Silyl peroxides as effective oxidants in the Baeyer-Villiger reaction with chloroaluminate(III) ionic liquids as catalysts

Article abstract of DOI:10.1016/j.molcata.2013.04.023

A new application of silyl peroxides as oxidants in the Baeyer-Villiger oxidation of cyclic ketones in chloroaluminate(III) ionic liquids is described. Among the silyl peroxides, the reactivity of two groups of peroxides was studied: bis(silyl) and t-butyl silyl peroxides possessing different structured substituents attached to the Si atom. It was shown that the acidic 1-hexyl-3-methylimidazolium chloroaluminate(III) ionic liquid (molar ratio of AlCl₃ in ionic liquid: 0.67) present in the oxidation of cyclic ketones with bis(silyl) peroxides acts as the catalyst. In this variant of the reaction, the reactivities of bis(silyl) peroxides decrease in the following order: bis(trimethylsilyl) peroxide > bis(vinyldimethylsilyl) peroxide > bis(phenyldimethylsilyl) peroxide > bis(diphenylmethylsilyl) peroxide. A variety of cyclic ketones such as cyclobutanone, 3-substituted cyclobutanones, cyclopentanone, cyclohexanone, 2-methylcyclohexanone, 4-methylcyclohexanone, 2-adamantanone and norcamphor were oxidised to their corresponding lactones with high yields (49-100%). When t-butyl silyl peroxides and neutral chloroaluminate(III) ionic liquids (molar ratio of AlCl₃ in ionic liquid: 0.5) were utilised in the Baeyer-Villiger oxidation, the studied ionic liquid acted as the reagent. Here, phenyldimethyl(t-butylperoxy)silane was the most efficient oxidant in the oxidation of cyclobutanone to γ- butyrolactone (70% yield). Other peroxides, including trimethyl(t-butylperoxy) silane, vinyldimethyl(t-butylperoxy)silane and diphenylmethyl-(t-butylperoxy) silane, were less reactive oxidants. Two variants of the Baeyer-Villiger reaction mechanism are postulated.

Full text of DOI:10.1016/j.molcata.2013.04.023

Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Silyl peroxides as effective oxidants in the Baeyer–Villiger reaction with
chloroaluminate(III) ionic liquids as catalysts
Stefan Baj, Roksana Słupska, Anna Chrobok∗, Agnieszka Droz˙ dz˙
Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new application of silyl peroxides as oxidants in the Baeyer–Villiger oxidation of cyclic
ketones in chloroaluminate(III) ionic liquids is described. Among the silyl peroxides, the reac-
tivity of two groups of peroxides was studied: bis(silyl) and t-butyl silyl peroxides possessing
different structured substituents attached to the Si atom. It was shown that the acidic 1-
hexyl-3-methylimidazolium chloroaluminate(III) ionic liquid (molar ratio of AlCl3 in ionic liquid:
0.67) present in the oxidation of cyclic ketones with bis(silyl) peroxides acts as the catalyst.
In this variant of the reaction, the reactivities of bis(silyl) peroxides decrease in the following
order: bis(trimethylsilyl) peroxide > bis(vinyldimethylsilyl) peroxide > bis(phenyldimethylsilyl) perox-
ide > bis(diphenylmethylsilyl) peroxide. A variety of cyclic ketones such as cyclobutanone, 3-substituted
cyclobutanones, cyclopentanone, cyclohexanone, 2-methylcyclohexanone, 4-methylcyclohexanone, 2-
adamantanone and norcamphor were oxidised to their corresponding lactones with high yields
(49–100%). When t-butyl silyl peroxides and neutral chloroaluminate(III) ionic liquids (molar ratio of
AlCl3 in ionic liquid: 0.5) were utilised in the Baeyer–Villiger oxidation, the studied ionic liquid acted as
the reagent. Here, phenyldimethyl(t-butylperoxy)silane was the most efficient oxidant in the oxidation of
cyclobutanone to ␥-butyrolactone (70% yield). Other peroxides, including trimethyl(t-butylperoxy)silane,
vinyldimethyl(t-butylperoxy)silane and diphenylmethyl-(t-butylperoxy)silane, were less reactive oxi-
dants. Two variants of the Baeyer–Villiger reaction mechanism are postulated.
