Exceptional activity of gallium(III) chloride and chlorogallate(III) ionic liquids for Baeyer–Villiger oxidation

Article abstract of DOI:10.1039/C6RA03435G

Baeyer–Villiger oxidation of cyclic ketones, using H₂O₂ as the oxidising agent, was systematically studied using a range of metal chlorides in different solvents, and in neat chlorogallate(III) ionic liquids. The extremely high activity of GaCl₃ in promoting oxidation with H₂O₂, irrespective of solvent, was reported for the first time. The activity of all other metal chlorides was strongly solvent-dependent. In particular, AlCl₃ was very active in a protic solvent (ethanol), and tin chlorides, SnCl₄ and SnCl₂, were active in aprotic solvents (toluene and dioxane). In order to eliminate the need for volatile organic solvent, a Lewis acidic chlorogallate(III) ionic liquid was used in the place of GaCl₃, which afforded typically 89–94% yields of lactones in 1–120 min, at ambient conditions. Raman and ⁷¹Ga NMR spectroscopic studies suggest that the active species, in both GaCl₃ and chlorogallate(III) ionic liquid systems, are chlorohydroxygallate(III) anions, [GaCl₃OH], which are the products of partial hydrolysis of GaCl₃ and chlorogallate(III) anions; therefore, the presence of water is crucial.

Full text of DOI:10.1039/C6RA03435G

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PAPER
Exceptional activity of gallium(III) chloride and
chlorogallate(III) ionic liquids for Baeyer–Villiger
oxidation†
Cite this: RSC Adv., 2016, 6, 30460
a
a
Magdalena Markiton, Anna Chrobok, Karolina Matuszek,a Kenneth R. Seddonb
*
b
*
and Małgorzata Swadzba-Kwasny
´
´
Baeyer–Villiger oxidation of cyclic ketones, using H2O2 as the oxidising agent, was systematically studied
using a range of metal chlorides in different solvents, and in neat chlorogallate(III) ionic liquids. The
extremely high activity of GaCl3 in promoting oxidation with H2O2, irrespective of solvent, was reported
for the first time. The activity of all other metal chlorides was strongly solvent-dependent. In particular,
AlCl3 was very active in a protic solvent (ethanol), and tin chlorides, SnCl4 and SnCl2, were active in
aprotic solvents (toluene and dioxane). In order to eliminate the need for volatile organic solvent, a Lewis
acidic chlorogallate(III) ionic liquid was used in the place of GaCl3, which afforded typically 89–94% yields
of lactones in 1–120 min, at ambient conditions. Raman and 71Ga NMR spectroscopic studies suggest
that the active species, in both GaCl3 and chlorogallate(III) ionic liquid systems, are
chlorohydroxygallate(III) anions, [GaCl3OH]ꢀ, which are the products of partial hydrolysis of GaCl3 and
chlorogallate(III) anions; therefore, the presence of water is crucial.
Received 5th February 2016
Accepted 17th March 2016
DOI: 10.1039/c6ra03435g
www.rsc.org/advances
polarisation of the C]O double bond.2,4 A number of Lewis
acidic metal chlorides (AlCl3, SnCl4, FeCl3, ZrCl4, TiCl4 and
Introduction
ZnCl2) have been used as catalysts in Baeyer–Villiger oxidation
with H2O2 (as aqueous solution) or with silyl peroxides (which is
a masked version of 100% H2O2).3,5–10 However, the literature
data are fragmentary, and it is not possible to directly compare
the catalysts due to the use of different solvents, substrates and
conditions. The most studied reaction is the oxidation of
ketones with H2O2 catalysed by AlCl3, both homogenously (in
ethanol), and heterogeneously (on alumina or silica
supports).5–7 Less encouraging results were found using tin
chlorides: SnCl4 in methyl t-butyl ether was very weakly active in
these oxidation of 2-adamantanone with H2O2, yielding only 3%
conversion,8 whereas SnCl2 in butanol was found inactive in the
oxidation with H2O2.9 In contrast, SnCl4, AlCl3 and FeCl3 were
all active catalysts in BV oxidation with bistrimethylsilyl
peroxide as an oxidising agent.10
Baeyer–Villiger (BV) oxidation, in which linear or cyclic ketones
are oxidised to esters or lactones, has been known for over 110
years.1 It is particularly useful to generate lactones, used as
intermediates for pharmaceuticals, herbicides and polymers, as
well as solvents, avours and fragrance additives.1,2 Common
oxidants in BV oxidation are peroxyacids, such as peroxyacetic
or m-chloroperoxybenzoic. They are very active oxidants, but
also intrinsically unstable and shock-sensitive, and thus
expensive to store and transport, hazardous to use, and they
produce acidic waste. All these factors drive the search for more
sustainable oxidising agents.
