- Email: [email protected]
Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols
Accepted Manuscript Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols Abhilash S. Singh, Suresh S. Shendage, Jayashree M. Nagarkar PII: DOI: Reference:
S0040-4039(14)01946-7 http://dx.doi.org/10.1016/j.tetlet.2014.11.053 TETL 45437
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
2 October 2014 10 November 2014 13 November 2014
Please cite this article as: Singh, A.S., Shendage, S.S., Nagarkar, J.M., Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/ j.tetlet.2014.11.053
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 To create your abstract, type over the instructions in the template box below. Fonts or abstract dimensions should not be changed or altered.
Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols Abhilash S.Singh, Suresh S. Shendage, Jayashree M. Nagarkar*
Leave this area blank for abstract info.
1
Tetrahedron Letters journal homepage: www.elsevier.com
Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols Abhilash S Singha , Suresh S Shendagea and Jayashree M Nagarkara a
Department of Chemistry, Institute of Chemical Technology (Deemed), N. M. Parekh Marg, Matunga, Mumbai – 400 019, India.
ARTICLE INFO
ABSTRACT
Article history: Received Received in revised form Accepted Available online
Deep eutectic solvents (such as the combination of urea and choline chloride) are found to be promising solvent and phase-transfer-media for benzylation of phenol. These methods avoided the complexity of multiple alkylations giving selectively O-alkylated aromatic products. Good to excellent yields of the corresponding benzyl phenyl ether were obtained. The non-toxic, biodegradable, inexpensive and recyclable nature of DES makes this protocol green and costeffective.
Keywords: Benzylation Phenols O-alkylation Deep eutectic solvent
Benzylation of phenol is an important synthesis reaction and was first published in 1974 by McKillop using dichloromethane as a solvent.1 These aromatic ethers are widely used in perfume, flavor, agriculture, and pharmaceutical industries. A variety of procedures have been developed in which various catalysts have been utilized some of these includes the use of crown ethers,2 phase-transfer catalysis (PTC),3 other methodologies includes use of ionic liquids,4 microwave method etc.5 However, most of the commonly used methods require costly alkali metal hydrides and phase-transfer catalysis conditions. Besides these the other reported methods for benzylation of phenols were carried out in volatile, toxic and flammable organic solvents such as acetonitrile, dichloromethane, 2-methyltetrahydrofuran and alcohols. The other polar aprotic solvents such as DMSO and DMF were also employed. However, extensive C-alkylation takes place in the presence of water and trifluoroethanol.6-7 Hence, these factors greatly limit the scope of selective procedures for O-alkylation of aromatic phenols. However, in most of the above described protocols it is very difficult to separate the catalyst and product from the reaction mixture. Therefore the development of alternative solvents which is environmentally benign, cheap, inexpensive, easily available and non-toxic is highly desirable. With the view of green chemistry concern in synthetic organic chemistry, new ‘‘green solvents’’ have been introduced.8 In this context, Ionic liquids (ILs) have appeared as a new class of promising solvents. However, recent studies have shown that there are several ILs which have hazardous toxicity and possess very poor biodegradability.9 To overcome the problem associated with ILs ———
2009 Elsevier Ltd. All rights reserved.
a greener solvent called DES (Deep Eutectic Solvents) was introduced. DES is similar to ILs which forms a eutectic by mixing two components with a melting point much lower than either of the individual components. DES is mainly prepared by mixing ammonium salt such as choline chloride with the hydrogen-bond donor like urea, or Lewis acids.10 DES possesses properties such as nontoxicity, biodegradability, low price and easy availability. DES is widely used in electrochemical applications and is devoted as promising solvents for catalysis. 11 They have also received substantial attention in the area of enzymatic synthesis as enzymes are highly stable and more active in DES compared to other solvents.12 At the same time, there has been relatively little work explored on the ability of DES to serve as catalyst and solvents in the field of synthetic organic chemistry.13 With our continued interest in the synthesis and application of DES,14 herein we wish to report benzylation of phenols using deep eutectic solvent of choline chloride and urea. Initially the reaction of benzyl bromide and phenol was selected as model reaction. The reactions were carried at different temperatures and the best yield was obtained at 80°C (Table 1, entries 1-4). It was found that the reaction proceeded smoothly in ChCl:urea based DES to afford the anticipated product in 96% yield after 2 h (Table 1, entry 3). Lower yield was obtained in other biodegradable media such as ChCl:glycerol and ChCl:ZnCl2 (Table 1, entry 5-6). The reaction was also investigated in the presence of various inorganic bases such as NaOH, K2CO3 and K3PO4 (Table 1, entries 7-9). Among all the bases, the KOH gave the highest yield of the desired product. No benzylation of phenol was observed when the reaction was
Corresponding author. Tel.: +0-000-000-0000; fax: +0-000-000-0000; e-mail: [email protected]
2
Tetrahedron
studied in the absence of DES, demonstrating the necessity of DES under given reaction conditions (Table 1, entry 10).
