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DOI: 10.1055/s-0033-1338386
Benzylation of Aromatic Compounds Catalyzed by 3-Methyl-1-sulfonic Acid Imidazolium Tetrachloroaluminate and Silica Sulfuric Acid under Mild Conditions
Publication History
Received: 01 February 2013
Accepted after revision: 24 March 2013
Publication Date:
16 April 2013 (online)
Abstract
In this work, efficient procedures for benzylation of a range of aromatic compounds by benzyl acetate in the presence of catalytic amounts of 3-methyl-1-sulfonic acid imidazolium tetrachloroaluminate ([Msim]AlCl4) or silica sulfuric acid (SSA) under mild conditions are described. Simple methodology, easy workup procedure, clean reaction and reusability of the catalyst are some advantages of this work.
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Key words
benzylation - benzyl acetate - 3-ethyl-1-sulfonic acid imidazolium tetrachloroaluminate - silica sulfuric acidDiarylmethanes are an interesting group of bioactive compounds, and are also very important and useful in organic synthesis.[1] [2] [3] Catalytic systems used for the benzylation of aromatic compounds include HfO2–HfCl4,[3] aliquat 336,[4] phosphomolybdic acid supported silica gel,[5] TMSOTf,[6] organozinc and organoboron derivatives, HCl, H2SO4, MeSO3H, HOTf and HNTf2.[7] However, some of these procedures suffer from various drawbacks, such as complex workup and purification, production of significant amounts of waste materials, use of strongly acidic conditions, and occurrence of side reactions with poor yields.
In recent years, silica sulfuric acid (SSA), as one of the more important solid acid catalysts,[8] [9] [10] [11] [12] and 3-methyl-1-sulfonic acid imidazolium tetrachloroaluminate ([Msim]AlCl4), as a new sulfonic acid functionalized imidazolium salts (SAFIS) with interesting applications, have been reported (Figure [1]).[13] [14] [15]
Herein, we describe the use of SSA and [Msim]AlCl4 as catalysts for benzylation of a range of aromatic compounds under mild conditions to give diarylmethanes, and compare their catalytic activity in this reaction (Scheme [1]). The promising aspects of this methodology are its efficiency, high yield, cleaner reaction profile, and simplicity, rendering it an attractive process for the preparation of diarylmethanes.




To optimize the reaction conditions, we selected the reaction of anisole and benzyl acetate as a model reaction (Scheme [2]).


We observed that the best results were obtained using 5 mol% of [Msim]AlCl4 at 70 °C or 7 mol% of SSA at 80 °C (Table [1]).
a Isolated yield.
In a further study, the model reaction was tested with [Msim]AlCl in several solvents; the corresponding results are displayed in Table [2], indicating that the best result was obtained under solvent-free conditions.
a Isolated yield.
b Reaction was carried out at 70 °C.
Subsequently we examined various aromatic substrates with benzyl acetate as electrophile under solvent-free conditions and with solvent using SSA or [Msim]AlCl4 as catalyst (Table [3]). In certain instances, reaction occurred in the absence of solvent (Table [3] entries 1–3, 7 and 8) and in others, CH2Cl2 was the reaction solvent. Substrates with electron-donating substituents gave high yields of benzylated products under mild conditions.
Mendoza et. al have reported benzylation of certain aromatic substrates using HOTf as a Brønsted acidic catalyst but indole was found to react poorly with the acid catalyst, leading to decomposition products and unreacted indole.[7] However, with our protocol, 3-benzyl indole was obtained in good yield using [Msim]AlCl4 or SSA as catalyst (Table [3], entry 6). Generally, our protocol is more efficient than previous methods;[16] with substrates possessing additional electron-donating groups, the yield increased and reaction time decreased (Table [3], entries 1 and 2) and, with phenol as substrate, the p-benzylphenol was produced rather than the o-benzylphenol. Furthermore, with indole, trimerization was not observed.[17]
Entry |
Nucleophile |
Main product |
Time (h)/ Yielda (%) {[Msim]AlCl4} |
Time (h)/ Yielda (%) (SSA) |
Time (h)/ Yielda,b (%) |
1 |
![]() |
![]() |
1/93 |
1/89 |
1/86[7] |
2 |
![]() |
![]() |
1/85 |
1/83 |
|
3 |
![]() |
![]() |
1.5/86 |
1.5/82 |
0.5/23[21] |
4c |
![]() |
![]() |
2/95 |
2/89 |
|
5c |
![]() |
![]() |
2/89 |
2/80 |
– |
6c |
![]() |
![]() |
3/87 |
3/80 |
18/69[16] |
7 |
![]() |
![]() |
2/86 |
2/81 |
40/63[3] |
8 |
![]() |
![]() |
2.5/92 |
3/87 |
40/87[3] |
9c,d |
![]() |
![]() |
3/95 |
4/90 |
– |
10c,e |
![]() |
![]() |
3.5/90 |
4.5/86 |
– |
a Isolated yield.
b Literature results compared with the presented results.
c These reactions were carried out in CH2Cl2 under reflux.
d Ratio of benzyl acetate to nucleophile was 4:1.
e Ratio of benzyl acetate to nucleophile was 2:1.
