Oxidation of alcohols to aldehydes and ketones using selenium ionic liquid

Article abstract of DOI:10.1016/j.cclet.2012.05.019

A new type of ionic liquid supported selenium reagents were synthesized and found to be an excellent catalyst in the oxidation of alcohols to aldehydes and ketones in the presence of 30% H₂O₂. The predictable solubility of ionic liquids allows an easy separation of the oxidation products from the reaction mixture. Furthermore, the oxidation reaction can be carried out using an ionic liquid as the solvent, and the ionic liquid-supported selenium reagents can be recycled and used for four times with a little decrease in catalytic performance.

Full text of DOI:10.1016/j.cclet.2012.05.019

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 801–804
www.elsevier.com/locate/cclet
Oxidation of alcohols to aldehydes and ketones using
selenium ionic liquid
Xian Yi Cheng a, Kang Feng Li a, Qiao Jian Wang a, Cong Ying Wang a, Tao Kai Ying a,b,
*
a College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China
b Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
Received 15 March 2012
Available online 9 June 2012
Abstract
A new type of ionic liquid supported selenium reagents were synthesized and found to be an excellent catalyst in the oxidation of
alcohols to aldehydes and ketones in the presence of 30% H2O2. The predictable solubility of ionic liquids allows an easy separation
of the oxidation products from the reaction mixture. Furthermore, the oxidation reaction can be carried out using an ionic liquid as
the solvent, and the ionic liquid-supported selenium reagents can be recycled and used for four times with a little decrease in
catalytic performance.
# 2012 Tao Kai Ying. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Selenium ionic liquids; Aldehydes; Ketones; Oxidation
The oxidation of alcohols into the corresponding aldehydes and ketones is a fundamental and pivotal class of
chemical transformations and is extensively used in both laboratory and industry [1,2]. In spite of their great
prevalence, these processes are not without drawbacks; they can implicate expensive reagents, large amount of waste,
and tedious purifications. Excellent transition-metal catalysts, based on, e.g., Cu, Pd, Ru, and Pt, have been developed
which allow the use of molecular oxygen as the terminal oxidant [3]. However, these metal catalysts are often
unsuitable for highly functionalized molecules which are used as intermediates in the pharmaceutical industry.
Although new oxidation methods have been developed over the past decades [4], novel methods permitting the
oxidation reaction in a simple and environmentally benign fashion to carry out these transformations are still highly
desirable because of the importance of the oxidative processes as well as the development of greener methodologies.
On the other hand, organoselenium chemistry has been developed rapidly over the past decades, and selenium-
based methodology is a powerful tool in organic synthesis now [5,6]. Selenium dioxide is widely used as a selective
and valuable oxidant. However, these reactions suffer a serious disadvantage of the formation of colloidal selenium,
being difficult to remove from the reaction mixture. These problems can be overcome by use of organoselenium
compounds. Selenoxide as a co-oxidant could oxidize alcohol to aldehydes by adding catalytic equivalent of selenium
dioxide in refluxed dioxane for 1 day [7]. But organic selenides are highly malodorous and generally unpleasant, and
difficult to handle with [8], especially the toxicity. These properties have reduced their attractiveness for applications.
* Corresponding author at: College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China.
E-mail address: sky50@zjnu.cn (T.K. Ying).
1001-8417/$ – see front matter # 2012 Tao Kai Ying. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.05.019