Aqueous hydrogen peroxide, a much ‘greener’ alternative: it
is more stable, therefore safer to use, and generates water as the
oxidation by-product. Although H2O2 is characterised by a high
content of active oxygen, it is kinetically inert, and requires an
acidic catalyst to oxidise ketones.2,3
Additionally, several organoselenium compounds, like bis[3,5-
bis(triuoromethyl)phenyl]diselenide,11
3,5-bis(peruorooctyl)
Lewis acids catalyse BV oxidation by coordinating to the
carbonyl group of a ketone, thereby activating it towards
the nucleophilic attack of hydrogen peroxide by increasing the
phenyl butylselenide,12 diselenide bearing bistriate13 and
dibenzyl diselenide14 were used as catalysts for the Baeyer–Villiger
oxidation with H2O2 (30 or 60% aq.). The oxidation of ketones
were carried out generally at room temperature from 2 (for
cyclobutanone) to 24 h (for larger cyclic ketones) giving high
yields of lactones.
aDepartment of Chemical Organic Technology and Petrochemistry, Silesian University
of Technology, Krzywoustego 4, 44-100 Gliwice, Poland. E-mail: anna.chrobok@polsl.
pl
The very promising results with H2O2 as an oxidant were also
achieved using molecular sieves modied with tin (Sn-Beta) or
Sn(IV) centres incorporated into mesoporous MCM-41 (Sn–
MCM-41) as catalysts, in dioxane at 90 ꢁC.8,15 Surprisingly,
bQUILL, The Queen's University of Belfast, Belfast, BT9 5AG, UK. E-mail: m.
swadzba-kwasny@qub.ac.uk
† Electronic supplementary information (ESI) available: NMR spectra of cyclic
ketones and lactones. See DOI: 10.1039/c6ra03435g
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homogenous BV oxidation in dioxane with Lewis acidic chlo-
rides has not been reported.
Raman analyses were performed using a Perkin-Elmer
Raman Station 400F, with a 785 nm focussed laser beam. All
In the search for a sustainable BV oxidation, we have studied samples were studied in quartz cuvettes, with forty 2 second
the use of ionic liquids, both as solvents and, more recently, as scans recorded for each sample.
catalysts. In studying the solvent effect, it has been demon-
strated that both conversion and reaction kinetics depended on
Synthetic procedures
the structure of ionic liquid or molecular solvent used.16,17
Chlorometallate(III) ionic liquids. Chlorometallate(III) ionic
liquids were synthesised in a nitrogen-lled glovebox (MBraun
labmaster dp, <0.3 ppm of H2O and O2). In a typical procedure,19
Mildly Brønsted acidic, supported ionic liquids based on
[HSO4]ꢀ, were active and recyclable catalysts for BV oxidation
with H2O2 in dichloromethane.16 Strongly Lewis acidic ionic
1-butyl-3-methylimidazolium chloride, [C4mim]Cl, was placed
liquids with chloroaluminate(III) anions were very potent cata-
in a vial (10 cm3) equipped with a stirring bar. Then, metal(III)
lysts for cyclobutanone oxidations with silyl peroxides.17 Despite
chloride (M ¼ Al or Ga) was added slowly, in portions, to achieve
promising results, this work remains the only example of BV
the desired composition, and allowed to react until a homoge-
oxidation catalysed with Lewis acidic ionic liquids reported to-
nous liquid was formed (3 h, 60 ꢁC, 1000 rpm). Exact amounts of
reactants are shown in Table 1.
Oxidation of ketones in the presence of metal chlorides. Into
a two-necked round-bottomed ask equipped with a septum,
a condenser with a balloon lled with nitrogen, and a magnetic
stirring bar, 2-adamantanone (0.1 g, 0.67 mmol) and metal
date.
Here, we aim to directly compare the catalytic performance
of various metal chlorides in BV oxidation of model cyclic
ketones with H2O2, and to investigate whether the performance
of the most active metal chlorides could be further enhanced by
using them in the form of chlorometallate ionic liquids, rather
than as a solution in a molecular solvent.
chloride (0.008–0.017 g, 0.067 mmol) were placed. Then,
a molecular solvent (5 cm3, see Table 2) was added, followed by
the slow addition of 30% aq. H2O2 (0.152 g, 1.34 mmol). The
ꢁ
Experimental
reactions were carried out at 40–90 C, and monitored by GC.
Materials and methods
All materials (metal chlorides, 1-butyl-3-methylimidazolium
Table 2 The influence of the metal chloride MClx structure on the
oxidation of 2-adamantanonea
chloride, ketones) were purchased from Sigma-Aldrich. Metal
chlorides were anhydrous (purity 99.99%, sealed under argon in
glass ampoules).
Solvent
MClx
Yield of lactoneb/%
Substituted cyclobutanones (3-phenylcyclobutanone and
3-butylcyclobutanone) were synthesised following a standard
two-step procedure: a [2 + 2] cycloaddition of dichloroketene to
the vinyl derivative, followed by the reduction of the resulting
dichloroketones with zinc in acetic acid (see ESI† for the NMR
spectra).18
Dichloromethane 40 ꢁ
C
GaCl3
AlCl3
SnCl2
SnCl4
InCl3
TiCl3
TiCl4
ZnCl2
FeCl3
GaCl3
AlCl3
SnCl2
SnCl4
InCl3
TiCl3
TiCl4
ZnCl2
FeCl3
GaCl3
AlCl3
SnCl2
SnCl4
InCl3
TiCl3
TiCl4
ZnCl2
FeCl3
SnCl2
SnCl4
96
32
57
65
43
25
12
1
GC analyses were performed using a PerkinElmer Clarus
500 gas chromatograph equipped with an SPB™-5 column (30 m
ꢂ 0.2 mm ꢂ 0.2 mm).
