Baeyer-Villiger oxidation of ketones in ionic liquids using molecular oxygen in the presence of benzaldehyde

Article abstract of DOI:10.1016/j.tet.2010.02.082

A new and efficient method for the synthesis of lactones involving the application of an oxygen/benzaldehyde system as the oxidant and ionic liquids as solvents is reported. A significant rate enhancement was observed at 90 °C when the oxidation of ketones was carried out in the presence of a free radical initiator. The oxidation of model cyclic ketones, such as cyclopentanone, cyclohexanone, 2-methylcyclohexanone, 2-norbornone, 2-adamantanone and cycloheptanone gave lactones in high yields (84-90%) within relatively short periods of time, with the possibility of effective ionic liquids recycling. Additionally, discussion of the free radical mechanism of this reaction is proposed.

Full text of DOI:10.1016/j.tet.2010.02.082

Tetrahedron 66 (2010) 2940–2943
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Baeyer–Villiger oxidation of ketones in ionic liquids using molecular oxygen
in the presence of benzaldehyde
Anna Chrobok *
Silesian University of Technology, Department of Chemical Organic Technology and Petrochemistry, ul. Krzywoustego 4, Gliwice 44-100, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A new and efficient method for the synthesis of lactones involving the application of an oxygen/benzal-
dehyde system as the oxidant and ionic liquids as solvents is reported. A significant rate enhancement was
observed at 90 C when the oxidation of ketones was carried out in the presence of a free radical initiator.
Received 9 November 2009
Received in revised form 3 February 2010
Accepted 22 February 2010
Available online 25 February 2010
ꢀ
The oxidation of model cyclic ketones, such as cyclopentanone, cyclohexanone, 2-methylcyclohexanone,
2-norbornone, 2-adamantanone and cycloheptanone gave lactones in high yields (84–90%) within rela-
tively short periods of time, with the possibility of effective ionic liquids recycling. Additionally, discussion
of the free radical mechanism of this reaction is proposed.
Keywords:
Oxidation
Ionic liquids
Ketones
Ó 2010 Elsevier Ltd. All rights reserved.
Lactones
Oxygen
1. Introduction
temperature range for liquid state, low vapour pressure and an
excellent ability to dissolve organic compounds, salts and metals.
6
The Baeyer–Villiger (BV) reaction is commonly used for the
synthesis of lactones and esters by the oxidation of cyclic and linear
ketones. This important reaction has been used in many different
applications, including the synthesis of antibiotics, steroids, pher-
There has been one promising report in the literature about using
m-chloroperoxybenzoic acid as an oxidant and an ionic liquid as
7
a solvent in the BV reaction. The authors report almost three- to
4
fourfold rate acceleration in bmimBF compared tothe same reaction
1
omones and monomers for polymerisation. The most often used
carried out in acetonitrile or chloroform. Nevertheless, this method
suffers from several disadvantages, in particular, the use of an
organic peroxyacid as the oxidant is relatively expensive and haz-
ardous, and limits the commercial application of this method.
In an effort to improve and simplify this procedure, a new and
efficient approach to the synthesis of lactones that involves a
oxidants in this process are organic peroxyacids and hydrogen
peroxide. However, Mukaiyama et al. found that in situ preparation
of organic peroxyacids by the oxidation of an aldehyde with mo-
lecular oxygen can be also an effective oxidation system for the BV
reaction.2 This reaction is carried out using benzaldehyde, acetal-
dehyde or isobutyraldehyde as the aldehyde component. The most
O
2
/benzaldehyde system as the oxidant and an ionic liquid as the
solvent is presented. These studies focused mainly on the oxidation
of cyclohexanone to -caprolactone. This process is of great impor-
tance in the chemical industry; currently, most -caprolactone is
/acetalde-
2 3
frequently used catalysts are Fe O , nickel or copper complexes, or
Fe/MgAl hydrotalcites.3 Dichloromethane, carbon tetrachloride,
benzene and acetonitrile typically are employed as the reaction
solvent. Although the BV reaction has been known for nearly
a hundred years, a large variety of new reaction systems (e.g.,
oxidant, catalyst, and solvent) continue to be investigated.4
In recent years, ionic liquids have become very popular as en-
3
3
prepared by oxidation of cyclohexanone using either O
hyde (BASF) or peroxyacetic acid (Perstorp).
