(467c) The Use of Dimethyl Carbonate for Etherification

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:

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.