7
Toluene 90 ꢁC
99
12
98
96
71
0
7
66
1
75
94
57
78
6
18
22
23
4
1H NMR spectra were recorded at 300 MHz or 600 MHz and
13C NMR at 75 or 150 MHz (Varian system), respectively.
71Ga NMR spectra were recorded at 300 K using a Bruker
AvanceIII 400 MHz spectrometer. Samples were analysed neat,
using d6-dimethylsulfoxide capillaries as an external lock. A
solution of Ga(NO3)3 hydrate in D2O was used as an external
reference (d ¼ 0 ppm).
Ethanol 70 ꢁC
Table 1 Compositions of chlorometallate ionic liquids
[C4mim]Cl
MClx
a
x
Ionic liquid
cMCl
mmol
g
mmol
g
1,4-Dioxane 90 ꢁC
83
83
[C4mim]Cl–GaCl3
[C4mim]Cl–GaCl3
[C4mim]Cl–GaCl3
[C4mim]Cl–AlCl3
0.50
0.67
0.75
0.67
5.70
3.80
2.85
4.53
0.9956
0.6637
0.4978
0.7913
5.70
7.59
8.54
9.06
1.0037
1.3364
1.5037
1.2081
a
Reaction conditions: 2-adamantanone (0.67 mmol), H2O2 30% aq. (1.34
mmol), solvent 5 cm3 catalyst 10 mol%, reaction time 6 h. Yields
b
a
cMCl – molar fraction of metal chlorides in ionic liquids.
determined by GC.
x
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(m, 2H), 4.21–4.26 (m, 2H). 13C-NMR (150 MHz, CDCl3): d/ppm
22.9, 28.9, 29.3, 34.5, 69.2, 176.4.
Oxidation of ketones in the presence of ionic liquids. Into
a two-necked round-bottomed ask equipped with a septum,
a condenser with a balloon lled with nitrogen, and a magnetic
stirring bar, a ketone (0.094–0.234 g, 1.34 mmol) and ionic
liquid (0.470–0.942 g, 1.34 mmol) were placed. Then, 30% aq.
H2O2 (0.304 g, 2.68 mmol) was slowly added into the ask using
a syringe. Aer the reaction completion (1–120 min), 2 cm3 of
water and 2 cm3 of dichloromethane were added, and stirred
vigorously (5 min). The organic phase was analysed by GC with
decane as an internal standard. For the isolation of product, the
reaction mixture was dissolved in water (2 cm3), extracted with
dichloromethane (6 ꢂ 5 cm3) and concentrated. The yields of
lactones aer the purication using column chromatography
with a short bed of Al2O3 (the conversion of ketone was 100%,
and only the isolation of traces of ionic liquid was necessary)
and dichloromethane as the eluent were 89–93%. 4-Oxatricyclo
[4.3.1.13.8]undecan-5-one was isolated by the extraction of the
post reaction mixture with toluene (10 ꢂ 2 cm3). Aer vacuum
drying, the lactone was puried by crystallisation from hexane–
ethyl acetate. The structure and purity of all synthesised ketones
and lactones were conrmed by 1H and 13C NMR analysis (ESI)
and were essentially identical with published spectral data for
authentic samples:
4-Methyl-3-caprolactone.21 (1H-NMR, 300 MHz, CDCl3, TMS): d/
ppm 0.94 (d, J ¼ 6.6 Hz, 3H), 1.80–1.92 (m, 5H), 2.55–2.71 (m,
2H), 4.15–4.25 (m, 2H). 13C-NMR (75 MHz, CDCl3): d/ppm 18.8,
21.6, 34.2, 36.7, 40.2, 67.5, 175.5.
4-Ethyl-3-caprolactone.22 (1H NMR, 600 MHz, CDCl3, TMS): d/
ppm 0.92 (t, J ¼ 7.5 Hz, 3H), 1.40–1.28 (m, 3H), 1.57–1.43 (m,
2H), 1.93 (dddt, J ¼ 15.1, 7.5, 3.6, 2.0 Hz, 1H), 2.03–1.96 (m, 1H),
2.61 (ddd, J ¼ 14.1, 12.4, 2.1 Hz, 1H), 2.69 (ddd, J ¼ 14.1, 7.6, 1.7
Hz, 1H), 4.18 (dd, J ¼ 12.8, 10.1 Hz, 1H), 4.31 (ddd, J ¼ 12.7, 5.9,
1.6 Hz, 1H). 13C NMR (150 MHz, CDCl3): d/ppm 11.14, 28.38,
28.98, 33.02, 34.83, 41.73, 68.05, 176.01.