2
2
2
. Results and discussion
5
vironmentally benign solvents for many oxidation reactions. Ionic
liquids are useful reaction media because they possess desirable
characteristics, such as thermal stability, low viscosity, a wide
.1. Ionic liquids as solvents for the Baeyer–Villiger oxidation
The room temperature ionic liquid 1-butyl-3-methyl imidazo-
lium bis(trifluoromethanesulfonyl)imide bmimNTF was selected as
2
a model hydrophobic, water- and air- stable solvent for BV reaction.
The effect of using different so-called sacrificial aldehydes has been
well documented. Based on the literature data, benzaldehyde was
*
Tel.: þ48 32 237 2917; fax: þ48 32 237 1032.
E-mail address: anna.chrobok@polsl.pl
0
040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.082

A. Chrobok / Tetrahedron 66 (2010) 2940–2943
2941
employed, which has been reported to be particularly effective in BV
reactions. All experiments were carried out in a well-stirred round-
bottom flask reactor with a blanket of oxygen at atmospheric pres-
sure according to the general Scheme 1.
in Table 1, the best yields were obtained when a 1:2 ratio cyclohex-
anone/benzaldehyde was used. Larger ratios do not influence the
reaction yield any further. Additionally, the optimum concentration
of ACHN was determined to be 0.033 mol dm with a fixed amount
of benzaldehyde (Table 2).
3
ꢁ3
o
O
90 C, 2.5 h
2
O , ACHN, IL
O
Table 2
O
The influence of ACHN additive on the oxidation of cyclohexanone (3 mmol) in the
presence of benzaldehyde (6 mmol) with oxygen in bmimNTf
2
(2 mL) as a solvent
PhCOH PhCOOH
ꢀ
at 90
C
Scheme 1.
Yielda of
after 30 min [%]
Yielda of
3-caprolactone
after 2.5 h [%]
Concentration of
ACHN [mol dm 3]
3
-caprolactone
ꢁ
Figure 1 (curves A, B, C) shows the reaction profiles for the se-
d
43
55
61
60
71
80
90
87
lective oxidation of cyclohexanone to
0 C. A two-fold molar excess of benzaldehyde was employed. High
3
-caprolactone at 40, 60 and
0.016
ꢀ
9
0.033
0.066
yields of lactone formation in a relatively short time period (even at
ꢀ
a
4
0 C) was observed. However, a significant rate enhancement was
Yields determined by GC.
ꢀ
observed at 90 C (Fig. 1, curve D) when the oxidation was carried
0
out in the presence of a free radical initiator, either 1,1 -azobis(cy-
The reaction also can be carried out using other ionic liquids as
solvents (Table 3). The use of three hydrophobic ionic liquids based
on NTf anions and other hydrophilic ones was explored. In all
cases, relatively high conversions and yields of -caprolactone were
observed. The best results were obtained for liquids with NTf an-
ions. Most likely, these highly hydrophobic liquids create an an-
hydrous reaction environment, so that hydrolysis of the lactone to
the corresponding hydroxyacid is less competitive. This phenom-
ꢀ
8
clohexanecarbonitrile) ACHN, (half-live at 90 C 469 min ) or azo-
ꢀ
8
isobutyronitrile AIBN (half-live at 90 C 26 min ). Nearly complete
conversion of cyclohexanone was observed after 2.5 h, providing
the desired lactone in 90% yield. The free radical nature of this
process will be discussed later.
2
3
2
1
00
9
(
D)
(C)
(C')
enon was also observed in our previous study.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(D')
Table 3
(B)
The influence of ionic liquid structure on oxidation of cyclohexanone (3 mmol) in
the presence of benzaldehyde (6 mmol) with oxygen and ACHN additive
(0.033 mol dm ) at 90 C over 2.5 h
(A)
ꢁ
3
ꢀ
Ionic liquid
Conversiona
%]
Yielda
of -caprolactone
%]
[
3
[
1-Butyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide
bmimNTf2
Butylmethylpyrrolidinium
bis(trifluoromethanesulfonyl)imide
96
95
95
90 (84)
89 (82)
88 (79)
bmpNTf
2
trimethylbutylammonium
0
1
2
3
4
5
6
7
8
bis(trifluoromethanesulfonyl)imide
Time [h]
tmbaNTf
-Butyl-3-methylimidazolium
tetrafluoroborate, bmimBF
1-Ethyl-3-methylimidazolium
methylsulfate, emimOSO Me
1-Butyl-3-methylimidazolium
trifluoroacetate, bmimCF COO
2
1
90
85
80
62
85 (76)
76 (65)
72(65)
55 (46)
Figure 1. Oxidation of cyclohexanone (3 mmol) in the presence of benzaldehyde
ꢀ
ꢀ
4
(
(
(
(
6 mmol) with oxygen in bmimNTf
2
(2 mL) as a solvent. Curve (A) at 40 C; (B) 60 C;
(1 mol %) at 90 C; (D) in the presence of ACHN
0.033 mol dm ) at 90 C; (D) in the presence of ACHN (0.033 mol dm ) and Fe
1 mol %) at 90 C. Yields determined by GC.
