2005, 231, 47-51; (b) Gopalakrishnan, M.; Sureshkumar, P.; Kanagarajan, V.; Thanusu, J Catal Commun 2005, 6, 753-756; (c) Deng, W.; Xu, Y.; Guo, Q X Chin Chem Lett 2005, 16, 327-330; (d) Flores, K O V.; de Aguiar, A P.; de Aguiar, M R M P.; de Santa Maria, L C Mater Lett 2007, 61, 1190-1196; (e) Berardi, S.; Conte, V.; Fiorani, G.; Floris, B.; Galloni, P J Organomet Chem 2008, 693, 3015-3020 (f) Wine, G.; Vanhaecke, E.; Ivanova, S.; Ziessel, R.; Pham-Huu, C Catal Commun 2009, 10, 477-480; (g) Mahdi, J.; Ankati, H.; Gregory, J.; Tenner, B.; Biehl, E R Tetrahedron Lett 2011, 52, 2594-2596 (h) Bai, G.; Li, T.; Yang, Y.; Zhang, H.; Lan, X.; Li, F.; Han, J.; Ma, Z.; Chen, Q.; Chen, G Catal Commun 2012, 29, 114-117; (i) Chandra Shekara, B M.; Jai Prakash, B S.; Bhat, Y S J Catal 2012, 290, 101-107; (j) Perrier, A.; Keller, M.; Caminade, A.-M.; Majoral, J.-P.; Ouali, A Green Chem 2013, 15, 2075-2080; (k) Reddy, K R.; Rajanna, K C.; Uppalaiah, K Tetrahedron Lett 2013, 54, 3431-3436; (l) Kumar, M S.; Rajanna, K C.; Venkanna, P.; Venkateswarlu, M Tetrahedron Lett 2014, 55, 1756-1759 24 Tran, P H.; Hansen, P E.; Pham, T T.; Huynh, V T.; Huynh, V H.; Thi Tran, T D.; Huynh, T V.; Le, T N Synth Commun 2014, 44, 2921-2929 25 The reactions were carried out with a CEM Discover oven, which offers microwave synthesis with safe pressure regulation, using a 10 mL pressurized glass tube fitted with a Teflon-coated septum and a vertically-focused IR temperature sensor controlling the reaction temperature General procedure: a mixture of Er(OTf)3 (0.0614 g, 0.1 mmol), anisole (0.5407 g, mmol) and benzoic acid (0.1221 g, mmol) was heated under microwave irradiation at 220 C for 30 in a CEM Discover apparatus After being cooled, the mixture was extracted with CH2Cl2 (3 × 15 mL) The organic layer was decanted, washed with H2O (10 mL), aqueous NaHCO3 (2 x 20 mL), and brine (10 mL), and dried over MgSO4 The solvent was removed on a rotary evaporator The crude product was purified by flash chromatography (n-hexane, then 10% EtOAc in n-hexane) to give 4-methoxybenzophenone (0.153 g, 72% yield) The purity and identity of the product were confirmed by GC-FID, and from GC-MS spectra which were compared with the spectra in the NIST library, and by 1H and 13C NMR spectroscopy 26 (a) Ross, J.; Xiao, J Green Chem 2002, 4, 129-133; (b) Gmouh, S.; Yang, H.; Vaultier, M Org Lett 2003, 5, 22192222; (c) Goodrich, P.; Hardacre, C.; Mehdi, H.; Nancarrow, P.; Rooney, D W.; Thompson, J M Ind Eng Chem Res 2006, 45, 6640-6647; (d) Zayed, F.; Greiner, L.; Schulz, P S.; Lapkin, A.; Leitner, W Chem Commun 2008, 79-81 27 Su, W.; Jin, C Synth Commun 2004, 34, 4199-4205 Figure Effect of side-products on the yield: (a) ratio of methyl benzoate to 4-methoxybenzophenone, (b) ratio of phenyl benzoate to 4-methoxybenzophenone anisole (5 mmol), benzoic acid (1 mmol) 220 oC, 30 72% 70% 70% mesitylene (1 mmol) benzoic acid (2 mmol) 180 oC, 20 89% 83% 82% toluene (1 mmol) benzoic acid (2 mmol) 180oC, 20 80% 77% 76% Scheme Recycling of erbium triflate over three consecutive cycles under microwave irradiation o Table Reaction of anisole and benzoic acid catalyzed by metal triflates under microwave irradiation at 220 C over 30 min.a a c Metal triflate Yield (%) LiOTf Cu(OTf)2 26 Bi(OTf)3 Pr(OTf)3 46 Er(OTf)3 78 (72)c La(OTf)3 50 Gd(OTf)3 70 Dy(OTf)3 50 Ce(OTf)3 54 10 Yb(OTf)3 24 11 Nd(OTf)3 56 12 Tm(OTf)3 60 Anisole (5 mmol), benzoic acid (1 mmol) b b Entry Yields were determined by GC analysis Isolated yield in parentheses Table Benzoylation of anisole with aromatic carboxylic acids catalyzed by Er(OTf)3 under microwave irradiation Entry Ar-COOH Product Yield (%)b 72c 27d 61e 80f 45 57 g h a i 