4-Propyl-3-caprolactone.22 (1H NMR, 600 MHz, CDCl3, TMS): d/
ppm 0.90 (t, J ¼ 7.2 Hz, 3H), 1.38–1.22 (m, 6H), 1.53–1.44 (m,
1H), 1.67–1.57 (m, 2H), 1.92 (dddd, J ¼ 16.0, 7.6, 3.8, 1.9 Hz, 1H),
2.01–1.95 (m, 1H), 2.61 (ddd, J ¼ 14.1, 12.3, 2.1 Hz, 1H), 2.68
(ddd, J ¼ 14.0, 7.6, 1.7 Hz, 1H), 4.18 (dd, J ¼ 12.9, 10.4 Hz, 1H),
4.30 (ddd, J ¼ 12.9, 6.0, 1.8 Hz, 1H). 13C NMR (150 MHz, CDCl3):
d/ppm 13.97, 19.70, 28.74, 33.04, 35.20, 38.50, 39.75, 68.05,
176.08.
4-tert-Butyl-3-caprolactone.23 (1H NMR, 600 MHz, CDCl3, TMS)
d/ppm 0.90 (s, 9H), 1.41–1.30 (m, 2H), 1.57–1.47 (m, 1H), 2.11–
1.99 (m, 2H), 2.62–2.53 (m, 1H), 2.71 (ddd, J ¼ 14.2, 7.5, 1.2 Hz,
1H), 4.16 (dd, J ¼ 12.9, 10.5 Hz, 1H), 4.34 (ddd, J ¼ 12.9, 6.0, 1.8
Hz, 1H). 13C NMR (150 MHz, CDCl3): d/ppm 23.76, 27.43, 30.33,
32.98, 33.45, 50.77, 68.62, 176.29.
3-Phenylcyclobutanone.18 (1H-NMR, 300 MHz, CDCl3, TMS):
d/ppm 3.22–3.27 (m, 2H), 3.44–3.50 (m, 2H), 3.66 (quint., J ¼
8.2 Hz, 1H), 7.20–7.38 (m, 5H). 13C-NMR (75 MHz, CDCl3): d/
ppm 31.1, 57.4, 129.2, 129.3, 131.4, 146.3, 209.2.
4-Phenyl-3-caprolactone.24 (1H NMR, 600 MHz, CDCl3, TMS): d/
ppm 1.89–1.80 (m, 1H), 2.11–1.99 (m, 2H), 2.17–2.11 (m, 1H),
2.88–2.73 (m, 3H), 4.32 (dd, J ¼ 12.8, 10.3 Hz, 1H), 4.39 (ddd, J ¼
13.0, 5.5, 2.0 Hz, 1H), 7.17–7.20 (m, 2H), 7.25–7.21 (m, 1H),
7.34–7.30 (m, 2H). 13C NMR (150 MHz, CDCl3): d/ppm 30.33,
33.69, 36.75, 47.25, 68.20, 126.57, 126.84, 128.75, 144.92,
175.58.
3-Butylcyclobutanone.18 (1H-NMR, 300 MHz, CDCl3, TMS): d/
ppm 0.92 (t, J ¼ 6.8 Hz, 3H), 1.20–1.40 (m, 4H), 1.55–1.62 (m,
1H), 2.28–2.43 (m, 1H), 2.50–2.70 (m, 2H), 3.10–3.19 (m, 2H).
13C-NMR (75 MHz, CDCl3): d/ppm 13.9, 22.4, 23.8, 30.4, 35.9,
52.4, 208.6.
3-Phenyl-g-butyrolactone.18 (1H NMR, 300 MHz, CDCl3, TMS):
d/ppm 2.66 (dd, J ¼ 17.5 Hz, 9.1 Hz, 1H), 2.92 (dd, J ¼ 17.5 Hz,
8.1 Hz, 1H), 3.77 (quint., J ¼ 8.4 Hz, 1H), 4.26 (dd, J ¼ 8.9 Hz, 8.0
Hz, 1H), 4.67 (dd, J ¼ 9.0 Hz, 7.9 Hz, 1H), 7.16–7.27 (m, 2H),
7.27–7.33 (m, 1H), 7.33–7.34 (m, 2H). 13C NMR (75 MHz, CDCl3):
d/ppm 35.7, 41.1, 74.0, 126.7, 127.7, 129.1, 139.4, 176.3.
3-Butyl-g-butyrolactone.18 (1H NMR, 300 MHz, CDCl3, TMS): d/
ppm 0.92 (t, J ¼ 6.9 Hz, 3H), 1.15–1.30 (m, 4H), 1.43–1.52 (m,
2H), 2.14–2.33 (m, 2H), 2.49–2.66 (m, 2H), 3.89–3.95 (m, 1H),
4.39–4.44 (m, 1H). 13C-NMR (75 MHz, CDCl3): d/ppm 13.8, 22.4,
29.4, 32.7, 34.4, 35.6, 73.3, 177.2.