ꢀ
0
ꢀ
C) 90 C; (C ) in the presence of Fe
2
O
3
ꢁ
3
ꢀ
ꢁ3
3
2
O
3
ꢀ
3
1
-Butyl-3-methylimidazolium
trifluoromethanesulfonate, bmimOTf
Additionally, the use of a metal catalyst in this process was in-
vestigated (Fig. 1, curves C and D ). Fe O d a commonly used
2 3
catalyst d does not influence the reaction course, regardless of
whether ACHN is presented.
The highest reaction rates were observed in the first 0.5 h of the
reaction. The influence of the concentration of benzaldehyde on the
0
0
a
Determined by GC; in parenthesis isolated yields.
The nature of the ionic liquid is quite important for the product
isolation step. For hydrophobic ionic liquids, the post-reaction
mixture was first washed with a saturated aqueous solution
yield of3-caprolactone was evaluated after 0.5 h and2 h. As indicated
3
NaHCO to remove benzoic acid and then the product was extracted
with diethyl ether. At the end of the study, the ionic liquids were
Table 1
The influence of benzaldehyde amount on the oxidation of cyclohexanone (3 mmol)
with oxygen in bmimNTf (2 mL) as a solvent, in the presence of ACHN additive
2
Table 4
Recycling of bmimNTf
ꢁ
3
ꢀ
(
0.033 mol dm ) at 90 C
2
(8 mL) in the oxidation of cyclohexanone (12 mmol) in the
presence of benzaldehyde (24 mmol) with oxygen and ACHN additive
Cyclohexanone/
benzaldehyde ratio
Yielda of
after 30 min [%]
3
-caprolactone
Yielda of
after 2.5 h [%]
3-caprolactone
ꢁ3
ꢀ
(
0.033 mol dm ) at 90 C over 2.5 h
Recycle of IL
Yielda of
Fresh, non-recycled IL 90
[mol/mol]
3
2
-caprolactone [%] bmimNTf Recovered [%]
1
1
1
1
/1
/1.5
/2
39
47
61
62
68
75
90
87
91
92
90
89
First
Second
Third
88
88
87
/3
a
a
Yields determined by GC.
Determined by GC.

2942
A. Chrobok / Tetrahedron 66 (2010) 2940–2943
recovered and reused in additional reactions, which gave nearly
identical results. Table 4 shows the results of reactions using three
acetonitrile).3 The first step d oxidation of aldehyde d is un-
doubtedly a free radical process. The newly formed acylperoxy rad-
recycles of bmimNTf
2
in the oxidation of cyclohexanone. Alterna-
ical abstracts a hydrogen atom from the aldehyde to give
tively, it is possible to distil the product from the crude reaction
mixture.
When hydrophilic ionic liquids were used as reaction solvents,
the product was first extracted with diethyl ether. Next, the organic
a peroxyacid. The second step is thought to proceed via attack of the
peroxyacid on the ketone (pathway 1), but the possibility of acyl-
peroxy radical participation in the formation of the lactone cannot be
3
excluded (pathway 2). It is difficult to confirm, which species attacks
extract was washed with a saturated aqueous solution NaHCO
dried and concentrated.
3
,
the ketone: a peroxyacid or an acylperoxy radical. Recently Takehira
et al. performed a BV reaction in acetonitrile using O /benzaldehyde
2
Other cyclic ketones, such as cyclopentanone, 2-methylcyclo-
hexanone, 2-norbornone, 2-adamantanone and cycloheptanone, can
system and Fe/MgAl hydrotalcite as the catalyst. They suggest that
the active oxygen species must be peroxyacid, which is activated on
3þ
3
be oxidised using
2
O /benzaldehyde as the oxidising agent,
the Fe clusters as the active Lewis acid site.