Received 30 November 2012
Received in revised form 15 April 2013
Accepted 21 April 2013
Available online 30 April 2013
Keywords:
Bis(silyl) peroxide
Alkyl silyl peroxide
Baeyer–Villiger reaction
Chloroaluminate(III) ionic liquids
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
catalyst such as SnCl4, BF3·OEt2 or trimethylsilyl trifluoromethane-
sulphonate [5,6]. It is also known that bis(trimethylsilyl) peroxide
Silyl peroxides are an organometallic group of compounds
in which the peroxy group is bonded directly to at least one
silicon atom; therefore, two groups of peroxides can be distin-
guished: bis(silyl) and alkyl silyl peroxides. Bis(silyl) peroxides
are widely used as oxidative agents in organic synthesis. These
compounds are known to generate peroxy anions, R3SiOO−, in con-
trast to dialkyl peroxides. In particular, bis(trimethylsilyl) peroxide
is considered to be an anhydrous, safe form of hydrogen perox-
ide that is very reactive and is often used in organic reactions
[1]. One of these reactions is the Baeyer–Villiger (BV) reaction,
which is based on the formation of esters and lactones by the
oxidation of ketones with peroxide derivatives. It remains one
of the most important reactions in organic chemistry, with a
large range of possible applications, including the synthesis of
antibiotics, steroids, pheromones, and monomers for polymeri-
sation [2–4]. Bis(trimethylsilyl) peroxide can be used as the
oxidant in the Baeyer–Villiger reaction, together with an acidic
can be effectively used in the BV reaction in the presence of 1-
butyl-3-methylimidazolium trifluoromethanesulphonate as both
the solvent and the catalyst [7]. Other bis(silyl) peroxides have not
been investigated in the BV reaction. Moreover, alkyl silyl peroxi-
des are known to be active oxidants in only a few reactions, such
as acyl halide or anhydride oxidations, where the ROO− anion is
generated [1].
Currently, the most frequently used solvents for BV reac-
tions include dichloromethane, chloroform, acetonitrile, and, more
recently, ionic liquids (ILs) [2–4]. Due to the properties of ILs, such as
undetectable vapour pressure, the ability to dissolve many organic
and inorganic substances, high thermal stability and broad liquid
range, ionic liquids have become a promising new reaction medium
[8,9]. The range of applications of ionic liquids is extremely large.
The application of ionic liquids in organic synthesis in both stoi-
chiometric and catalytic reactions has demonstrated their ability
to improve yields and selectively enhance reaction rates [10–12].
The last decade has shown that there is significant interest in task-
specific ionic liquids, which refer to the potential “design” capacity of
ionic liquids for chemical tasks. Recently, chlorometallate(III) ionic
liquids possessing high Lewis acidity have generated attention as
∗
Corresponding author. Fax: +48 32 237 10 32.
E-mail address: Anna.Chrobok@polsl.pl (A. Chrobok).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.04.023

S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
121
χ
AlCl3
0.25 0.33
Cl-, [AlCl4]- [AlCl4]- [AlCl4]-, [Al2Cl7]-
0.50
0.67
0.75
[Al2Cl7]-, [Al3Cl10]-,[Al2Cl6]
>0.75
[Al3Cl10]-, [Al4Cl13]-
,[Al2Cl6]
χ
Where means molar ratio of AlCl3 in the ionic liquid.
Scheme 1. Major anions present in chloroaluminate(III) ionic liquids as a function of the molar ratio of AlCl3 [16].
catalysts and co-catalysts in many organic reactions. Among them,
chloroaluminates (III) are the most frequently used ionic liquids,
e.g., in Diels–Alder, Friedel–Crafts and polymerisation reactions
[13–15]. The structure of the major anions present in chloroa-
luminate(III) systems depends on the molar ratio of substrates
(Scheme 1) [16]. When a high molar ratio of AlCl3 is used, the
participation of [Al2Cl6] is observed as indicated by the arrow in
Scheme 1.