Isopropyl propanoate.25 (1H NMR, 600 MHz, CDCl3, TMS): d/
ppm 1.11 (t, J ¼ 7.6 Hz, 3H), 1.22 (d, J ¼ 6.3 Hz, 6H), 2.28 (q, J ¼
7.6 Hz, 2H), 5.00 (dt, J ¼ 12.5, 6.2 Hz, 1H). 13C NMR (150 MHz,
CDCl3): d/ppm 9.11, 21.78, 27.93, 67.45, 176.11–174.37.
p-Methoxyphenyl acetate.26 (1H NMR, 600 MHz, CDCl3, TMS):
d/ppm 2.28 (s, 2H), 3.79 (s, 2H), 6.89 (d, J ¼ 9.1 Hz, 1H), 7.00 (d, J
¼ 9.1 Hz, 1H). 13C NMR (150 MHz, CDCl3): d/ppm 21.02, 55.55,
114.44, 122.28, 144.17, 157.23, 169.87.
g-Butyrolactone.20 (1H-NMR, 300 MHz, CDCl3, TMS): d/ppm
2.13–2.24 (m, 2H), 2.38–2.44 (m, 2H), 4.27 (t, J ¼ 7.4 Hz, 2H).
13C-NMR (75 MHz, CDCl3): d/ppm 22.3, 28.0, 68.8, 178.1.
4-Oxatricyclo[4.3.1.13.8]undecan-5-one.20 (1H-NMR, 300 MHz,
CDCl3, TMS): d/ppm 1.70–2.15 (m, 12H), 3.01–3.12 (m, 1H),
4.39–4.52 (m, 1H). 13C NMR (75 MHz, CDCl3): d/ppm 25.8, 30.9,
33.7, 35.7, 41.2, 73.1, 178.2.
Results and discussion
Screening of metal chlorides
Metal chlorides were used as Lewis acidic catalysts in a model
oxidation of 2-adamantanone (Scheme 1) with a two-fold molar
excess of H2O2 (30% aq.), at 10 mol% metal chloride loading.
2-Oxabicyclo[3.2.1]octan-3-one.20 (1H-NMR, 300 MHz, CDCl3,
TMS): d/ppm 1.65–2.25 (m, 7H), 2.40–2.80 (m, 2H), 4.82–4.89
(m, 1H). 13C NMR (75 MHz, CDCl3): d/ppm 29.1, 31.7, 32.4, 35.7,
40.5, 80.9, 170.7.
3-Caprolactone.20 (1H-NMR, 600 MHz, CDCl3, TMS): d/ppm
1.72–1.87 (m, 4H), 1.88 (dd, J ¼ 5.3 Hz, 2.6 Hz, 2H), 2.62–2.67
Scheme 1 Model Baeyer–Villiger oxidation of 2-adamantanone.
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Each metal chloride was tested in three solvents: dichloro- by acceptor number (AN) approach,27 and are more resistant to
methane, ethanol and toluene. Additionally, tin chlorides were hydrolysis. Neutral chlorogallates(III) (cGaCl ¼ 0.50) have been
3
tested in 1,4-dioxane.
used as solvents and co-catalysts in palladium-catalysed
The observed catalytic activity was strongly dependent the hydroethoxycarbonylation28 and acetal formation,29 whereas
metal chloride, the solvent, and the reaction temperature (Table Lewis acidic systems (cGaCl ¼ 0.67, 0.75) catalysed arene
3
2). In agreement with the literature, high yield of lactone was carbonylation,30 and oligomerisation of alkenes to lubricant
obtained using AlCl3 in ethanol, but not with less polar and base oils.31 In general, however, applications of chlorogallate(III)
aprotic solvents.5 In contrast, the previously unstudied GaCl3 systems are relatively poorly explored compared to their chlor-
was the most active catalyst under all conditions. Moreover, oaluminate(III) analogues.
SnCl2 and SnCl4 were very active in toluene and 1,4-dioxane, but
Three compositions of chlorogallate(III) ionic liquids were
not in ethanol and dichloromethane. These observations may used: cGaCl ¼ 0.50, 0.67 and 0.75, which corresponds to three
3
explain the previously reported low activity of tin chlorides.8,9 different chlorogallate(III) anions, dominant in the respective
Other metal chlorides (InCl3, ZnCl2, TiCl3, TiCl4, FeCl3) compositions: [GaCl4]ꢀ (neutral, as a benchmark), [Ga2Cl7]ꢀ
exhibited much lower activity, with an unpredictable depen- (Lewis acidic), and [Ga3Cl10]ꢀ (Lewis acidic). All ionic liquids
dence on the solvent.
were based on 1-butyl-3-methylimidazolium cation, [C4mim]+.
The high catalytic activity of GaCl3 was most likely related to Results of a model oxidation of 2-adamantanone with a two-fold
its high solubility in the reaction mixtures, compared to other molar excess of H2O2 (30% aq.) are presented in Table 3. In
metal halides, which resulted in favourable reaction kinetics. order to form a homogenous reaction medium, equimolar
Kinetic studies of selected catalytic systems (Fig. 1) revealed amounts of ketone and ionic liquid (based on cation) were
superior performance of GaCl3, even at a lower temperature (40 typically required. Depending on the anionic speciation, this
ꢁC).
resulted in molar excess of gallium metal to substrate between 1
and 3. Hence, the reactions in the ionic liquids contained at
least ten times the concentration of metal than the equivalent
reactions in molecular solvents. Nevertheless, this does not
account for the signicantly greater activity of the ionic liquid.