The recyclability of the catalyst was also investigated with the reaction of anisole and benzyl acetate as a model reaction. The reaction mixture was extracted with CH2Cl2 to separate products form the [Msim]AlCl4 or with ethanol to separate from the SSA. Subsequently, the catalyst was employed for a second cycle reaction. We observed that the catalytic activity of [Msim]AlCl4 (Table [4]) and SSA (Table [5]) was retained, within the limits of experimental error, for five and four successive runs, respectively.
a Isolated yield.
a Isolated yield.
In summary, we have reported two new procedures[22] for the preparation of diarylmethanes by the benzylation of a range of aromatic compounds using 3-methyl-1-sulfonic acid imidazolium tetrachloroaluminate {[Msim]AlCl4} or silica sulfuric acid (SSA) as heterogeneous catalysts with high yields under mild conditions.
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Acknowledgement
The authors gratefully acknowledge partial support of this work by the Research Affairs Office of Bu-Ali Sina University (Grant number 32-1716: Development of Chemical Methods, Reagents and molecules), and the Center of Excellence in Development of Chemical Method (CEDCM), Hamedan, I. R. Iran.
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References and Notes
- 1 Choudary BM, Mulukutla RS, Klabunde KJ. J. Am. Chem. Soc. 2003; 125: 2020
- 2 Martínez AG, Barcina JO, Heras Md. R. C, Cerezo ÁD. F. Org. Lett. 2000; 2: 1377
- 3 Zhang CY, Gao XQ, Zhang JH, Peng XJ. Chin. Chem. Lett. 2009; 20: 913
- 4 Yang H.-M, Lin C.-L. J. Mol. Catal. A: Chem. 2003; 206: 67
- 5 Yadav JS, Reddy BV. S, Reddy AS. J. Mol. Catal. A: Chem. 2008; 280: 219
- 6 Theerthagiri P, Lalitha A. Tetrahedron Lett. 2010; 51: 5454
- 7 Mendoza O, Rossey G, Ghosez L. Tetrahedron Lett. 2011; 52: 2235
- 8 Mohammadpoor-Baltork I, Moghadam M, Tangestaninejad S, Mirkhani V, Mohammadiannejad-Abbasabadi K, Zolfigol MA. Compt. Rend. Chim. 2011; 14: 934
- 9 Veisi H. Tetrahedron Lett. 2010; 51: 2109
- 10 Shirini F, Sadeghzadeh P, Abedini M. Chin. Chem. Lett. 2009; 20: 1457
- 11 Zolfigol MA. Tetrahedron 2001; 57: 9509
- 12 Khalafi-Nezhad A, Parhami A, Soltani Rad MN, Zolfigol MA, Zare A. Tetrahedron Lett. 2007; 48: 5219
- 13 Zolfigol MA, Kazaei A, Moosavi-Zare AR. J. Org. Chem. 2012; 77: 3640
- 14 Zolfigol MA, Khazaei A, Moosavi-Zare AR, Zare A, Khakyzadeh V. Appl. Catal. A: Gen. 2011; 400: 70
- 15 Zolfigol MA, Khazaei A, Moosavi-Zare AR, Zare A. Org. Prep. Proced. Int. 2012; 42: 95
- 16 Zhu Y, Rawal VH. J. Am. Chem. Soc. 2011; 134: 111
- 17 Valentine RA, Whyte A, Awaga K, Robertson N. Tetrahedron Lett. 2012; 53: 657
- 18 Silva M, Costa C, Pinto M, Lachter E. Reactive Polymers 1995; 25: 55
- 19 Podder S, Roy S. Tetrahedron 2007; 63: 9146
- 20 Guo Q, Li L, Chen L, Wang Y, Ren S, Shen B. Energy Fuels 2009; 23: 51
- 21 Keglevich G, Balint E, Karsai E, Grün A, Balint M, Greiner S. Tetrahedron Lett. 2008; 49: 5039
- 22 General Procedure for the Synthesis of Diarylmethanes Using [Msim]AlCl4 : To a mixture of aromatic substrate (1 mmol) and benzyl acetate (1 mmol) was added [Msim]AlCl4 (0.017 g, 5 mol%) and the mixture was heated to 70 °C for the appropriate time (Table 3). After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to r.t. and CH2Cl2 (2–5 mL) was added to separate the catalyst. After filtration, the pure product was obtained using plate chromatography on silica gel with n-hexane–EtOAc (10:2) as eluent. Note: In some cases, the reaction was carried out in CH2Cl2 (5 mL) at reflux (Table 3, entries 4–6, 9 and 10). General Procedure for the Synthesis of Diarylmethanes Using SSA: To a mixture of aromatic substrate (1 mmol), and benzyl acetate (1 mmol), silica sulfuric acid (0.054 g, 7 mol%) was added and the mixture was heated to 80 °C for the appropriate time (Table 3). After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to r.t. Absolute EtOH (2–5 mL) was added and the mixture was filtered to separate the catalyst. The pure product was obtained using plate chromatography on silica gel with n-hexane–EtOAc (10:2) as eluent. Note: In some cases, the reaction was carried out in CH2Cl2 (5 mL) at reflux (Table 3, entries 4–6, 9 and 10).