(m, 2H); 4.19 (t, 2H, J = 7.4 Hz); 3.96 (s, 3H); 1.81–1.97 (m, 2H);
1.25–1.36 (m, 6H); 0.96 (t, 3H, J = 7.3 Hz); 13C NMR (125 MHz,
CD3OD) ı 137.83, 124.91, 123.63, 50.52, 36.54, 30.93, 29.89, 25.72,
22.24, 13.76. MS: Peaks found in positive ESI (+) mode Mass Spec-
tra: 167 ([hmim]+); 369 ([hmim]2Cl+); and negative ESI (−) mode
Mass Spectra:169 ([AlCl4]); 237 ([hmim]Cl2−); 466 ([hmim]Al2Cl7).
2.3. Stability of lactones in ionic liquids
Herein, we present a new application of bis(silyl) peroxides and
alkyl silyl peroxides as oxidants in BV reactions with chloroalumi-
nate(III) ionic liquids as Lewis acid catalysts and reaction media.
A sample of lactone (1.5 mmol) was added to the freshly syn-
thesised acidic 1-hexyl-3-methylimidazolium chloroaluminate(III)
(2 mmol), and the solution was stirred under a nitrogen atmosphere
at room temperature for 3 h. Next, the process was quenched with
5 ml of water. The mixture was extracted with methylene chloride
or diethyl ether (10× 5 ml). The organic layer was washed with 5 ml
of water, dried over anhydrous MgSO4, filtered and concentrated
in a vacuum.
2. Experimental
2.1. Materials and procedures
1-Hexyl-3-methylimidazolium chloride was supplied by
Merck; anhydrous aluminium chloride (packed in ampules)
was supplied by Sigma Aldrich. The commercially available
cyclic ketones: cyclobutanone, cyclopentanone, cyclohex-
2.4. General procedure for cyclic ketones oxidation
The ketone (1.5 mmol) and silyl peroxide (2.25 mmol for reac-
tions with bis(silyl) peroxides and 3 mmol for reactions with t-butyl
silyl peroxides) were added dropwise at 0 ◦C into the freshly
synthesised [hmim][AlxCly] (2 mmol for reactions with bis(silyl)
peroxides and 3 mmol for reactions with t-butyl silyl peroxides).
Next, the solution was stirred under a nitrogen atmosphere at room
temperature for 1–24 h (depending on the reaction rate). After
this time, 5 ml of water was added to the post-reaction mixture,
and the water phase was extracted with methylene chloride or
diethyl ether (10× 5 ml). The organic layer was washed with 5 ml
of water, dried over anhydrous MgSO4, filtered, concentrated in
a vacuum (50 ◦C, 100 mbar) and purified by column chromatogra-
phy (hexane:ethyl acetate (4:1)) when necessary. All products were
characterised by comparison of their NMR spectra (see Supplemen-
tary data) with authentic samples [20].
anone,
2-adamantanone
Acros Organics.
phenylcyclobutanone,
2-methylcyclohexanone,
and norcamphor
Three-substituted
4-methylcyclohexanone,
were
supplied
by
cyclobutanones
(3-
3-(4ꢀ-methoxyphenyl)cyclobutanone,
3-(4ꢀ-chlorophenyl)cyclobutanone and 3-butylcyclobutanone)
were synthesised according to known procedures in two steps via
a [2 + 2] cycloaddition of dichloroketene to the vinyl derivative,
followed by the reduction of the resulting dichloro ketones with
zinc in AcOH [17]. Bis(trimethylsilyl) peroxide was synthesised
in a reaction of N,Nꢀ-bis(trimethylsilyl)urea supplied by Aldrich
with a urea hydrogen peroxide adduct supplied by Acros Organics
[18]. Other bis(silyl) peroxides were synthesised from a 1,4-
diaza[2,2,2]bicyclooctane hydrogen peroxide adduct (Aldrich) and
chlorosilanes (Acros Organics) [19]. The structure and purity of all
synthesised substances were confirmed by NMR analysis. 1H NMR
and 13C NMR spectra were recorded at 300 MHz in CDCl3 (Varian
Unity Inova plus, internal TMS) and are provided in the Supple-
mentary data. Electrospray ionisation mass spectroscopy (ESI-MS)
experiments were carried out using a Waters Xevo G2 QTOF
with injection system (cone voltage 50 V; source 120 ◦C). Fourier
transform infrared absorption (FT-IR) spectra were recorded with
a Fourier transform infrared spectrometer (IC10, Mettler-Toledo
Co., Ltd.) with an ATR diamond probe in the transmission mode.