Table 1. Optimization of reaction conditionsa
Entry
DES
Temp.(°C)
Base
Yieldb (%)
1
ChCl:urea
40
KOH
46
2
ChCl:urea
60
KOH
75
3
ChCl:urea
80
KOH
96
4
ChCl:urea
100
KOH
97
5
Chcl:glycerol
80
KOH
84
6
ChCl:ZnCl2
80
KOH
67
7
ChCl:urea
80
K2CO3
88
8
ChCl:urea
80
K3PO4
84
9
ChCl:urea
80
NaOH
94
10c
-
80
KOH
N.R
10
94
11
95
12
93
13
92
14
75
15
82
16
81
17
85
18
87
19
90
20
90
a
Reaction condition: Phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL), base (2.0 equiv), 2 h and Temperature (80° C) b
Isolated yields
c
Without DES
Table 2. Benzylation of phenols with benzyl bromide by using deep eutectic solvent (choline chloride: urea)a Entry
Benzyl bromide
Phenol
Product
b
Yield (%)
a
Reaction conditions: phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL) and base (2.0 equiv), 2h, Temperature (80° C) b
Isolated yields
1
96
2
95
3
90
4
90
5
80
6
84
7
85
8
89
9
95
With the optimized conditions at hand, the DES-catalyzed reactions were further explored for the substrate scope of different aromatic phenols and the results were summarized in table 2.15 Structurally diverse phenols with different functional groups such as methyl, methoxy, chloro, bromo, tertiary butyl, acyl, and trifluoromethyl groups underwent benzylation with various aromatic bromides under mild reaction conditions and the product were isolated in good to excellent yield. The highest yield of the desired product was observed for the sterically hindered substrates (Table 2, entries 2-4). Moderate to good yield were obtained for electron-donating substituent on the aromatic ring (Table 2, entries 5-8). The deactivated phenols were found to be more active for O-alkylation (Table 2, entries 9-13). The etherification products of naphthol and heterocycle phenol were also studied and moderate yields was obtained (Table 2, entries 14-15). The study was further continued using different substituents on the phenyl ring of benzyl bromide. Different functional group has little effect at the aryl moiety of benzyl bromides. It was observed that the benzyl bromides bearing electron-withdrawing groups produced the corresponding products in slightly higher yields electron-donating groups. Good to excellent yield were observed for different electronic substituents on benzyl bromide (Table 2, entries 16-20). Hence an electron-withdrawing group favours the formation of the benzyl phenyl ethers, in contrast to those with electron-donating groups.
3 Recyclability of the DES was examined for the benzylation of phenol using benzyl bromide under optimized reaction parameters. After completion, the reaction mixture was cooled to room temperature and water (2mL) was added. The product was extracted with ethyl acetate and DES was dried was under vacuum at 80 °C. The reusability of the deep eutectic solvent was tested for five consecutive cycles but a decrease in the catalytic activity of DES was observed after the fourth cycle (Table 3).
References and notes 1. 2. 3.
Table 3. Recyclability of DES solventa Entry
Recycling
Yieldb (%)
1
1st
94
2
2nd
93
3
rd
3
90
4
4th
88
5
th
72
5
4.
5.
a
Reaction condition: Phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL), base (2.0 equiv), 2 h and Temperature (80° C) b
Isolated yields
Table 4. Comparison of different methodology benzylation of phenols with benzyl bromide
for
Entry
External Catalyst
Solvent
Time(h)/ Temp.(°C)
Yield (%)
Reusa -bility
Ref.
