An environmentally benign benzylic oxidation catalyzed by hypervalent iodine intermediate in water

Article abstract of DOI:10.1016/j.cclet.2012.06.002

An effective and environmentally benign benzylic oxidation for transition of alkylarenes into the corresponding carbonyl compounds was reported. Alkylarenes were mixed and stirred with potassium bromide, m-chloroperbenzoic acid and a catalytic amount of iodobenzene in water at 60 °C for several hours, a series of the corresponding carbonyl compounds was obtained in moderate to good yields. In the reaction, iodobenzene was first oxidized by m-chloroperbenzoic acid into the hypervalent iodine intermediate which then reacted with potassium bromide to form the key radical initiator for the benzylic oxidation.

Full text of DOI:10.1016/j.cclet.2012.06.002

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 891–894
www.elsevier.com/locate/cclet
An environmentally benign benzylic oxidation catalyzed by
hypervalent iodine intermediate in water
*
Yuan Xu, Jian Tao Hu, Jie Yan
College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310032, China
Received 9 March 2012
Available online 4 July 2012
Abstract
An effective and environmentally benign benzylic oxidation for transition of alkylarenes into the corresponding carbonyl
compounds was reported. Alkylarenes were mixed and stirred with potassium bromide, m-chloroperbenzoic acid and a catalytic
amount of iodobenzene in water at 60 8C for several hours, a series of the corresponding carbonyl compounds was obtained in
moderate to good yields. In the reaction, iodobenzene was first oxidized by m-chloroperbenzoic acid into the hypervalent iodine
intermediate which then reacted with potassium bromide to form the key radical initiator for the benzylic oxidation.
# 2012 Jie Yan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Benzylic oxidation; Hypervalent iodine intermediate; Catalytic reaction; Synthesis
Benzylic oxidation of alkylarenes into ketones is one of the most significant and synthetically useful methods in
organic chemistry. These are numerous methods for the benzylic oxidation, and the use of stoichiometric quantities of
oxidants such as potassium permanganate or potassium dichromate in these reactions is the classical process [1].
During the last decades, a number of oxidation catalysts have been described [2], but most of them involve toxic
metals, such as Cr, Co, Ru, Mn, Bi and Rh, and their applicability is limited [3–10]. Having low toxicity, mild
reactivity and high stability, the hypervalent iodine reagents-induced benzylic oxidations are new improvement
processes; however, in these reactions the stoichiometric hypervalent iodine reagents are needed and most of the
reactions occur in organic solvents [11–15]. Kita group reported a new green benzylic oxidation using water as solvent
in 2008, in which 3 equiv. of hypervalent iodine reagent iodosobenzene was used [16].
Recently, the catalytic utilization of hypervalent iodine reagents is increasing in importance, with the growing
interest in the development of environmentally benign synthetic transformations [17–21]. In these catalytic reactions,
a catalytic amount of an iodine-containing molecule together with a stoichiometric oxidant is used. The oxidant
generates the hypervalent iodine reagent in situ, and after the oxidative transformation, the reduced iodine-containing
molecule is re-oxidized. Ochiai et al. reported the a-acetoxylation of ketones with iodobenzene as catalyst and
metachloroperbenzoic acid (mCPBA) as oxidant [22]. Kita’s group demonstrated the catalytic spirocyclization of
phenol derivatives using hypervalent iodine reagents as catalysts [23]. In order to extend the scope of catalytic use of
hypervalent iodine reagents in organic synthesis, we have explored some reactions with catalytic hypervalent iodine
* Corresponding author.
E-mail address: jieyan87@zjut.edu.cn (J. Yan).
1001-8417/$ – see front matter # 2012 Jie Yan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.06.002