In experiments with the FT-IR probe placed in the reaction mix-
ture, the reactions were scaled up by a factor of 10. The reactions
were performed in the same manner as described above.
3. Results and discussion
By using silyl peroxides as oxidants and chloroaluminate(III)
ionic liquids as Lewis acids, we aimed to develop a clean BV
oxidation process (Scheme 2). The synthesis of ␥-butyrolactones,
␦-valerolactones and ␧-caprolactones was chosen as the target
reaction.
Accordingly, a range of chloroaluminate(III) ionic liquids (acidic,
basic and neutral based on the 1-hexyl-3-methylimidazolium
[hmim] cation) were used. Chloroaluminate(III) ionic liquids are
2.2. Synthesis of basic, neutral and acidic chloroaluminate(III)
ionic liquids
The chloroaluminate ionic liquids were prepared by mixing
appropriate amounts of 1-methyl-3-hexylimidazolium chloride
(2 mmol) and aluminium chloride (1.0–4.0 mmol). Both substrates
were weighed in a two-necked 10 ml round bottom flask in a glove
bag under a nitrogen atmosphere. Next, the flask was equipped
with a balloon filled with nitrogen, closed with a septum and then
stirred for 1 h. The obtained mixtures were clear, light to dark yel-
low liquids and were immediately used for further reactions.
1-Hexyl-3-methylimidazolium chloroaluminate(III) (ꢀAlCl3 =
0.67): 1H-NMR (300 MHz, CD3OD, TMS): ı = 9.05 (s, 1H); 7.58–7.67
Scheme 2. Oxidation of cyclic ketones via the Baeyer–Villiger reaction.

122
S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
Scheme 3. The structures of the investigated silyl peroxides.
extremely sensitive to moisture and can easily undergo hydroly-
sis; therefore, these compounds were freshly synthesised under a
nitrogen atmosphere (dry conditions) in a glove bag immediately
before each reaction. It was interesting to study both bis(silyl) and
alkyl silyl peroxides as oxidants (Scheme 3). As a model represen-
tative of alkyl silyl peroxides, t-butyl silyl peroxides with different
alkyl or aryl substituents attached to the Si atom were used.
Lactones are known to undergo open-ring polymerisation in the
presence of acids. Therefore, the preliminary studies were aimed at
confirming the stability of lactones chosen as targets in this work
by mixing them with acidic chloroaluminate(III) ionic liquid and
isolating intact.
[Al2Cl7]− are effective catalysts. Basic and neutral ionic liquids with
[AlCl4]− anions were also active in the investigated reaction, but
the yields of ␥-butyrolactone obtained after 3 h were moderate
(approximately 77%).
We assume that chloroaluminate(III) ionic liquid can serve as
both the Lewis acid catalyst and the solvent. Based on the fact that
the oxidation reaction of cyclic ketones with bis(silyl) peroxides in
[hmim][Cl] as the reaction medium did not occur and that the reac-
tion was most efficient in acidic chloroaluminate(III) ionic liquids,
we propose the following mechanism for the reaction (Scheme 4).