Using the ionic liquid with the highest GaCl3 concentration
Screening of chlorogallate(III) ionic liquids
Seeking to additionally improve the reaction sustainability by
the removal of volatile organic solvents, and in the hope of
further enhancement in the reaction rate, chlorogallate(III) ionic
liquids were studied as Lewis acidic catalysts.
Chlorogallate(III) ionic liquids are synthesised by the reaction
of anhydrous GaCl3 with an organic chloride salt. Various molar
(cGaCl ¼ 0.75) and the highest Lewis acidity (expressed as
3
acceptor number, AN ¼ 107.5), the oxidation was remarkably
fast, reaching 99% yield of lactone and ketone conversion, in
less than 1 min at ambient temperature. This contrasts with
solutions of GaCl3 in molecular solvents, which took more than
two orders of magnitude longer, at elevated temperatures, to
produce comparable yields.
fractions of GaCl3, expressed as cGaCl , can be used to prepare
3
homogenous ionic liquids.19 For cGaCl < 0.50, a neutral [GaCl4]ꢀ
3
anion coexists with Lewis basic chloride anion; for cGaCl > 0.50,
3
multinuclear chlorogallate(III) anions: [Ga2Cl7]ꢀ and [Ga3Cl10]ꢀ
The chlorogallate(III) ionic liquid with lower GaCl3 concen-
are formed, which impart strong Lewis acidity to these
tration (cGaCl ¼ 0.67) and slightly lower Lewis acidity (AN ¼
3
systems.27 At the ideally neutral composition, c
¼ 0.50, only
GaCl3
99.5) was also slightly less active. Nevertheless, it still displayed
extremely high catalytic activity, yielding 93% lactone yield aer
1 min at ambient temperature. In contrast, no conversion was
[GaCl4]ꢀ is present. Compared to chloroaluminates(III), chlor-
ogallates(III) offer equal or stronger Lewis acidity, as quantied
Table 3 The influence of chlorometallate(III) ionic liquids as catalysts
for 2-adamantanone oxidation with H2O2 30% aq., at ambient
temperaturea
Ionic liquid
ILb/mol%
Time/min
Yield of lactonec/%
[C4mim]Cl–GaCl3,
c ¼ 0.75
25
50
100
100
1
1
1
1
99
99
99
93
[C4mim]Cl–GaCl3,
c ¼ 0.67
[C4mim]Cl–GaCl3,
c ¼ 0.50
100
100
1
60
300
1
60
300
0
15
43
0
11
11
[C4mim]Cl–AlCl3,
c ¼ 0.67
Fig. 1 Kinetics of the oxidation of 2-adamantanone (0.67 mmol), with
H2O2 30% aq. (1.34 mmol) in solvent (5 cm3), in the presence of MClx
(10 mol%).
a
Reaction conditions: 2-adamantanone (0.67 mmol), H2O2 30% aq.
(1.34 mmol), RT. Relative to ketone. c Yields determined by GC.
b
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Table 4 The influence of oxidation agent on the oxidation of 2-ada-
two concentrations of H2O2 in water (30 and 60%) were
compared to oxidations using the anhydrous urea$H2O2
complex (UHP), cumyl hydroperoxide (neat), and tert-butyl
hydroperoxide, used as solutions in water and in decane (Table
4). Equimolar amounts of ionic liquid (based on the cation) and
ketone were used in all cases.
mantanone in the presence of the chlorogallate(III) ionic liquida
Oxidant
Oxidantb/mol% Time/min Yield of lactonec/%
H2O2 30% aq.
200
150
110
200
200
200
200
200
1
1
1
1
60
1
60
60
60
99
98
87
99
80
92
3
Using decreasing amounts of H2O2 (30% aq.), it was
demonstrated that the ratio of oxidising agent could be
decreased from two-fold to nearly equimolar, without signi-
cant product loss. Decreasing water content (i.e. increasing
H2O2 concentration to 60%) had no observable effect on
conversion and yield, compared to using 30% aqueous solution
of H2O2. However, replacing aqueous solution with anhydrous
UHP complex (with formal H2O2 concentration of 36%), led to
a dramatic decrease in the lactone yield (80% aer 60 min). To
conrm the benecial role of water, the reaction with UHP was
repeated in the presence of 0.1 g of water (giving formal H2O2
concentration of ca. 30%), with resulted in 92% lactone yield
aer 1 min. This conrmed that the presence of water is crucial
for the observed high reaction rates in gallium(III)-promoted BV
oxidation with H2O2. Interestingly, this is in direct opposition to
earlier views, whereby water was detrimental for metal-catalysed
BV oxidation, which was considered suitable only for anhydrous
conditions.2
In BV oxidation, H2O2 is known to be a less active oxidant
than peracids, but more active than hydroperoxides.2,3 Consid-
ering the excellent performance of gallium(III) catalysts in H2O2
oxidation, it was anticipated that they may also catalyse oxida-
tion by hydroperoxides. However, irrespective of the solvent
(neat, water, aprotic solvent – see Table 4), hydroperoxides
remained inactive in the model oxidation catalysed by the
chlorogallate(III) ionic liquids.
