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 4 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 2 to 6 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.
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