In the first step, the ionic liquid can activate the carbonyl group in
the ketone molecule. Next, the trialkylsilyl peroxy anion is gener-
ated by the reaction with the chloroaluminate(III) anion [Al2Cl7]−
and attacks the carbonyl group. The Criegee adduct A is formed in
parallel with the ionic species [R3Si]+[Al2Cl7]−, which is involved
in the rearrangement of B to the final ester and the stable hex-
aalkylsiloxane C. At the same time, the catalyst [hmim][Al2Cl7]
is regenerated. The hexamethylsiloxane (CH3)3SiOSi(CH3)3 was
insoluble in the reaction mixture and was isolated as a separate
liquid layer. The structure of hexamethylsiloxane was determined
by NMR analysis (see Supplementary data).
3.1. Application of bis(silyl) peroxides in the Baeyer–Villiger
reaction
The three compositions of AlCl3 and [hmim][Cl] at the molar
ratios of 0.67, 0.50 and 0.33 were prepared under anhydrous condi-
tions. These systems were tested as media for the model oxidation
of cyclobutanone to ␥-butyrolactone with bis(trimethylsilyl) per-
oxide (1) as the oxidant. After the appropriate period of time, the
reaction mixtures were quenched with water, and the products
were extracted from the water phase with methylene chloride.
As can be observed in Table 1, the Baeyer–Villiger oxidation pro-
ceeded in the presence of chloroaluminate(III) ionic liquids and in
the presence of a classical Lewis acid, AlCl3, in dichloromethane
as a solvent. ␥-Butyrolactone was not observed in the reaction
performed in the presence of [hmim][Cl]. This fact confirms the
catalytic effect of chloroaluminate(III) ionic liquids. The most
active system was the acidic one, which indicates that ionic
liquids possessing mainly anionic polynuclear species such as
The course (the formation and/or disappearance of charac-
teristic bands) of the above-mentioned reaction was observed
with FT-IR spectroscopy. For this reason, the spectrum of the
reaction mixture (cyclobutanone oxidation in acidic ionic liquid
(ꢀAlCl3 = 0.67) with Me3SiOOSiMe3) was registered with the IR
probe every 15 s. Generally, the whole reaction mixture spectrum
is complicated (see Supplementary data). However, the appear-
ance of a 991.7 cm−1 band during the reaction (characteristic
for ␥-butyrolactone) is noticeable. Additionally, the characteristic
1059.1 cm−1 band of a peroxy group
O
O
(Me3SiOOSiMe3) can
be observed. This band decreased in intensity during the reaction,
while the band in the region of 1038.3 cm−1 appeared. This peak can
be assigned to the by-product Me3SiOSiMe3. The frequencies of the
characteristic bands are compatible with literature data [21,22].
The creation of the lactone and the hexamethylsiloxane in this
reaction can be regarded as a strong argument for the generation
of the trialkylsilyl peroxy anion instead of a homolytic cleavage of
bis(silyl) peroxide.
At this stage, we also checked the reactivity of different
structured bis(silyl) peroxides (Table 2). As expected, the reac-
tivity of bis(silyl) peroxides depended on the structure of the
alkyl substituents connected to the Si atom. The most effi-
cient oxidant among them was bis(trimethylsilyl) peroxide.
The small methyl substituents connected to the silicon atom
cause low steric hindrance that enables the effective attack of
the newly created silyl peroxy anion on the carbonyl group.
As a result, the high yield of ␥-butyrolactone was obtained.
Table 1
Oxidation of cyclobutanone (1.5 mmol) with peroxide (1) (2.25 mmol) in the pres-
ence of [hmim][AlxCly] (2.25 mmol) in various compositions at room temperature.
Entry
Molar ratio
[hmim][Cl]:AlCl3
Reaction time (h)
␥-Butyrolactone
yield (%)
1:2 (ꢀAlCl3 = 0.67,
acidic)
1
0.5
99
74
1
2
3
1:1 (ꢀAlCl3 = 0.50,
neutral)
1
3
51
77
1:0.5 (ꢀAlCl3 = 0.33,
basic)
1
3
49
75
4
5
1:0
3
3
0
0:1 (CH2Cl2 as
a solvent)
95

S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
123
Scheme 4. Proposed mechanism of the Baeyer–Villiger oxidation of cyclic ketones with bis(silyl) peroxides in the presence of acidic [hmim][AlxCly] as the solvent and the
catalyst.