H2O2 60% aq.
UHP
UHP 30% aq.d
PhC(CH3)2OOH
tBuOOH in water
5
0
tBuOOH in decane 200
a
Reaction conditions: 2-adamantanone (0.67 mmol), [C4mim]Cl–GaCl3,
cGaCl ¼ 0.75 (0.67 mmol), RT. b Relative to ketone. c Yields determined
3
by GC. d 30% concentration of H2O2 in the solution.
detected under the same reaction conditions in the presence of
the neutral chlorogallate(III) ionic liquid (cGaCl ¼ 0.50). Upon
3
prolonged reaction time some reactivity occurred (43% lactone
yield aer 300 min), possibly due to hydrolysis of [GaCl4]ꢀ,
resulting in catalysis by the evolved hydrogen chloride.
For comparison, strongly Lewis acidic chloroaluminate(III)
ionic liquids were used as catalysts (cAlCl ¼ 0.67, AN ¼ 95.6), at
3
the same temperature. Very low yields of lactone (11%) were
obtained, even aer 300 min. This is in agreement with some of
the results recorded for catalysis with metal chlorides (Table 2),
where the performance of AlCl3 was inferior to that of GaCl3.
Screening of oxidising agents
The catalytic activity of [C4mim]Cl–GaCl3 (cGaCl ¼ 0.75) was
3
tested in the presence of a range of oxidising agents. Results for
Fig. 2 (A) Raman spectra of [C4mim]Cl–GaCl3 cGaCl ¼ 0.50 (green), water phase (navy blue) and organic phase (red) after reaction with H2O2; (B)
3
71Ga NMR spectra of water phase (navy blue) and organic phase (red) of [C4mim]Cl–GaCl3 cGaCl ¼ 0.50 after addition of H2O2.
3
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Fig. 3 (A) Raman spectra of [C4mim]Cl–GaCl3 cGaCl ¼ 0.75 (orange), [C4mim]Cl–GaCl3 cGaCl ¼ 0.75 after reaction with H2O2 (blue) and of the
3
3
mixture GaCl3–H2O2 (green); (B) 71Ga NMR spectra of [C4mim]Cl–GaCl3 cGaCl ¼ 0.75 after reaction with H2O2 (blue) and of the mixture GaCl3–
3
H2O2 (green).
The catalytically active ionic liquid, [C4mim]Cl–GaCl3 (cGaCl
Mechanistic considerations
3
¼ 0.75), in its neat state, contained oligonuclear chlor-
ogallate(III) anions, such as [Ga2Cl7]ꢀ and [Ga3Cl10]ꢀ, as indi-
cated by a group of Raman signals between 500 and 100 cmꢀ1
(Fig. 3A), that correspond to vibrations of terminal and bridging
chlorides in oligomeric chlorogallate(III) anions, in agreement
with the literature.33
Screening studies revealed that gallium(III) chloride and chlor-
ogallate(III) ionic liquids were much more active catalysts in BV
oxidation with aqueous H2O2 than their aluminium(III)
analogues. A combination of Raman and 71Ga NMR spectro-
scopic studies was used to shed some light on the activation
process. Two active systems were investigated: GaCl3 and the
Upon the addition of the ionic liquid to aqueous hydrogen
peroxide, a single phase was formed in a highly exothermic
process. The same was observed upon the addition of solid
GaCl3 to aqueous H2O2. For both solutions, Raman spectra
exhibited a Ga–Cl stretching frequency at 348 cmꢀ1, corre-
sponding to monomeric chlorogallate(III) units, and signal at
876 cmꢀ1, corresponding to the O–O stretch in H2O2 (Fig. 3A).
The 71Ga NMR spectra both contained two signals (Fig. 3B). A
broad peak ca. 240 ppm corresponded to tetracoordinate gal-
lium(III) anions. The broadening and a slight upeld shi
compared to that for [GaCl4]ꢀ may suggests the partial
replacement of a chloride with a hydroxide ligand (71Ga NMR
signal for [GaOH4]ꢀ is at 225 ppm).34 It may also indicate the
chlorogallate(III) ionic liquid (cGaCl ¼ 0.75), both as mixtures
3
with aqueous H2O2 (molar ratio 1 : 1). As a comparison,
aqueous H2O2 solutions of the inactive, neutral chlorogallate(III)
ionic liquid (cGaCl ¼ 0.50), and of the less active AlCl3, have also
3
been studied.
The inactive mixture of [C4mim]Cl–GaCl3, cGaCl ¼ 0.50, and
3
hydrogen peroxide was biphasic, comprising an aqueous phase
and an organic phase. In the organic phase, the only detected
gallium(III) species was the neutral tetrachlorogallate(III) anion,
conrmed by the presence of a Raman Ga–Cl stretching
frequency at 349 cmꢀ1 (Fig. 2A),32 and a narrow 71Ga NMR signal
at 250 ppm (Fig. 2B),19 both characteristic of the tetrahedral
[GaCl4]ꢀ. The aqueous phase contained H2O2 (Raman O–O
stretching frequency at 876 cmꢀ1, Fig. 2A), and the product of
complete hydrolysis of the chlorogallate(III) anion, [Ga(H2O)6]3+,
characterised by a narrow 71Ga NMR signal at 0 ppm (Fig. 2B), as
the only observable gallium species.