The oxidation of benzaldehyde with molecular oxygen is
bmimNTF as the solvent, and ACHN as the free radical initiator.
2
10
These reactions proceeded efficiently without any additional cata-
lysts, and the corresponding lactones were obtained in high yields
strongly solvent dependent. Peroxybenzoic acid (PBA) was iso-
lated even in high yields (90%) as the primary product of benzal-
dehyde oxidation with oxygen in classical organic solvents.
However, depending on the reaction conditions, the peroxyacid,
that is, formed can react with another molecule of benzaldehyde to
(
77–90%) within short times (Table 5). Only the oxidation of cyclo-
heptanone afforded the corresponding lactone in insufficient yield
2
(
40%) what was also observed by other researchers.
11
give benzoic acid as the main product (Scheme 3).
Table 5
Oxidation of ketone (3 mmol) in the presence of benzaldehyde (6 mmol) with
O
O
O
ꢁ3
ꢀ
C
oxygen and ACHN additive (0.033 mol dm ) in bmimNTf
2
(2 mL) as a solvent at 90
+
H
1
R1
Reaction Conversiona Yield of
R
OOH R
H
R
OO
O
Ketone
Lactone
a
time [h] [%]
lactone [%]
OH
O
2
99
85 (77)
O
O
O
O
H
+
R1
OH
2
R
OH
R
O
O
O
O
O
2.5
96
99
90 (84)
Scheme 3. Possible reaction pathway of peroxybenzoic acid during the oxidation of
11
O
benzaldehyde with molecular oxygen.
O
O
2
4
95 (90)
96 (89)
O
When ionic liquids are used as the reaction medium, the situ-
ation is different. Only one literature report describes the oxidation
of benzaldehyde with oxygen using an ionic liquid and a Ni catalyst
12
100
(
bmimPF
6
/Ni(acac)
2
). Howarth observed the formation of benzoic
ꢀ
O
O
O
acid as the main product (66% yield) at 60 C after 24 h.
O
O
However, we know from our previous study that it is possible to
carry out free radical oxidation processes in ionic liquids. For this
reason, additional experiments were performed. A variety of other
13
4
100
49
94 (86)
(40)
ionic liquids (bmimNTf
bmimOSO Oc, bmimOSO
CoCl , MnCl , CuCl , NiCl
2
, tmbaNTf
Me, bmimCF
, Ni(acac) in the oxidation of benzalde-
2
2 4
, bmpNTf , bmimBF , bmimOTf,
O
3
3
3
COO) and catalysts Fe
2 3
O ,
O
2
2
2
2
2
O
10
O
hyde were explored. These systems were homogenous or partially
homogenous. The oxidations were carried out at 20–60 C for 2–
ꢀ
a
24 h, with or without a metal catalyst, and with or without the free
Determined by GC; in parenthesis isolated yields.
ꢀ
8
radical initiator AIBN (half-live at 60 C 1219 min ). In all cases we
observed high conversions of benzaldehyde and the formation of
benzoic acid as a main product (40–70%); only traces of PBA were
found (0.5–4%). For comparison, the yield of PBA after 1 h of
2
.2. Discussion of the reaction mechanism
In the literature, two possible reaction pathways (Scheme 2) are
postulated for the BV reaction using O /benzaldehyde system in
classical organic solvents (e.g., dichloromethane, chloroform,
2 4
benzaldehyde oxidation in bmimNTf is only 1% while in CCl is
2
26%. The data presented above clearly illustrate that it is not pos-
sible to obtain peroxybenzoic acid in ionic liquids under the re-
action conditions that we used for the BV reaction. The stability of
this relatively unstable peroxy compound was also checked. As
pathway 1
R2
shown in Table 6, PBA is relatively stable in bmimNTf
2
solution at
+
RCHO
-
RCO H
O
3
ꢀ
ꢀ
20 C but after stirring it at 90 C for 3 h 75% decomposed. This
R1
RCO
result may be caused by the influence of temperature d thermal
R1
O
-
RCOOH
O2
OH
OOCOR
RCHO
RCO3
R1 OR
R2
Table 6
R1
R1
Stability of peroxybenzoic acid (1.5 mmol) in bmimNTf
various temperatures
2
(2 mL) after 3 h of stirring at
O
+
RCHO/-RCO
O
R2
R2
OOCOR
pathway 2
ꢀ
Temp [ C]
20
1
40
6
60
13
90
75
Decomposition of PBA [%]a
a
Scheme 2. Mechanism of BV reaction with O
2
/benzaldehyde system.