With the increase in size of the substituents on the sili-
con atom, the steric hindrance increased and the reactivity of
bis(silyl) peroxide decreased as follows: bis(trimethylsilyl) perox-
ide > bis(vinyldimethylsilyl) peroxide) > bis(phenyldimethylsilyl)
peroxide > bis(diphenylmethylsilyl) peroxide. All investigated
peroxides could be used as effective oxidants in this reaction.
Finally, the most reactive peroxide (1) with acidic ionic liquid
was examined in the BV oxidation of various ketones to determine
its practical potentials (Table 3).
In practical terms, cyclobutanones (Table 3, entries 1–5) were
readily oxidised to their corresponding lactones in high yields
(80–99%) within a relatively short time. In the cases of full con-
versions of cyclobutanones (˛ = 100%, Table 3, entry 1–3) just after
the extraction and evaporation of solvent, the pure products were
obtained. The oxidation of cyclopentanone proceeded at a high
conversion rate, but under these conditions, the resulting lactone
underwent hydrolysis to 5-hydroxyvaleric acid. All ␧-caprolactones
were obtained in moderate yields. 2-Methylcyclohexanone could
be oxidised to give 34% of the regioisomer.
few examples of heterolytic cleavage of R3Si OO CR3 are available
in the literature [1].
In this work, we present the first application of alkyl silyl per-
oxides in the BV reaction in the presence of basic, neutral and
acidic 1-methyl-3-hexylimidazolium chloroaluminates (III). As a
model peroxide, phenyldimethyl(t-butylperoxy)silane (7) was cho-
sen (Table 4).
Surprisingly, the presence of ␥-butyrolactone was observed
under all investigated reaction conditions for which chloroalumi-
nate(III) ionic liquids were used. The highest yields of product were
obtained in the reaction with the neutral ionic liquid possessing
mainly [AlCl4]− anions.
To propose the mechanism of this reaction, we first determined
which of the two possible peroxy anions (R3SiOO− or R3COO−) is
created during the reaction course. For this purpose, the in situ FT-IR
analysis of the reaction of PhMe2SiOOtBu with neutral ionic liquid
(ꢀAlCl3 = 0.50) was studied. The peroxide was added dropwise at
0 ◦C under anhydrous conditions to the ionic liquid placed in the
flask equipped with an IR probe. After addition of the peroxide, the
reaction was performed at RT.
The spectrum of the reaction mixture was registered every 15 s.
For clarity of data, only the first scan after peroxide addition and
the last scan after 8 h are presented in Fig. 1. For comparison, the
spectrum of pure PhMe2SiOOtBu (standard) was added.
3.2. Application of t-butyl silyl peroxides in the BV reaction
It is known that dialkyl peroxides are not active as oxidants in
the Baeyer–Villiger reaction. Moreover, alkyl silyl peroxides have
not been investigated as oxidants in this reaction at all. However, a
The characteristic band of
a
peroxy group
O O
(PhMe2SiOOtBu) can be observed in the region 902.5 cm−1
,
which completely decays during the reaction. At the same time,
the growth of a broad 1051.4 cm−1 band, which can be assigned to
the anion COO− (t-BuOO−), appears. Additionally, the characteris-
tic band for an OH group, which could be attributed to t-BuOOH,
was not observed in the FT-IR spectrum (3383.4 cm−1). The fre-
quencies of characteristic bands are compatible with literature
data [21–23].
Table 2
Oxidation of cyclobutanone (1.5 mmol) with bis(silyl) peroxides (2.25 mmol) in the
presence of acidic ionic liquid [hmim][AlxCly] (2.0 mmol) at room temperature. The
reaction time was 1 h.