1
-
DES
2 /(80)
96
Present work
2
-
[bmIm] OH
2 /(80)
92
4b
3
CS2CO3
CH3CN
5/(80)
92
5a
4
PEGAmmon ium salt
DCM
1/(30)
89
3b
5
TBAB
Water
2/(28)
97
3h
6. 7. 8. 9.
10. 11.
The efficacy of the DES for benzylation of phenols with benzyl bromide was compared with some earlier reported catalysts as demonstrated in Table 4. Even though TBAB gives better result at lower temperature it suffers from the drawbacks of reusability of catalyst and solvent which is desirable from economic and environmental point of view (Table 4, entry 5). In conclusion, the greener protocol described in this paper offer a new scope for benzylation of various aromatic phenols using the deep eutectic mixture as green catalyst/solvent. The DES was used as the catalyst and solvent there by giving good to excellent yield of the desired product. This protocol offers advantages with regard to high yields of products and the greenness of procedure as no external hazardous organic solvents and toxic catalysts was used. The DES is eco-friendly, safe, biodegradable, non-toxic, inexpensive, easily available and recyclable solvent. Hence this protocol offers marked improvements in terms of simplicity, recyclability and procedure for the preparation of benzyl phenyl ethers.
Acknowledgments The author gratefully acknowledges the financial assistance from the Green Tech. U.P.E. Government of India.
12.
13.
14. 15.
McKillop, A.; Fiaud, J. C.; Hug, R. P. Tetrahedron 1974, 30, 1379-1382. Lissel, M.; Schimidt, S.; Neumann, B. Synthesis 1986, 5, 382-383. (a) Srivastava, P.; Srivastava, R. Tetrahedron Lett. 2007, 48, 4489-4493; (b) Benaglia, M.; Cinquini, M.; Cozzi, F.; Tocco, G. Tetrahedron Lett. 2002, 43, 3391-3393; (c) Albanese, D.; Benaglia, M.; Landini, D.; Maia, A.; Lupi, V.; Penso, M. Ind. Eng. Chem. Res. 2002, 41, 4928-4935; (d) Tamami, B.; Ghasemi, S. J. Iran. Chern. Soc. 2008, 5, 26-32; (e) Chen, Z. X.; Xu, G. Y.; Yang, G. C.; Wang, W. React. Funct. Polym. 2004, 61, 139-146; (f) Denmark, S. D.; Weintraub, R. C.; Gould, N. D. J. Am. Chem. Soc. 2012, 134, 13415-13429; (g) Coleman, M. T.; LeBlanc, G. Org. Process Res. Dev. 2010, 14, 732-736; (h) Wang, H.; Ma, Y.; Tian, H.; Yu, A.; Chang, J.; Wu, Y. Tetrahedron 2014, 70, 26692673. a) Badri, M.; Brunet, J. J. Tetrahedron Lett. 1992, 33, 4435-4438; (b) Mohanazadeh, F.; Aghvami, M.; Monatsh. Chem. 2007, 138, 47-49; (c) Lourenco, N. M. T.; Afonso, C. A. M. Tetrahedron 2003, 59, 789-794. (a) Lee, J. C.; Yuk, J. Y.; Cho, S. H. Synth. Commun. 1995, 25, 1367-1370; (b) Godfrey, J. D.; Mueller, R. H.; Sedergran, T. C.; Soundararajan, N.; Colandrea, V. J. Tetrahedron Lett. 1994, 35, 6405-6408; (c) Taniguchi, H.; Nomura, E. Chem. Lett. 1988, 1773-1776; (d) Zhang, M.; Flynn, D. L.; Hanson, P. R. J. Org. Chem. 2007, 72, 3194-3198; (e) Tanigushi, H.; Otsuji, Y.; Nomura, E. Bull. Chem. Soc. Jpn. 1995, 68, 3563-3567; (f) Shah, S.T. A.; Khan, K. M.; Hussain, H.; Anwar, M. U.; Fecker, M.; Voelter, W. Tetrahedron 2005, 61, 6652-6656; (g) Shah, S. T. A.; Khan, K. M.; Heinrich, A. M.; Choudhary, M. I.; Voelter, W. Tetrahedron Lett. 2002, 43, 8603-8606; (h) Brown Ripin, D. H.; Vetelino, M. Synlett. 2003, 2353-2353; (i) Kendall, J. T. J. Labelled Compd. Radiopharm. 