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References and Notes
- 1 Choudary BM, Mulukutla RS, Klabunde KJ. J. Am. Chem. Soc. 2003; 125: 2020
- 2 Martínez AG, Barcina JO, Heras Md. R. C, Cerezo ÁD. F. Org. Lett. 2000; 2: 1377
- 3 Zhang CY, Gao XQ, Zhang JH, Peng XJ. Chin. Chem. Lett. 2009; 20: 913
- 4 Yang H.-M, Lin C.-L. J. Mol. Catal. A: Chem. 2003; 206: 67
- 5 Yadav JS, Reddy BV. S, Reddy AS. J. Mol. Catal. A: Chem. 2008; 280: 219
- 6 Theerthagiri P, Lalitha A. Tetrahedron Lett. 2010; 51: 5454
- 7 Mendoza O, Rossey G, Ghosez L. Tetrahedron Lett. 2011; 52: 2235
- 8 Mohammadpoor-Baltork I, Moghadam M, Tangestaninejad S, Mirkhani V, Mohammadiannejad-Abbasabadi K, Zolfigol MA. Compt. Rend. Chim. 2011; 14: 934
- 9 Veisi H. Tetrahedron Lett. 2010; 51: 2109
- 10 Shirini F, Sadeghzadeh P, Abedini M. Chin. Chem. Lett. 2009; 20: 1457
- 11 Zolfigol MA. Tetrahedron 2001; 57: 9509
- 12 Khalafi-Nezhad A, Parhami A, Soltani Rad MN, Zolfigol MA, Zare A. Tetrahedron Lett. 2007; 48: 5219
- 13 Zolfigol MA, Kazaei A, Moosavi-Zare AR. J. Org. Chem. 2012; 77: 3640
- 14 Zolfigol MA, Khazaei A, Moosavi-Zare AR, Zare A, Khakyzadeh V. Appl. Catal. A: Gen. 2011; 400: 70
- 15 Zolfigol MA, Khazaei A, Moosavi-Zare AR, Zare A. Org. Prep. Proced. Int. 2012; 42: 95
- 16 Zhu Y, Rawal VH. J. Am. Chem. Soc. 2011; 134: 111
- 17 Valentine RA, Whyte A, Awaga K, Robertson N. Tetrahedron Lett. 2012; 53: 657
- 18 Silva M, Costa C, Pinto M, Lachter E. Reactive Polymers 1995; 25: 55
- 19 Podder S, Roy S. Tetrahedron 2007; 63: 9146
- 20 Guo Q, Li L, Chen L, Wang Y, Ren S, Shen B. Energy Fuels 2009; 23: 51
- 21 Keglevich G, Balint E, Karsai E, Grün A, Balint M, Greiner S. Tetrahedron Lett. 2008; 49: 5039
- 22 General Procedure for the Synthesis of Diarylmethanes Using [Msim]AlCl4 : To a mixture of aromatic substrate (1 mmol) and benzyl acetate (1 mmol) was added [Msim]AlCl4 (0.017 g, 5 mol%) and the mixture was heated to 70 °C for the appropriate time (Table 3). After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to r.t. and CH2Cl2 (2–5 mL) was added to separate the catalyst. After filtration, the pure product was obtained using plate chromatography on silica gel with n-hexane–EtOAc (10:2) as eluent. Note: In some cases, the reaction was carried out in CH2Cl2 (5 mL) at reflux (Table 3, entries 4–6, 9 and 10). General Procedure for the Synthesis of Diarylmethanes Using SSA: To a mixture of aromatic substrate (1 mmol), and benzyl acetate (1 mmol), silica sulfuric acid (0.054 g, 7 mol%) was added and the mixture was heated to 80 °C for the appropriate time (Table 3). After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to r.t. Absolute EtOH (2–5 mL) was added and the mixture was filtered to separate the catalyst. The pure product was obtained using plate chromatography on silica gel with n-hexane–EtOAc (10:2) as eluent. Note: In some cases, the reaction was carried out in CH2Cl2 (5 mL) at reflux (Table 3, entries 4–6, 9 and 10).

