Entry
Bis(silyl) peroxide
␥-Butyrolactone yield (%)
The 1051.4 cm−1 band width is most likely influenced by the
stabilisation of the above-mentioned anion by the large cation
[bmim]+. The stability of the generated t-BuOO− anion was also
confirmed by iodometric titration of the studied mixture after 12 h
of mixing. The decrement of the concentration of the peroxy group
was not detected. Additionally, the NMR analysis of the isolated
products from the post-reaction mixture confirmed the genera-
tion of PhMe2SiCl (see Supplementary data). It was not possible
to observe when the IR probe was used (the characteristic Si Cl
bond could oscillate in a range of 625–425 cm−1 [21]).
Based on the results presented above, we propose the following
mechanism of the Baeyer–Villiger reaction with t-butyl silyl peroxi-
des in the presence of chloroaluminate(III) ionic liquids (Scheme 5).
In the first step, the decomposition of peroxide to R3COO− and R3Si+
occurs. Next, the attack of the peroxy anion R3COO− on the carbonyl
O
Si
Si
O
O
(1)
1
2
3
99
85
75
Si
O
Si
(2)
Ph
Ph
Si
Si
O
O
Si
Ph
(3)
(4)
Ph
O
Ph
Si
O
Ph
4
72

124
S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
Table 3
Oxidation of cyclic ketones (1.5 mmol) with peroxide (1) (2.25 mmol) in the presence of acidic ionic liquid [hmim][AlxCly] (2.0 mmol) as the solvent and the catalyst.
Entry
Ketone
Lactone
Reaction time (h)
Conversiona (%)
Yieldb (%)
O
O
O
1
1
100
97
O
O
O
nBu
nBu
O
2
3
4
3
3
3
100
100
89
93
83
80
O
O
O
Ph
Ph
O
O
p-Cl-Ph
p-Cl-Ph
O
O
O
p-OMe-Ph
O
p-OMe-Ph
O
5
6
3
3
90
70
83
O
34c
O
O
O
7
24
60
55
O
O
O
O
O
(34%)
8
9
24
24
69
67
66
61
O
O
O
O
O
O
10
24
24
50
49
44
41
O
O
O
11
a
Determined by NMR analysis.
Isolated yields.
The part of created lactone undergoes the reaction of hydrolysis.
b
c

S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
125
Fig. 1. FT-IR spectra of the reaction course of PhMe2SiOOtBu (30 mmol) with neutral ionic liquid [hmim][AlxCly] (30 mmol) – initial and final state (8 h) and standard spectrum
of PhMe2SiOOtBu.
where [AlCl4]− is the dominant anion. In this step, the role of the
ionic liquid is essential; therefore, the BV reaction with t-butyl silyl
peroxides as oxidants in the presence of AlCl3 alone in classical sol-
Table 4
Oxidation of cyclobutanone (1.5 mmol) with peroxide (7) (3.0 mmol) in the presence
of ionic liquid [hmim][AlxCly] in various compositions at room temperature. The
reaction time was 3 h.
vents, e.g., methylene chloride, does not occur (Table 4, entry 5).
Entry
Molar ratio
[hmim][Cl]:AlCl3
Molar ratio
IL:ketone
␥-Butyrolactone
yield (%)
In the last step, water was added to quench the reaction. Thus, the
proton that is necessary to create t-BuOH was inserted into the reac-
tion system. The NMR analysis of the products of this reaction after
isolation with water showed the presence of R3SiCl and R3SiOH as
by-products (see Supplementary data).
1
2
3
4
5
1:2 (ꢀAlCl3 = 0.67,
acidic)
1:1 (ꢀAlCl3 = 0.50,
neutral)
1:1 (ꢀAlCl3 = 0.50,
neutral)
1:0.5 (ꢀAlCl3 = 0.33,
basic)
2
33
70
40
30
0
2
The reaction between the R3Si+ cation and a chloroaluminate(III)
ionic liquid was also observed by Yin et al. during the study of
the [BMIM]Cl–nAlCl3 ionic liquid-catalysed redistribution reac-
tion between methyltrichlorosilane and a low-boiling residue to
dimethyldichlorosilane [24].