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Y. Xu et al. / Chinese Chemical Letters 23 (2012) 891–894
Table 1
Optimization of the iodobenzene catalyzed oxidation of tetrahydronaphthalene.[TD$INLE]
O
H2O
R.T.
+PhI
Oxidant
+KBr+
Entry
Oxidant (equiv.)
KBr (equiv.)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
mCPBA
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.2
1.2
0.1
0.1
0.1
0.1
0.1
0.3
0.3
0.3
0.3
24
24
24
24
30
18
18
18
18
28
Oxone1
t-BuOOH
K2S2O8
29
8
Trace
Trace
48b
48b
51b
52b
NaBO3Á4H2O
mCPBA
Oxone1
mCPBA
Oxone1
Determined by 1H NMR spectroscopy.
a
b
Reactions were performed at 60 8C.
reagents generated in situ, and reported the a-phosphoryloxylation of ketones [24] and sulfonyloxylactonization of
alkenoic acids [25]. More recently we have investigated the new environmentally benign benzylic oxidation using
iodobenzene as catalyst and mCPBA as oxidant in the present of potassium bromide in water, a series of the
corresponding carbonyl compounds was obtained in moderate to good yields. Now we would like to report the new
environmentally benign benzylic oxidation, and this catalytic process in water has never been reported before.
Firstly, tetrahydronaphthalene was chosen as the model substrate to investigate the catalytic benzylic oxidation.
When 1.0 equiv. of tetrahydronaphthalene was treated with 2.0 equiv. of mCPBA, 0.1 equiv. of potassium bromide and
iodobenzene in water at room temperature, it was found that after 24 h the reaction provided the desired product of 3,4-
dihydronaphthalen-1(2H)-one in 28% yield. Prompted by the result, a series of experiments was performed to
determine the suitable reaction conditions, the results are summarized in Table 1.
It is shown from Table 1 that when another oxidant Oxone1(KHSO4Á2KHSO5ÁK2SO4) was used in place of
mCPBA, the reaction gave the nearly same yield (entries 1, 2). Other oxidants, such as t-BuOOH, K2S2O8 and
NaBO3Á4H2O were not active in the reaction (entries 3–5). It was found that the amount of KBr and reaction
temperature influenced on the yield greatly. When 0.3 equiv. of KBr was added and the reaction was performed at
60 8C, the reaction yield was increased to 48% after 18 h either mCPBA or Oxone1 was used as oxidant (entries 6 and
7). Due to part of product can be future oxidized into the by-product of ester by more oxidant, the amount of oxidant
was also investigated and 1.2 equiv. of oxidant should be suitable to the reaction (entries 8 and 9).
Having establishing optimal conditions, the catalytic benzylic oxidation of a series of alkylarenes (1) with
1.2 equiv. of mCPBA or Oxone1 and 0.3 equiv. of KBr in the presence of 0.1 equiv. of iodobenzene in water at 60 8C
was investigated, and several corresponding carbonyl compounds (2) were obtained (Scheme 1), the results are
summarized in Table 2.
It is notable that the cyclic and liner alkylbenzenes gave the corresponding arylketones in moderate to good yields
(Table 2, entries 1–5). Both diarylmethanes 1f and 1g provided the products in quite different yields: the
dibenzenemethane was oxidized into the ketone in excellent yield of 84%; while 1g which had an electron-
withdrawing group at benzene ring and an electron-withdrawing pyridine ring only let to the corresponding product
[(Scheme_1)TD$FIG]
O
H2O
+
Oxidant
+
+
PhI KBr
Ar
R
Ar
R
60 °C
2
1
Scheme 1.

Y. Xu et al. / Chinese Chemical Letters 23 (2012) 891–894
893
Table 2
The result of catalytic benzylic oxidation of alkylarenes.
Entry
1
Alkylarenes (1)
Carbonyl compounds (2)
Yield (%)a
51
O
[TD$INLINE]
[DT$INLE]
1a
2a
O
2
55
[TD$INLINE]
[DT$INLE]
1b
2b
3
4
5
6
PhCH2CH3 1c
PhCH2CH2CH3 1d
PhCH2CH2CH2CH3 1e
PhCOCH3 2c
PhCOCH2CH3 2d
PhCOCH2CH2CH3 2e
69
55
81
84b
O
[TD$INLINE]
[DT$INLE]
1f
2f
7
8
9
19
47
27
O
O
[TD$INLINE]
N
[DT$INLE]
NO2
1g
N
2g
NO2
O
[TD$INLINE]
[DT$INLE]
1h
2h
O
O
O
[TD$INLINE]
[DT$INLE]
1i
O
2i
10
52c
O
[TD$INLINE]
[DT$INLE]
1j
2j
a
Isolated yield.
Oxone1 was used as oxidant.
b
c
MeCN was used as solvent.
in 19% of yield (entries 6 and 7). These results clearly show this method is not suitable to the alkylbenzenes which
having electron-withdrawing groups at aryl rings, and compounds 1h and 1i resulted in the products 2h and 2i also
agreed with the above conclusion (entries 8 and 9).
The proposed mechanism for the catalytic benzylic oxidation is shown in Scheme 2. Iodobenzene is first oxidized
by mCPBA into the hypervalent iodine intermediate, and then it is transformed to the active key species by treatment
with KBr, following a radical reaction on tetrahydronaphthalene is happened, and finally the corresponding ketone is
obtained by future oxidation.
A typical procedure for the catalytic benzylic oxidation tetrahydronaphthalene 1a (1.0 mmol), mCPBA
(1.2 mmol), KBr (0.3 mmol) and iodobenzene (0.1 mmol) were added in water (3 mL). The mixture was stirred at

