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Notes and references
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Selected traditional esterication methods: (a) J. C. Lee,
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Scheme 3 The proposed mechanism.
(
1
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Based on the results of control experiments and previous
5,11b,12
studies,
a possible radical mechanism is proposed
(
{
Scheme 3). Initially, [BPy]I is oxidized by TBHP to generate
+
À
+
À
2
[BPy] [IO] } A or {[BPy] [IO ] } B species. Subsequently, the
benzyl radical C is obtained from the hemolytic cleavage of
a benzylic C–H bond in the presence of A or B, followed by
combination with benzoic acid to give the benzyl ester radical
anion F. Finally, the desired product benzyl ester is produced by
losing an electron from F with the assistance of hydroxyl radical
2009, 130, 505.
3
4
K. M. Khan, G. M. Maharvi, S. Hayat, Z. Ullah, M. I. Choudhary
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Selected esterication protocols through C–H activation: (a)
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Soc., 2006, 128, 6790; (b) A. R. Dick, K. L. Hull and
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D. J. Covell and M. C. White, Angew. Chem., Int. Ed., 2008,
(
Scheme 3, path A). Furthermore, the benzyl radical is easily
oxidized by active iodine species A or B to give the benzyl cation
D and the hydroxide ion simultaneously. Subsequently, benzoic
acid is deprotonated by the hydroxide ion to form the benzoate
anion E. At last, the electrostatic attraction between the
benzoate anion with specie E to yield corresponding product 3a
(
Scheme 3, path B).
It's worth noting that ionic liquid could be easily recycled. 2-
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Chlorobenzoic acid and methylbenzene were chosen as model
substrates to study the reusability of the catalyst [BPy]I under
standard reaction conditions. Aer completion of the reaction,
ethyl acetate and water were added, and then the ionic liquid
[BPy]I was recovered from the aqueous phase and reused
without the signicant loss of its activity (Fig. 1).
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4
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In summary, we have developed classic heterocyclic ionic liquid-
catalyzed oxidative esterication of carboxylic acid derivatives
with benzylic hydrocarbons. Most importantly, a inexpensive and
recyclable ionic liquid [BPy]I is reused for at least four cycles with
similar catalytic activity. Ionic liquid catalyzed C–H bond acti-
vation reactions are ongoing in our laboratory.
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Acknowledgements
This work is supported by Grants from Key Laboratory of Xin-
jiang Uyghur Autonomous Region (2015KL014) and the
National Natural Science Foundation of China (21572195 and
21262035).
23044 | RSC Adv., 2017, 7, 23041–23045
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