0.5
2
0:1 (CH2Cl2 as a
solvent)
–
At this stage, the course of the cyclobutanone oxidation in
the neutral ionic liquid with PhMe2SiOOtBu as the oxidant was
also registered with the IR probe every 15 s. Unfortunately, we
could not observe the changes in the characteristic 1050 cm−1
band (t-BuOO−), which is overlapped by the groups coming from
cyclobutanone and ␥-butyrolactone that oscillate in the same
group of the ketone takes place. At the same time, the cation R3Si+
can be stabilised to R3SiCl as a consequent product of the reaction
with [AlCl4]−. This stabilisation process is likely to be most effi-
cient in the presence of a neutral chloroaluminate(III) ionic liquid,
Scheme 5. Proposed mechanism of the Baeyer–Villiger oxidation of ketones by t-butyl silyl peroxides in the presence of neutral [hmim][AlxCly].

126
S. Baj et al. / Journal of Molecular Catalysis A: Chemical 376 (2013) 120–126
Table 5
Two different mechanisms of the Baeyer–Villiger reaction were
proposed. For the reaction performed with bis(silyl) peroxides,
chloroaluminate(III) ionic liquids can act as the catalyst and the
solvent. The highest yields of lactones were obtained in an acidic
ionic liquid system. In the case of applying t-butyl silyl peroxides
for this reaction, the studied chloroaluminate(III) ionic liquids can
also serve as the catalyst and the solvent, but the consumption of
the chloroaluminate(III) anion was observed during the reaction.
The best results were obtained in neutral ionic liquids.
In summary, the method presented above is the most efficient
for the synthesis of substituted ␥-butyrolactones, which can be
easily obtained in high yields (80–97%). Other lactones, such as
␧-caprolactones, were obtained in moderate yields. In the case of
the oxidation of cyclopentanone, the newly created lactone imme-
diately undergoes consecutive reactions. All results presented in
this study confirmed the generation of peroxy anions, R3SiOO−
or R3COO− from corresponding peroxides under the influence of
chloroaluminate(III) ionic liquids.
Oxidation of cyclobutanone (1.5 mmol) with t-butyl silyl peroxides (3 mmol) in the
presence of neutral [hmim][AlxCly] (3 mmol) at room temperature. The reaction time
was 3 h.
Entry
t-Butyl silyl peroxide
␥-Butyrolactone yield (%)
O
Si
O
Ph
(7)
1
2
3
4
70
50
49
36
O
Si
O
O
(5)
Si
O
(6)
Ph
O
Si
O
Ph
(8)
Acknowledgements
This work was financed by the Ministry of Science and Higher
Education (Grant no. N N209 021739). Authors of this publication
are scholars in SWIFT project POKL.08.02.01-24-005/10 which is
co-financed by European Union within European Social Fund.
region. However, we still were able to observe the decrease of the
peroxy band (902.5 cm−1).
Additional proof of this mechanism is the fact that performing
this reaction with lowers than stoichiometric amounts of the ionic
liquid leads to a decrease in the ␥-butyrolactone yield (Table 4,
entry 3). This phenomenon confirms the consumption of the ionic
liquid during the reaction.
The influence of other t-butyl silyl peroxides on the
Baeyer–Villiger oxidation of cyclobutanone was studied (Table 5).
The model lactone was obtained in the range of 36–70%. The
following order of reactivity of t-butyl silyl peroxides was
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
molcata.2013.04.023.
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In this study,
a new general method for the oxidation
of cyclic ketones with silyl peroxides in the presence of
chloroaluminate(III) ionic liquids was presented. Various systems
(acidic, neutral and basic) based on 1-hexyl-3-methyl-imidazolium
chloroaluminate(III) were tested as solvents and catalysts.
Bis(silyl) and t-butyl silyl peroxides were used as the oxi-
dants.