Introduction
In this work, we present the use of dimethyl carbonate (DMC) as an O-alkylating agent to prepare ethers from the reaction of alcohol and DMC (Sheme 1).[1] DMC is considered a green alternative for alkylation's reactions, once the final sub products are carbon dioxide and methanol, which can be latter, recovered to DMC.[2]
Scheme 1
Usually, those reactions undergo over high temperature and high pressure, to overcome these hard conditions we have used ultrasound as energy source. Ultrasound has been utilized recently to accelerate a number of synthetically useful reactions.[3],[4],[5],[6] The use of ultrasound in chemistry is called sonochemistry and the ultrasound effects observed on organic reactions are due to cavitation, a physical process that create, enlarge, and implode gaseous and vaporous cavities in an irradiated liquid. Cavitation induces very high local temperatures and pressure inside the bubbles (cavities), leading to a turbulent flow in the liquid and enhanced mass transfer.
Methodology
In a conical flask phenol (5 mmol), DMC (50 mmol), base and additive (see table 1) were added in quickly agitated for homogenization. After 1 minute, an ultrasound horn was introduced into the flask and ultrasound wave was irradiated for 2 hours in the reaction medium (Amplitude= 40; Pulse-on=15 seconds and pulse-off= 10 seconds). The reaction was monitored by GC-MS.
Results and Discussion
The O-alkylation reaction between phenol and dimethyl carbonate was chosen as the model reaction and a variety of conditions were screened (Table 1). Inorganic base such as KOH, t-BuOK and K2CO3 were tested in the solvent dimethylformamide (DMF) (Table 1, entries 6-8), however no products were observed. Potassium carbonate was also tested in the presence of phase transfer catalyst such as tetrabutylammonium chloride (TBACl) and tetrabutylammonium bromide (TBABr) (Table 1, entries 9 and10) and the product was observed in 8 and 15% GC yield, respectively.
Table 1: Screening condition of the O-alkylation of phenol using DMC and ultrasound:
Entry
| Phenol (mmol)
| DMC (mmol)
| Base (mmol)
| Additive (mmol)
| Solvent (ml)
| GC Yield (%)
|
1
| 5
| 60
| DBU (0.5)
| -
| -
| ND
|
2
| 5
| 10
| DBU (0.5)
| -
| DMF (5)
| 9
|
3
| 5
| 60
| DABCO (5)
| -
| -
| 46
|
4
| 5
| 10
| DABCO (5)
| -
| DMF (5)
| ND
|
5
| 5
| 10
| TMEDA (5)
| -
| DMF (5)
| ND
|
6
| 5
| 60
| KOH (5)
| -
| -
| ND
|
7
| 5
| 10
| K2CO3 (5)
| -
| DMF (5)
| ND
|
8
| 5
| 10
| t-BuOK (5)
| -
| DMF (5)
| ND
|
9
| 5
| 10
| K2CO3 (5)
| TBABr (1)
| DMF (3)
| 8
|
10
| 5
| 10
| K2CO3 (5)
| TBACl (1)
| DMF (3)
| 15
|
11
| 5
| 10
| t-BuOK (5)
| TBACl (1)
| DMF (3)
| ND
|
Better results were observed when organic base were used (Table 1, entries 1-5). We have tested 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO) and tetramethylethylenediamine (TMEDA). The best condition was using DMC as the solvent and DABCO in 1 equimolar quantity, the GC yield achieve was 46% (Table 1, entry 3).
Conclusion
In summary, we have showed our initial results in the O-alkylation of phenol using DMC as an alkylating agent using sonochemistry as a new tool to overcome hard conditions. Despite the fact that the yields are still low, the reaction is showing potential. The screening for better conditions is ongoing in our lab.
Acknowledgments
The authors are grateful to FAPESP (Grant 2013/11822-1 and Fellowship 2014/21897-1) for financial support.
[1] TUNDO, P.; ROSSI, L.; LORIS, A. J. Org. Chem., v. 70, p. 2219, 2005.
[2] DELLEDONNE, D.; RIVETTI, F.; ROMANO, U. J. Organomet. Chem. v. 488, p. C15, 1995.
[3] MARGULIS, M. A. High Energ. Chem., v. 38, p. 135, 2004.
[4] Mason, T. J. Chem. Soc. Rev., v. 26, p. 443, 1997.
[5] Cravotto, G.; Cintas, P. Chem. Soc. Rev., v. 35, p. 180, 2006.
[6] Suslick, K. S. Science, v. 247, p.1439, 1990.
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