802
X.Y. Cheng et al. / Chinese Chemical Letters 23 (2012) 801–804
Ionic liquids have received much attention in recent years as environmentally benign reaction media for organic
synthesis [9]. They possess many interesting properties such as wide liquid range, negligible vapor pressure, high
thermal stability and high loading capacity. They have also been refereed to as ‘‘designable solvents’’ because their
chemical and physical properties can be adjusted by modifying the structure of the cation or the anion.
Thus, we aim to render the organoselenium compounds non-volatile, easily recoverable and recyclable by
anchoring the selenium function to the ionic liquid moiety, and to study the use of selenium ionic liquid as a solvent
and catalyst in the oxidation of alcohols to aldehydes and ketones.
1. Results and discussion
The details of the preparation of the ionic liquids were outlined in Scheme 1. As shown in Scheme 1, the preparation
of the ionic liquid-supported selenium compounds started from Na2Se2 1 and aryl halide 2 (Scheme 1). The diaryl
diselenides 3b (this compound was synthesized following the literature procedure [10]) were formed in 84% yield (3a
yield only 17%) by simple extraction using dichloromethane. The arylselenide anion (ArSeÀ) was readily prepared by
reduction of 3 with NaBH4, followed by reaction with chloralkane bromide gave the selenide compound 4 in 87%
yield. The reactivity of halides ranked in the order Br > Cl. The following substitution by N-methylimidazol led to the
ionic liquid-grafted selenium 5 at 80 8C in 98% yield with no need of chromatographic purification of intermediates
and there was no foul smell released in any of the operations. All new ionic liquids described in this paper were
1
characterized by H, 13C NMR and mass spectrum.
Following our previouswork, the catalyst 5a, however, proved to be difficult to prepare. Therefore, 5b was chosen as a
more suitable precursor of the same active species. We decided to explore the utility of the new ionic liquid 5b in the
alcohol oxidation reaction. Initially, we chose benzyl alcohol as a substrate to screen the optimized conditions for the
reaction inthepresenceof1.0 mLof5b(1.35 g, 4.1 mmol). Afterextensivetrialsanderrors(Table 1), highertemperature
and longer reaction time had positive effects on the conversion rate of benzyl alcohol. It should be mentioned that further
increasing of the reactiontemperaturewas followedby a reduction in the conversion rate of benzylalcohol (Table 1, entry
5), owing to the oxidative decomposition of Se catalyst. (At higher temperatures, the syn-elimination reaction occurs
[11].) High amount of oxidant had also negative effects on the conversion rate of benzyl alcohol (Table 1, entry 9), we
found that facile b-elimination have occurred leading to the fragmentation of the imidazolium moiety from the
selenoxides (Scheme 2). Ionic liquid 5c also had been completed to test (entry 11), the result is not good, b-elimination
have also occurred leading to the fragmentation of the imidazolium moiety from the selenoxides.
The optimal conditions consisted of 1 equiv. of benzyl alcohol as substrates and 1.0 mL of 5b as solvent and
catalyst, 8.2 mmol of H2O2 as oxidant at 60 8C for 1 day, and no over-oxidation products are observed even after
extended reaction times (Table 1, entry 8).
Under these conditions, a variety of alcohols were oxidized into the corresponding carbonyl compounds in
moderate to good yields. The electron-withdrawing group decreased the reaction yields such as 4-nitrobenzyl alcohol
gave p-nitrobenzaldehyde in only 32% yield (entry 1). Furthermore the reactivity of the secondary benzylic alcohols
was affected by R substituent groups probably due to the steric hindrance, and moderate yields were attained (Table 2).
Allylic alcohols are more readily oxidized than other alcohols (entries 3 and 10). The extension of this catalytic system
to the oxidation of saturated aliphatic alcohols was examined (entry 11), and achieve better result.
In addition, a study regarding the recovering and reusing of the ionic liquid was also performed. After the full
conversion, the product was extracted with petroleum ether, the inferior, ionic liquid phase was dried under vacuum
Cl
NaBH
MeIm
MeIm
4
N
N
SeR
R-Se-Se-R
R=phenyl
RSe
Cl
89%
n
n
98%
n
n=3
Br
Cl
Cl
4a. n=3
5a. n=3
Na Se
2
+
2 R-X
3a
2
Cl
R=phenyl,benzyl
NaBH
4
1
2
R-Se-Se-R
R=benzyl
RSe
Cl
87%
N
N
SeR
n
n
98%
n
Br
4b. n=6
4c. n=3
5b. n=6
5c. n=3
3b
n=3 or 6
Scheme 1. Preparation of ionic liquid-grafted selenium.

X.Y. Cheng et al. / Chinese Chemical Letters 23 (2012) 801–804
803
Table 1
Oxidation conditions of benzyl alcohol.
O
OH
1. 5b
.
% H O
2 2
2
30
.
a
b
Entry
T (8C)
Time (h)
Molar ratio
Product yield
1
2
25
25
40
60
80
60
60
60
60
60
4
4
0:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.5
1:2
1:4
0%
15%
20%
24%
19%
41%
51%
53%
67%
60%
3
4
4
4
5
4
6
12
24
24
24
24
7
8
9
10
a
Molar ratio: 5b/hydrogen peroxide.
b
Yields are given for pure isolated products. 6-phenyl-1-hexene was found as byproduct.
Table 2
Oxidation of several alcohols.
Entrya
1
Product
Yieldb (%)
32
Entrya
7
Product
Yieldb (%)
43
CHO
O
O2N
Cl
2
3
62
65
8
9
47
51
O
CHO
OH
CHO
O
4
5
61
10
11
57
41
CHO
O
Br
(1) 59c
O
CHO
(2) 56d
(3) 57e
6
49
O
a
Reaction performed on a 1 mmol scale.
Yields are given for pure isolated products.
The second run after Table 1, entry 9.
The third run after entry 5 (1).
b
c
d
e
The fourth run after entry 5 (2).