44 15j 20 k a Anisole (5 mmol), aromatic carboxylic acid (1 mmol), erbium triflate (0.1 mmol), microwave irradiation (CEM Discover), 220 °C, 30 unless otherwise noted Aromatic carboxylic acids such as 2-methoxybenzoic acid, 2-nitrobenzoic acid and 3nitrobenzoic acid did not give rise to ketones b Isolated yield c Side products are methyl benzoate and phenyl benzoate d Side products are phenyl 4-methoxybenzoate and methyl 4-methoxybenzoate, and 2- and 4-methylanisole e Side products are phenyl 2-methylbenzoate and methyl 2-methylbenzoate, and 2- and 4-methylanisole f Side products are phenyl 2-chlorobenzoate and 2- and 4-methylanisole In addition, 9H-xanthen-9-one was formed g Side products are phenyl 2-fluorobenzoate and 2- and 4-methylanisole In addition, 9H-xanthen-9-one was formed h Side products are phenyl 2-bromobenzoate and 2- and 4-methylanisole In addition, 9H-xanthen-9-one was formed i Side products are phenyl 4-bromobenzoate and 2- and 4-methylanisole j The major product is fluorenone k Side product is methyl naphthalene-2-carboxylate Table Acylation of aromatic compounds with benzoic acids catalyzed by Er(OTf)3 under microwave irradiation.a Entry Aromatic compound Product Yield (%)b 72 24c 23d 41e 15f 65g X traceh O tracei a Arene (5 mmol), aromatic carboxylic acid (1 mmol), erbium triflate (0.1 mmol), microwave irradiation, 220 °C, 30 b i Isolated yield c Side products are 2-methoxyphenol, methyl benzoate, 3,4-dimethoxytoluene and 2-methoxyphenyl benzoate d Side products are methyl benzoate, 2,4-dimethoxytoluene and 3-methoxyphenyl benzoate e Side products are methyl benzoate, 2,5-dimethoxytoluene and 4-methoxyphenyl benzoate f Ratio of o-/m-/p- isomers = 15/1/84 as determined by GC g Ratio of o-/p- isomers = 6/94 as determined by GC h No reaction took place with halobenzenes (X: F, Cl, Br, I) The nitro compound decomposed Table Benzoylation of aromatic compounds with benzoic acids catalyzed by Er(OTf)3 under microwave irradiation.a Entry Aromatic compound Conditions Product Yield (%)b Isomer ratioc 190 °C, 20 95 6/0/94 180 °C, 20 80 22/4/74 180 °C, 20 78 14/7/79 220 °C, 20 87 14/9/77 90 8/11/81 90 100 83 100 O i-Pr 220 °C, 20 Ph i-Pr Me Ph 180 °C, 20 Me O Me 180 °C, 20 O i-Pr i-Pr Me Ph 220 °C, 20 80 18/82 78 100 80 >95 85 41/59 82 >95 Me Ph Ph 250 °C, 30 Ph O 10 220 °C, 20 O 11 218 °C, 30 Ph Ph 12 a 220 °C, 20 O Arene (1 mmol), benzoic acid (2 mmol), erbium triflate (0.1 mmol) Anisole and dimethoxybenzenes were demethylated using this method and did not give rise to ketones (see text) b Isolated yield c Isomer ratio was determined by GC Table Benzoylation of toluene and mesitylene with aromatic carboxylic acids catalyzed by Er(OTf)3 under microwave irradiation.a Yield (%)b Isomer ratioc 81 20/16/64 85 15/21/64 78 22/8/70 69 20/7/73 79 10/19/71 Entry Substrate Aromatic acid COOH Cl COOH Br Product 55 24/4/72 85 100 76 100 78 100 80 100 11 86 100 12 80 100 COOH F COOH Cl 10 COOH Me a Toluene (1 mmol), mesitylene (1 mmol), aromatic carboxylic acid (2 mmol), erbium triflate (0.1 mmol) b Isolated yield c Isomer ratio was determined by GC  ...Graphical Abstract Erbium trifluoromethanesulfonate catalyzed Friedel-Crafts acylation using aromatic carboxylic acids as acylating agents under monomode-microwave irradiation Leave this area... homepage: www.elsevier.com Erbium trifluoromethanesulfonate catalyzed Friedel–Crafts acylation using aromatic carboxylic acids as acylating agents under monomodemicrowave irradiation Phuong Hoang Trana,... of erbium triflate was successful in Friedel–Crafts acylations of activated aromatic compounds with aromatic carboxylic acids under microwave irradiation The recycling of erbium triflate was