2000, 43, 505-514; (j) Bu, X. L.; Jing, H. W.; Wang, L.; Chang, T.; Jin, L. L.; Liang, Y. M. J. Mol. Catal. A: Chem. 2006, 259, 121-124; (k) Gathirwa, J. W.; Maki, T. Tetrahedron 2012, 68, 370-375; (l) Alauddin, M. M.; Miller, J. M.; Clark, J. H.; Can. J. Chem. 1984, 62, 263-265; Kornblum, N.; Berrigan, P. J.; Le Noble, W. J. J. Am. Chem. Soc. 1963, 85, 1141-1147. Kornblum, N.; Seltzer, R.; Paul, H. J. Am. Chem. Soc.1963, 85, 1148-1154. Sheldon, R. A., Green Chem. 2005, 7, 267-278. (a) Romero, A.; Santos, A.; Tojo, J.; Rodriguez, A. J. Hazard. Mater. 2008, 151, 268-273; (b) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123-150. Abbott, A.P, Davies, D. L.; Capper, G.; Rasheed, R. K.; Tambyrajah, V.; US Patent 7,183, 433, 2007. (a) Jhong, H. R.; Wong, D. S. H.; Wan, C. C.; Wang, Y. Y.; Wei, T. C.; Electrochem. Commun. 2009, 11, 209-211 (b) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Ryder, K. S.; J. Electroanal. Chem. 2007, 599, 288-294. (a) Gorke, J. T.; Srienc, F.; Kazlauskas, R. J. Chem. Commun. 2008, 1235-1237; (b) Zhao, H.; Baker, G. A.; Holmes, S. Org. Biomol. Chem. 2011, 9, 1908-19016. (a) Abbott, A. P.; Bell, T. J.; Handa, S.; Stoddart, B. Green. Chem. 2005, 7, 705-707; (b) Morales, R. C.; Tambyrajah, V; Jenkins, P. R.; Davies, D. L.; Abbott, A. P. Chem. Commun. 2004, 2, 158159; (c) Sonawane, Y. A.; Phadtare, S. B.; Borse, B. N.; Jagtap, A. R.; Shankarling, G. S. Org. Lett. 2010, 12, 1456-1459; (d) Phadtare, S. B.; Shankarling, G. S. Green Chem. 2010, 12, 458462. Patil, U. B.; Singh, A. S.; Nagarkar, J. M.; RSC Advances, 2014, 4, 1102-1106. Representative reaction conditions: Phenol (0.5 mmol), benzyl bromide (1.2 mmol) and KOH (2.0 mmol) was added to the DES (1mL) and heated at temperature (80° C) for 2hr. After cooling to room temperature water was added and the product was extracted with ethyl acetate (1x 3 mL) and analyzed by GC-MS after the addition of hexamethylbenzene as an internal standard. The product was purified using column chromatography on silica gel (hexane/EtOAc :: 99:1). The pure product was characterized by 1H NMR and 13C NMR.
S0040-4039(14)01946-7 http://dx.doi.org/10.1016/j.tetlet.2014.11.053 TETL 45437
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
2 October 2014 10 November 2014 13 November 2014
Please cite this article as: Singh, A.S., Shendage, S.S., Nagarkar, J.M., Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/ j.tetlet.2014.11.053
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 To create your abstract, type over the instructions in the template box below. Fonts or abstract dimensions should not be changed or altered.
Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols Abhilash S.Singh, Suresh S. Shendage, Jayashree M. Nagarkar*
Leave this area blank for abstract info.
1
Tetrahedron Letters journal homepage: www.elsevier.com
Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols Abhilash S Singha , Suresh S Shendagea and Jayashree M Nagarkara a
Department of Chemistry, Institute of Chemical Technology (Deemed), N. M. Parekh Marg, Matunga, Mumbai – 400 019, India.
ARTICLE INFO
ABSTRACT
Article history: Received Received in revised form Accepted Available online
Deep eutectic solvents (such as the combination of urea and choline chloride) are found to be promising solvent and phase-transfer-media for benzylation of phenol. These methods avoided the complexity of multiple alkylations giving selectively O-alkylated aromatic products. Good to excellent yields of the corresponding benzyl phenyl ether were obtained. The non-toxic, biodegradable, inexpensive and recyclable nature of DES makes this protocol green and costeffective.