[(Scheme_2)TD$FIG]
894
Y. Xu et al. / Chinese Chemical Letters 23 (2012) 891–894
[O]
H2O
-Br
Br
Ph
KBr
I (III)
Br
Br
OH
O
PhI
Ph
I (III)
mCBA
mCPBA
Scheme 2.
60 8C for 18 h and then separated on a silica gel plate using (4:1 hexane–ethyl acetate) as eluant to give 3,4-
dihydronaphthalen-1(2H)-one 2a in 51% of yield. Oil; 1H NMR (500 MHz, CDCl3): d 8.04 (d, 1H, J = 3.0 Hz), 7.46–
7.50 (m, 1H), 7.32 (t, 1H, J = 7.6 Hz), 7.26 (d, 1H, J = 8.5 Hz), 2.98 (t, 2H, J = 6.1 Hz), 2.67 (t, 2H, J = 6.6 Hz), 2.13–
2.18 (m, 2H); 13C NMR (125 MHz, CDCl3): d 198.35, 190.53, 144.46, 133.36, 132.61, 128.75, 127.13, 126.60, 39.14,
29.68, 23.27.
In conclusion, we have achieved an aqueous benzylic oxidation using catalytic amount of iodobenzene combined
with mCPBA or Oxone1 as terminal oxidants. This method has some advantages such as mild reaction conditions,
simple procedure and environmentally benign. Further investigation of the catalytic benzylic oxidation will be
reported in due course.
Acknowledgments
Financial support from the Natural Science Foundation of China (No. 21072176) and the Zhejiang Province
Natural Science Foundation of China (No. Y4100231) are greatly appreciated.
References
[1] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph No. 186, American Chemical Society, Washington DC, 1990.
[2] J.E. Backvall (Ed.), Modern Oxidation Methods, Wiley-VCH, Weinheim, 2004.
[3] B. Choudary, A.D. Prasad, V. Bhuma, et al. J. Org. Chem. 57 (1992) 5841.
[4] E. Modica, G. Bombieri, D. Colombo, et al. Eur. J. Org. Chem. (2003) 2964.
[5] C. Jin, L. Zhang, W.K. Su, Synlett (2011) 1435.
[6] M.D. Nikalje, A. Sudaldai, Tetrahedron 55 (1999) 5903.
[7] G. Blay, I. FernOndez, T. Gimenez, et al. Chem. Commun. (2001) 2102.
[8] Y. Bonvin, E. Callens, I. Larrosa, et al. Org. Lett. 7 (2005) 4549.
[9] A.J. Catino, J.M. Nichols, H. Choi, et al. Org. Lett. 7 (2005) 5167.
[10] M. Nakanishi, C. Bolm, Adv. Synth. Catal. 349 (2007) 861.
[11] O. Ochiai, T. Ito, Y. Masaki, et al. J. Am. Ceram. Soc. 114 (1992) 6269.
[12] O. Ochiai, T. Ito, H. Takahashi, et al. J. Am. Ceram. Soc. 118 (1996) 7716.
[13] K.C. Nicolaou, Y.L. Zhong, P.S. Baran, J. Am. Ceram. Soc. 123 (2001) 3183.
[14] K.C. Nicolaou, T. Montagnon, P.S. Baran, et al. J. Am. Ceram. Soc. 124 (2002) 2245.
[15] M.S. Yusubov, A.A. Zagulyaeva, V.V. Zhdankin, Chem. Eng. J. 15 (2009) 11091.
[16] T. Dohi, N. Takenaga, A. Goto, et al. J. Org. Chem. 73 (2008) 7365.
[17] R.D. Richardson, T. Wirth, Angew. Chem. Int. Ed. 45 (2006) 4402.
[18] T. Dohi, Y. Minamitsuji, A. Maruyama, et al. Org. Lett. 10 (2008) 3359.
[19] M. Ochiai, Chem. Rec. 7 (2007) 13.
[20] M. Uyanik, K. Ishihara, Chem. Commun. (2009) 2086.
[21] T. Dohi, Y. Kita, Chem. Commun. (2009) 2073.
[22] M. Ochiai, Y. Takeuchi, T. Katayama, et al. J. Am. Ceram. Soc. 127 (2005) 12244.
[23] T. Dohi, A. Maruyama, M. Yoshimura, et al. Angew. Chem. Int. Ed. 44 (2005) 6193.
[24] Y. Pu, L.M. Gao, H.J. Liu, et al. Synthesis 44 (2012) 99.
[25] J. Yan, H. Wang, Z.P. Yang, et al. Synlett (2009) 2669.