D
Synlett
S. Nandy et al.
Letter
ry alcohol was further established through acetylation of 8
References and Notes
(
Scheme 4) which was confirmed by a singlet at δ = 2.10
(
1) Verma, S.; Baig, R. B. N.; Han, C.; Nadagouda, M. N.; Varma, R. S.
Green Chem. 2016, 18, 251.
1
ppm (in the H NMR spectrum) as well as a signal at δ =
1
70.9 ppm (in the 13C NMR spectrum) in compound 9.
(2) Miyamura, H.; Yasukawa, T.; Kobayashi, S. Green Chem. 2010, 12,
7
76.
OH
TBHP (2 mol equiv)
TBAI (20 mol%)
Imidazole (2 mol equiv)
COOMe
O
O
COOMe
(3) Kiyooka, S.; Ueno, M.; Ishii, E. Tetrahedron Lett. 2005, 46, 4639.
4) Guha, N. R.; Sharma, S.; Bhattacherjee, D.; Thakur, V.; Bharti, R.;
Reddy, C. B.; Das, P. Green Chem. 2016, 18, 1206.
5) Yoo, W. J.; Li, C. J. Tetrahedron Lett. 2007, 48, 1033.
(6) Oda, Y.; Hirano, K.; Satoh, T.; Kuwabata, S.; Miura, M. Tetrahe-
dron Lett. 2011, 52, 5392.
(
Me
O
Me
MeOH
H2O, 8 h, 80 °C
NaOAc, 80 °C, 5 h
(
O
O
O
OH
OH
OAc
7
8 (86%)
9 (82%)
(
7) Rajbongshi, K. K.; Sarma, M. J.; Phukan, P. Tetrahedron Lett.
Scheme 4 Chemoselective oxidation of 1°-benzylic alcohol
2014, 55, 5358.
(
(
8) Tan, L.; Chen, C.; Liu, W. Beilstein J. Org. Chem. 2017, 13, 1079.
9) Li, P.; Zhao, J.; Lang, R.; Xia, C.; Li, F. Tetrahedron Lett. 2014, 55,
3
90.
10) Liu, L.; Yun, L.; Wang, Z.; Fu, X.; Yan, C. Tetrahedron Lett. 2013,
4, 5383.
Similar chemoselectivity was also observed with the
compound 10 where the secondary benzylic alcohol was
selectively oxidized to the ketomethyl moiety under the
present oxidative protocol to yield the compound 11 leav-
ing the aliphatic primary alcohol group intact (Scheme 5).
The occurrence of aromatic ketomethyl moiety in 11 was
(
5
(
(
11) Hackbusch, S.; Franz, A. H. Tetrahedron Lett. 2016, 57, 2873.
12) Guo, S.; Yu, J. T.; Dai, Q.; Yang, H.; Cheng, J. Chem. Commun.
2014, 50, 6240.
(13) Wang, H.; Chen, C.; Liu, W.; Zhu, Z. Beilstein J. Org. Chem. 2017,
13, 2023.
1
confirmed by the singlet at δ = 2.54 ppm (in the H NMR
spectrum) as well as a signal at δ = 200.4 ppm (in the 13
(14) (a) Ghatak, A.; Khan, S.; Roy, R.; Bhar, S. Tetrahedron Lett. 2014,
C
55, 7082. (b) Khan, S.; Ghatak, A.; Bhar, S. Tetrahedron Lett.
NMR spectrum). Survival of the 1°-aliphatic alcohol moiety
was further substantiated through the acetylation of 11 to 12.
2
015, 56, 2480 .
(
(
15) Ghatak, A.; Khan, S.; Bhar, S. Adv. Synth. Catal. 2016, 358, 435.
16) (a) Rout, S. K.; Guin, S.; Ghara, K. K.; Banerjee, A.; Patel, B. K. Org.
Lett. 2012, 14, 3982. (b) Majji, G.; Guin, S.; Gogoi, A.; Rout, S. K.;
Patel, B. K. Chem. Commun. 2013, 49, 3031. (c) Majji, G.; Rout, S.
K.; Rajamanickam, S.; Guin, S.; Patel, B. K. Org. Biomol. Chem.
Me
OH
Me
O
Me
O
TBHP (2 mol equiv)
TBAI (20 mol%)
O
O
Imidazole (2 mol equiv)
Me
O
Me
H2O, 8 h, 80 °C
OH
NaOAc, 80 °C, 5 h
2
016, 14, 8178.
O
O
O
(17) Representative Procedure for the Reaction
OH
OAc
To a mixture of benzyl alcohol (108 mg, 1.0 mmol) and TBHP
1
0
11 (88%)
12 (85%)
(180 mg, 2.0 mmol) in water (5 ml), the catalyst TBAI (73.8 mg,
Scheme 5 Chemoselective oxidation of 2°-benzylic alcohol
0.2 mmol) and imidazole (136 mg, 2.0 mmol) were added, and
the mixture was stirred at 80 °C for 8 h. The progress of the
reaction was monitored by thin layer chromatography (TLC).
After completion of reaction, the reaction mixture was cooled
to room temperature. Then the organic product was extracted
with ethyl acetate (3 × 10 ml), repeatedly washed with distilled
water (4 × 5 ml) to remove the unreacted TBHP, dried with
anhydrous sodium sulfate, and the solvent was evaporated
under reduced pressure to afford benzyl benzoate (182 mg,
yield 86%).
A cost-effective, operationally simple, transition-metal
free, chemoselective, eco-compatible protocol in aqueous
medium has been developed for the synthesis of a variety
of aryl esters directly from primary benzylic alcohols where
17,18
aliphatic alcoholic moiety remained unaffected.
Such
chemoselectivity was also observed during the reaction of
secondary benzylic alcohols where the corresponding ke-
tones were obtained. Investigations towards the extension
of this protocol towards other oxidative transformations are
recently in progress.
To a mixture of benzyl alcohol (108 mg, 1.0 mmol) and TBHP
(180 mg, 2.0 mmol) in water (3 ml), the catalyst TBAI (73.8 mg,
0.2 mmol), imidazole (136 mg, 2.0 mmol), and MeOH (2 ml)
were added, and the mixture was stirred at 80 °C for 8 h. The
progress of the reaction was monitored by TLC. After comple-
tion of reaction, the reaction mixture was cooled to room tem-
perature. Then MeOH was distilled out, and the organic product
was extracted with ethyl acetate (3 × 10 ml), repeatedly washed
with distilled water (4 × 5 ml) to remove the unreacted TBHP,
dried with anhydrous sodium sulfate, and the solvent was evap-
orated under reduced pressure to afford methyl benzoate (112
mg, yield 82%).
Funding Information
S. N. thanks DST-INSPIRE, India for junior research fellowship. Finan-
cial support from UGC-CAS-II program in Chemistry and DST-PURSE-
II program at Jadavpur University as well as Infrastructural support
from DST-FIST program are gratefully acknowledged.
)(
(
18) Spectral and Analytical Data of some Representative Com-
pounds
(
Thiophen-2-yl) Methyl Thiophene-2-carboxylate (2c)
Acknowledgment
1
Yellow oil (yield 85%). H NMR (300 MHz, CDCl ): δ = 5.47 (2 H,
3
s), 7.00–7.02 (1 H, m), 7.07–7.10 (1 H, m), 7.17–7.18 (1 H, m),
The authors thank Mr. N. Dutta of IACS for necessary assistance.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E