Determined by iodometric titration.

A. Chrobok / Tetrahedron 66 (2010) 2940–2943
2943
decomposition. PBA is moderately stable at room temperature but
NaHCO
bmimNTf
3
solution, and then with diethyl ether (6ꢂ5 mL). Next,
ꢀ
ꢀ
when is heated to 80–100 C, it decomposes smoothly into benzoic
2
was concentrated and dried in a vacuum (60 C, 8 h).
11
acid and gases containing oxygen. Therefore, even if peroxy-
benzoic acid forms during the oxidation of benzaldehyde in
4.4. Oxidation of benzaldehyde in ionic liquids
2
bmimNTf , it is thermally unstable or immediately reacts with
another benzaldehyde molecule to form benzoic acid.
Additionally, the Baeyer–Villiger oxidation of ketones with
2
O /benzaldehyde system in ionic liquids is accelerated by the
A mixture of benzaldehyde (6.0 mmol) and ionic liquid (2 mL)
was placed into the round bottom flask equipped with condenser
and balloon with oxygen. The reaction mixture was stirred with
addition of a free radical initiator (Fig. 1).
ꢀ
magnetic stirrer at 20–60 C in time periods 2–24 h with or without
Based on these data, the mechanism of the BV reaction in ionic
liquids occurs mainly via free radical pathways, which is in contrast
to the reaction using typical organic solvents. An acylperoxy radical
is generated in the first step by the oxidation of benzaldehyde; this
radical is a key intermediate that then undergoes facile reaction
with the ketone to give the corresponding lactone in high yield
a metal catalyst (0.5–1 mol %), with or without a free radical initi-
ꢁ3
ator AIBN (0.033 mol dm ). Reaction products were extracted with
diethyl ether (6ꢂ5 mL), concentrated and analysed by the aim of GC
and iodometric titration.
4
.5. Stability of PBA in bmimNTf
2
(Scheme 2, pathway 2).
A mixture of PBA (1.5 mmol) and bmimNTf
at 20, 40, 60, 90 C for 3 h. After this time the content of PBA was
determined by iodometric titration.
2
(2 mL) was stirred
3
. Conclusion
ꢀ
In summary, an efficient method for the synthesis of lactones
2
that utilises O /benzaldehyde system as the oxidant and ionic liq-
uids as solvents was reported. This method avoids the use of very
often expensive catalysts. The influences of reaction conditions
4.6. Analysis
(
temperature, initiator concentration, structure of ionic liquid and
All products were characterised by comparison of their NMR
the amount of benzaldehyde) were described. The use of hydro-
phobic ionic liquids based on NTf
free radical initiator gave
This new method is attractive also for the oxidation of other cyclic
ketones, since it gives lactones in high yields. The ionic liquid sol-
vent can be efficiently recycled. Additionally, the mechanism of this
reaction in ionic liquids was discussed and the free radical nature of
this reaction was postulated.
16 1
13
spectra with authentic samples.
were recorded at 300 MHz in CDCl
H NMR and C NMR spectra
3
(Varian Unity Inova plus, in-
ꢀ
2
anions at 90 C with ACHN as the
3
-caprolactone in 84% yield after 2.5 h.
ternal TMS). GC analysis was performed using Perkin–Elmer chro-
matograph and decane as the external standard.
Acknowledgements
This work was financially supported by the Ministry of Science
and Higher Education (Grant no N N209 149236).
4
4
. Experimental
.1. Materials
References and notes
1
. (a) Krow, G. Org. React. 1993, 43, 251–798; (b) Renz, M.; Meunier, B. Eur. J. Org.
Chem. 1999, 737–750.
. Yamada, T.; Takahashi, K.; Kato, K.; Takai, T.; Inoki, S.; Mukaiyama, T. Chem. Lett.
1991, 641–644.
2 3 3
Ionic liquids: bmpNTf , bmimOTf, bmimOSO Oc, emimOSO Me,
bmimCF
3
COO (Merck) and ketones (Acros Organics), oxygen (BOC,
, bmimNTf
peroxybenzoic acid were prepared according to the
known procedures. Benzaldehyde (Acros Organics), was purified by
2
purity 99.99%) were commercial materials; bmimBF
4
2
14
15
tmbaNTf
2
,
3. (a) Kawabata, T.; Fujisaki, N.; Shishida, T.; Nomura, K.; Sano, T.; Takehira, K.