Keywords: Benzylation Phenols O-alkylation Deep eutectic solvent
Benzylation of phenol is an important synthesis reaction and was first published in 1974 by McKillop using dichloromethane as a solvent.1 These aromatic ethers are widely used in perfume, flavor, agriculture, and pharmaceutical industries. A variety of procedures have been developed in which various catalysts have been utilized some of these includes the use of crown ethers,2 phase-transfer catalysis (PTC),3 other methodologies includes use of ionic liquids,4 microwave method etc.5 However, most of the commonly used methods require costly alkali metal hydrides and phase-transfer catalysis conditions. Besides these the other reported methods for benzylation of phenols were carried out in volatile, toxic and flammable organic solvents such as acetonitrile, dichloromethane, 2-methyltetrahydrofuran and alcohols. The other polar aprotic solvents such as DMSO and DMF were also employed. However, extensive C-alkylation takes place in the presence of water and trifluoroethanol.6-7 Hence, these factors greatly limit the scope of selective procedures for O-alkylation of aromatic phenols. However, in most of the above described protocols it is very difficult to separate the catalyst and product from the reaction mixture. Therefore the development of alternative solvents which is environmentally benign, cheap, inexpensive, easily available and non-toxic is highly desirable. With the view of green chemistry concern in synthetic organic chemistry, new ‘‘green solvents’’ have been introduced.8 In this context, Ionic liquids (ILs) have appeared as a new class of promising solvents. However, recent studies have shown that there are several ILs which have hazardous toxicity and possess very poor biodegradability.9 To overcome the problem associated with ILs ———
2009 Elsevier Ltd. All rights reserved.
a greener solvent called DES (Deep Eutectic Solvents) was introduced. DES is similar to ILs which forms a eutectic by mixing two components with a melting point much lower than either of the individual components. DES is mainly prepared by mixing ammonium salt such as choline chloride with the hydrogen-bond donor like urea, or Lewis acids.10 DES possesses properties such as nontoxicity, biodegradability, low price and easy availability. DES is widely used in electrochemical applications and is devoted as promising solvents for catalysis. 11 They have also received substantial attention in the area of enzymatic synthesis as enzymes are highly stable and more active in DES compared to other solvents.12 At the same time, there has been relatively little work explored on the ability of DES to serve as catalyst and solvents in the field of synthetic organic chemistry.13 With our continued interest in the synthesis and application of DES,14 herein we wish to report benzylation of phenols using deep eutectic solvent of choline chloride and urea. Initially the reaction of benzyl bromide and phenol was selected as model reaction. The reactions were carried at different temperatures and the best yield was obtained at 80°C (Table 1, entries 1-4). It was found that the reaction proceeded smoothly in ChCl:urea based DES to afford the anticipated product in 96% yield after 2 h (Table 1, entry 3). Lower yield was obtained in other biodegradable media such as ChCl:glycerol and ChCl:ZnCl2 (Table 1, entry 5-6). The reaction was also investigated in the presence of various inorganic bases such as NaOH, K2CO3 and K3PO4 (Table 1, entries 7-9). Among all the bases, the KOH gave the highest yield of the desired product. No benzylation of phenol was observed when the reaction was
Corresponding author. Tel.: +0-000-000-0000; fax: +0-000-000-0000; e-mail: [email protected]
2
Tetrahedron
studied in the absence of DES, demonstrating the necessity of DES under given reaction conditions (Table 1, entry 10).