804
X.Y. Cheng et al. / Chinese Chemical Letters 23 (2012) 801–804
Cl
O
Cl
equiv H O
2 2
1
N
N
Se OH
N
N
Se
Ph
6
N
N
SeR
equiv H O
2
1
2
6
Cl
7
5b
6
O
O
Cl
N
N
Se OH
2
1
R (H)
R
8
1
R
O
N
N
Se OCH
OH
2
Cl
R
2
9
1
R (H)
R
Scheme 2. Proposed catalytic cycle.
and reused. The selenium ionic liquid maintained its good level of oxidation activity even after being recycled four
times as shown in Table 2, entry 5.
Although we have found no evidence for the generation of selenoxide 7, there appear to be one possible routes for
the formation of the aldehydes as outlined in Scheme 2. The oxidation of the selenide 5b with 1 equiv. H2O2 generated
the selenenic acid 7 [12a], and then the selenenic acid 7 oxidations with another 1 equiv. H2O2 gave the corresponding
selenenic acid 8 and byproduct of 6-phenyl-1-hexene. And then react with alcohols via the formation of intermediate
seleninate ester 9. This type of intermediate has been proposed in several oxidation reactions to form aldehydes [12].
In summary, we have shown that the ionic liquid-supported selenium reagent 5b could be easily prepared, and that
5b could be used in oxidation reactions in an ionic liquid medium, affording carbonyl compounds in moderate to good
yields. Additional advantages of 5b are: easy to handle, environmentally benign, and easy recovery and recycling,
although the scope of this reaction was somewhat narrow. Further investigations on developing more active selenium
reagents and the mechanism for the oxidation of alcohols are now in progress.
Acknowledgment
This research was financially supported by National Science Foundation of Zhejiang province of China (No.
Y404358).
References
[1] (a) J.C. Collins, W.W. Hess, F.J. Frank, Tetrahedron Lett. 30 (1968) 3363;
(b) E.J. Corey, J.W. Suggs, Tetrahedron Lett. 31 (1975) 2647.
[2] (a) T.T. Tidwell, Org. React. 39 (1990) 297;
(b) S. De Munari, M. Frigerio, M. Santagostino, J. Org. Chem. 61 (1996) 9272;
(c) A. DeMico, R. Margarita, L. Parlanti, et al. J. Org. Chem. 62 (1997) 6974;
(d) P.L. Anelli, F. Montanari, S. Quici, Org. Synth. 69 (1990) 212.
[3] (a) K. Hallman, C. Moberg, Adv. Synth. Catal. 343 (2001) 260;
(b) P. Nagaraju, N. Pasha, P.S. Sai Prasad, et al. Green Chem. 9 (2007) 1126;
(c) C.X. Miao, L.N. He, J.L. Wang, J. Org. Chem. 75 (2010) 257.
[4] M. Hudlicky, Oxidation in Organic Chemistry, ACS, Washington, DC, 1990.
[5] C. Paulmier, Selenium Reagents and Intermediates in Organic Synthesis, Pergamon, Oxford, 1986.
[6] T.G. Back, Organoselenium Chemistry: A Practical Approach, Oxford University Press, Oxford, 1999.
[7] F. Ogura, T. Otsubo, K. Ariyoshi, et al. Chem. Lett. 12 (1983) 1833.
[8] A.J. Mukherjee, S.S. Zade, H.B. Singh, et al. Chem. Rev. 110 (2010) 4357.
[9] (a) P. Wasserscheid, W. Keim, Angew. Chem., Int. Ed. 39 (2000) 3773;
(b) L. Sanggi, Chem. Commun. 348 (2006) 1049.
[10] L. Syper, J. MLochowski, Synthesis 5 (1984) 439.
[11] D. Liotta, Organoselenium Chemistry, Wiley, New York, 1987.
[12] (a) G.J. Brink, Tetrahedron 58 (2002) 3977;
(b) I. Kuwajima, S. Makoto, U. Hirokazu, J. Org. Chem. 47 (1982) 837;
(c) C.T. John, J. Org. Chem. 74 (2009) 3085.