J. Mol. Catal. A: Chem. 2006, 253, 279–289; (b) Chisem, I. C.; Chisem, J.; Clark, J. H.
New J. Chem. 1998, 22, 81–82; (c) Wang, X.; Li, F.; Zhang, H.; Wang, C.; Song, G.
Synth. Commun. 1996, 26, 1613–1616; (d) Kaneda, K.; Ueno, S.; Imanaka, T.;
Shimotsuma, E.; Nishiyama, Y.; Ishii, Y. J. Org. Chem. 1994, 59, 2915–2917; (e)
Bolm, C.; Schlingloff, G.; Weickhardt, K. Tetrahedron Lett. 1993, 34, 3405–3408;
extraction with 10% aqueous solution Na
bp¼79–80 C/25 mmHg).
2
CO
3
and distillation
ꢀ
(
(
f) Murahashi, S.-I.; Oda, Y.; Naota, T. Tetrahedron Lett. 1992, 33, 7557–7560.
4
.2. Typical procedure for BV oxidation
4
5
6
. Brink, G.; Arends, I.; Sheldon, R. A. Chem. Rev. 2004, 104, 4105–4123.
. Muzart, J. Adv. Synth. Catal. 2006, 348, 275–295.
. (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Wein-
heim, 2007; (b) Binnemans, K. Chem. Rev. 2005, 105, 4148–4204; (c) Parvulescu,
V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615–2665.
A mixture of benzaldehyde (6.0 mmol), ketone (3.0 mmol), ionic
ꢁ
3
liquid (2 mL) and ACHN (0.033 mol dm ) was placed into the
5 mL round bottom flask equipped with condenser and balloon
2
7. Yadav, J. S.; Redy, B. V. S.; Basak, A. K.; Narasiah, A. V. Chem. Lett. 2004, 33,
48–249.
2
with oxygen. The reaction mixture was stirred with magnetic stir-
rer at 90 C for 2.5 h. The progress of the reaction was monitored by
GC. After this time, post-reaction mixture was washed with a sat-
8
. (a) Overberger, C. G.; Biletch, H.; Finestone, A. B.; Lilker, J.; Herbert, J. J. Am.
Chem. Soc. 1953, 75, 2078–2082; (b) Van Hook, J. P.; Tobolsky, A. V. J. Am. Chem.
Soc. 1958, 80, 779–782.
ꢀ
urated aqueous NaHCO
with diethyl ether (6ꢂ5 mL). The organic phase was dried over
anhydrous MgSO , filtered and concentrated in a vacuum. The
yields of lactones after the purification by column chromatography
with hexane/ethyl acetate (4:1) as an eluent were 46–90%.
3
. Next, the ionic liquid layer was extracted
9. Baj, S.; Chrobok, A.; S1upska, R. Green Chem. 2009, 11, 279–282.
0. Dick, C. R.; Hanna, R. F. J. Org. Chem. 1964, 29, 1218–1220.
1
1
1. Denisov, E. T.; Afanas’ev, I. B.; Denisova, T. T.; Trepalin, S. V.; Drozdova, T. I. In
Oxidation and Antioxidants in Organic Chemistry and Biology; Denisov, E. T.,
Afanas’ev, I. B., Eds.; Taylor&Francis Group: Boca Raton, 2005; pp 299–329.
4
12. Howarth, J. Tetrahedron Lett. 2000, 41, 6627–6628.
13. Baj, S.; Chrobok, A. Int. J. Chem. Kinet. 2008, 40, 287–293.
1
4. (a) Bonhote, P.; Dias, A.; Papageorgiou, N.; Kalyanasundaram, K.; Armand, M.;
Graetzel, M. Inorg. Chem. 1996, 35, 1168–1178; (b) Shang, H.; Wu, J.; Zhou, Q.;
Wang, L. J. Chem. Eng. Data 2006, 51, 1286–1288.
4
.3. Recycling of bmimNTf
2
1
5. Moyer, J. R.; Manley, N. C. J. Org. Chem. 1964, 29, 2099–2100.
2
After BV reaction, bmimNTf was purified for recycling tests by
16. Gonzalez-Nunez, M. E.; Mello, R.; Olmos, A.; Asensio, G. J. Org. Chem. 2006, 71,
extractions of post-reaction mixture: first with a saturated aqueous
6432–6436.