Table 1. Optimization of reaction conditionsa
Entry
DES
Temp.(°C)
Base
Yieldb (%)
1
ChCl:urea
40
KOH
46
2
ChCl:urea
60
KOH
75
3
ChCl:urea
80
KOH
96
4
ChCl:urea
100
KOH
97
5
Chcl:glycerol
80
KOH
84
6
ChCl:ZnCl2
80
KOH
67
7
ChCl:urea
80
K2CO3
88
8
ChCl:urea
80
K3PO4
84
9
ChCl:urea
80
NaOH
94
10c
-
80
KOH
N.R
10
94
11
95
12
93
13
92
14
75
15
82
16
81
17
85
18
87
19
90
20
90
a
Reaction condition: Phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL), base (2.0 equiv), 2 h and Temperature (80° C) b
Isolated yields
c
Without DES
Table 2. Benzylation of phenols with benzyl bromide by using deep eutectic solvent (choline chloride: urea)a Entry
Benzyl bromide
Phenol
Product
b
Yield (%)
a
Reaction conditions: phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL) and base (2.0 equiv), 2h, Temperature (80° C) b
Isolated yields
1
96
2
95
3
90
4
90
5
80
6
84
7
85
8
89
9
95
With the optimized conditions at hand, the DES-catalyzed reactions were further explored for the substrate scope of different aromatic phenols and the results were summarized in table 2.15 Structurally diverse phenols with different functional groups such as methyl, methoxy, chloro, bromo, tertiary butyl, acyl, and trifluoromethyl groups underwent benzylation with various aromatic bromides under mild reaction conditions and the product were isolated in good to excellent yield. The highest yield of the desired product was observed for the sterically hindered substrates (Table 2, entries 2-4). Moderate to good yield were obtained for electron-donating substituent on the aromatic ring (Table 2, entries 5-8). The deactivated phenols were found to be more active for O-alkylation (Table 2, entries 9-13). The etherification products of naphthol and heterocycle phenol were also studied and moderate yields was obtained (Table 2, entries 14-15). The study was further continued using different substituents on the phenyl ring of benzyl bromide. Different functional group has little effect at the aryl moiety of benzyl bromides. It was observed that the benzyl bromides bearing electron-withdrawing groups produced the corresponding products in slightly higher yields electron-donating groups. Good to excellent yield were observed for different electronic substituents on benzyl bromide (Table 2, entries 16-20). Hence an electron-withdrawing group favours the formation of the benzyl phenyl ethers, in contrast to those with electron-donating groups.
3 Recyclability of the DES was examined for the benzylation of phenol using benzyl bromide under optimized reaction parameters. After completion, the reaction mixture was cooled to room temperature and water (2mL) was added. The product was extracted with ethyl acetate and DES was dried was under vacuum at 80 °C. The reusability of the deep eutectic solvent was tested for five consecutive cycles but a decrease in the catalytic activity of DES was observed after the fourth cycle (Table 3).
References and notes 1. 2. 3.
Table 3. Recyclability of DES solventa Entry
Recycling
Yieldb (%)
1
1st
94
2
2nd
93
3
rd
3
90
4
4th
88
5
th
72
5
4.
5.
a
Reaction condition: Phenol (1.0 mmol), benzyl bromide (1.2 equiv), DES (1.0 mL), base (2.0 equiv), 2 h and Temperature (80° C) b
Isolated yields
Table 4. Comparison of different methodology benzylation of phenols with benzyl bromide
for
Entry
External Catalyst
Solvent
Time(h)/ Temp.(°C)
Yield (%)
Reusa -bility
Ref.
1
-
DES
2 /(80)
96
Present work
2
-
[bmIm] OH
2 /(80)
92
4b
3
CS2CO3
CH3CN
5/(80)
92
5a
4
PEGAmmon ium salt
DCM
1/(30)
89
3b
5
TBAB
Water
2/(28)
97
3h
6. 7. 8. 9.
10. 11.
The efficacy of the DES for benzylation of phenols with benzyl bromide was compared with some earlier reported catalysts as demonstrated in Table 4. Even though TBAB gives better result at lower temperature it suffers from the drawbacks of reusability of catalyst and solvent which is desirable from economic and environmental point of view (Table 4, entry 5). In conclusion, the greener protocol described in this paper offer a new scope for benzylation of various aromatic phenols using the deep eutectic mixture as green catalyst/solvent. The DES was used as the catalyst and solvent there by giving good to excellent yield of the desired product. This protocol offers advantages with regard to high yields of products and the greenness of procedure as no external hazardous organic solvents and toxic catalysts was used. The DES is eco-friendly, safe, biodegradable, non-toxic, inexpensive, easily available and recyclable solvent. Hence this protocol offers marked improvements in terms of simplicity, recyclability and procedure for the preparation of benzyl phenyl ethers.
Acknowledgments The author gratefully acknowledges the financial assistance from the Green Tech. U.P.E. Government of India.
12.
13.
14. 15.
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