DOI: 10.1039/C7CC05383E
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ChemComm
with GPP-0303 (YSS/2014/001018) project. We are grateful to
the Director, CSIR-NEIST for his keen interests.
60 Notes and references
5
Applied Organic Chemistry, Chemical Sciences & Technology Division,
CSIR-North East Institute of Science and Technology, Jorhat-785006,
AcSIR, India, Fax: +913762370011 Tel.: +91 3762372948;
65 .†Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization data, 1H and 13C NMR spectra of
synthesized compounds. See DOI:
10
Scheme 2. Possible reaction mechanism
1. a) J. Otera and J. Nishikido, Esterification: Methods, Reactions, and
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Application; Wiley-VCH: Weinheim, 2003; b) M. Hosseini Sarvari
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bromo and nitro were found to be very good substrates to provide
dibromoacetophenone 4a-i. In the reaction mixture of 1a and 2a,
the presence of 2,2-dibromoacetophenone, phenylglyoxalic acid
15 and benzoic acid was determined by thin layer chromatography
after four hours. In the absence of the directing group, reaction of
simple phenol with 2a could not provide the corresponding aryl
ester. Furthermore, the reaction of 1a and 2a under the standard
conditions and in the presence of argon provided 43% yield of
20 3aa, indicating the requirement of oxygen for the reaction to
progress. Again, the same reaction when performed under oxygen
balloon, surprisingly, could not provide the desired compound
3aa, instead benzoic acid was isolated as the main product. In a
separate experiment of compound 4a under air, with one
25 equivalent of Cu(OAc)2 in tAmOH, provided a mixture of
phenylglyoxalic acid and benzoic acid (14%) in six hours at 100
oC. Probably, the Cu(II)-mediated conversion of 2-
bromoacetophenone to phenyl glyoxalic acid and finally to
benzoic acid via 2,2-dibromoacetophenone is faster in the
30 presence of excess oxygen. Again, the reaction of 1a and 2,2-
dibromoacetophenone (4a) under the optimized reaction
conditions afforded the same aryl ester 3aa in 70% yield. Under
the same conditions, reaction of 1a with phenylglyoxalic acid
afforded only 11% of 3aa and reaction of 1a with benzoic acid
35 did not work. Based on these observations and literature
precedents,11 a possible mechanism is proposed for this reaction
which is shown in Scheme 2. In the presence of Cu(II), initially,
two molecules of 2 form Cu(II)-complex A, which on subsequent
cleavage affords complex B and compound 4. In the presence of
40 air, complex B forms phenylglyoxalic acid and benzoic acid.11a-c
The carbonyl directed activation of phenolic hydroxyl group in
the presence of Cu(II) and removal of the acidic proton of 4
affords complex C. Further hydrolysis of dibromo compound C
in the presence of Cu and oxygen provides dicarbonyl Cu(II)-
45 complex D,11f which on decarbonylation and reductive
elimination of Cu affords the final compound 3.
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In summary, we have developed a new method for the
directing group assisted synthesis of aryl esters. In this
copper(II)-mediated reaction, ortho-formyl and ortho-acetyl
50 substituted phenols were converted to aryl esters by using 2-
bromoacetophenones as novel but readily available aroylating
agent. In addition, we developed a new copper(II)-mediated
bromine free reaction conditions for the synthesis of very
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important
motif
2,2-dibromoacetophenones
from
2-
55 bromoacetophenones, which should be of synthetic utility.
Acknowledgements
Authors thank SERB, New Delhi, for financially supporting us
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