The mixture of AlCl3 and hydrogen peroxide formed
a homogenous liquid, in a very exothermic mixing process. It
contained products of AlCl3 hydrolysis, but no H2O2. Hydrogen
peroxide was assumed to have undergone a rapid decomposi-
tion due to temperature and/or released HCl, which may be the
main factor lowering the activity of AlCl3.
Scheme 2 Electrophilic activation of (a) ketone by Lewis acid, (b) the
intermediate by Lewis acid, and (c) ketone by Brønsted acid in Baeyer–
Villiger oxidation with hydrogen peroxide. Adapted from ref. 2.
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Table 5 Oxidation of selected cyclic ketones to lactonesa
Table 5 (Contd.)
Time/min Yieldb/%
Entry Ketone
Lactone
Time/min Yieldb/%
Entry Ketone
Lactone
1
1
5
99 (94)
99 (90)
13
120
50
a
Reaction conditions: ketone (1.34 mmol), 30% aq. H2O2 (2.68 mmol),
b
2
[C4mim]Cl–GaCl3, cGaCl ¼ 0.75 (1.34 mmol), RT. Yields determined
using GC, isolated yield3s given in parenthesis.
3
4
1
1
99 (92)
99 (93)
exchange with other gallium species. The signal at 0 ppm was
assigned to [Ga(H2O)6]3+, but again – the broadening indicates
exchange with other species. These results are in agreement
with prior NMR studies on GaCl3 hydrolysis.33,34 In addition,
complementary FAB-MS studies on hydrolysis of chlor-
ogallate(III) ionic liquids revealed that the hydrolysis proceeds
via chlorohydroxygallate(III) anions, such as [GaCl3(OH)]ꢀ and
[GaCl(OH)3]ꢀ,35 which is in agreement with observed here small
upeld shi in the 71Ga NMR spectrum.
In summary, both catalytically active systems contain
mixtures of [GaCl4]ꢀ, chlorohydroxygallate(III) anions, and
[Ga(H2O)6]3+, coexisting in the same phase with H2O2. The
monomeric [GaCl4]ꢀ anion it is not known to either undergo
facile chloride replacement, or to expand its coordination to
accommodate a h ligand; therefore it is commonly described
as ‘neutral’, as opposed to ‘Lewis acidic’.27 In contrast, chlor-
ohydroxygallate(III) anions, such as [GaCl3(OH)]ꢀ, are less
sterically crowded and have been demonstrated to expand to
penta- or hexacoordinate motif in the presence of O-donors, L,
e.g. {GaCl3L2}.36 Therefore, chlorohydroxygallate(III) complexes
are proposed to be the active Lewis acidic species in BV oxida-
tion (Scheme 2a and b), which also explains the benecial
inuence of water in this reaction. Finally, a very slow conver-
sion of ketone to lactone (43% aer 5 h) detected in the pres-
5
1
99 (89)
6
7
1
26
30
99 (91)
8
9
30
15
99 (90)
99 (93)
ence of the ‘inactive’ [C4mim]Cl–GaCl3 (cGaCl ¼ 0.50), was
3
probably caused by hydrolytically-generated HCl, protonating
the ketone and resulting in its mild activation (Scheme 2c).2
10
15
99 (92)
99 (91)
Substrate scope
To determine the catalytic potential of the chlorogallate(III)
ionic liquid, [C4mim]Cl–GaCl3 (cGaCl ¼ 0.75), it was used as
3
a catalyst for the synthesis of a range of lactones from the cor-
responding ketones (Table 5). Numerous cyclic ketones were
readily oxidised to the corresponding lactones in high yields
(93–99%) under the optimised reaction conditions, which were
in general remarkably better than those reported in the
literature.4–7
11
12
120
120
240
15
15
Very reactive, strained cyclobutanones were oxidised to g-
butyrolactones in 1 min with 99% yield. High yields and short
reaction times were also achieved for the oxidation of 2-ada-
mantanone and norcamphor. Non-strained cyclohexanones,
which are much less reactive, were also successfully oxidised to
corresponding 3-caprolactones at 99% yields. Reaction times
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varied from 15 to 120 min, depending on the substituent in the 11 G. J. ten Brink, J. M. Vis, I. W. C. E. Arends and R. A. Sheldon,
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with full conversion of ketone. This anomaly could be caused by Tetrahedron, 2002, 58, 3977.
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˙ ˙
17 S. Baj, R. Słupska, A. Chrobok and A. Drozdz, J. Mol. Catal. A:
The gallium(III) chloride systems provide the fastest known
homogenous activation of the Baeyer–Villiger oxidation with
aqueous hydrogen peroxide to produce near quantitative yields
(89–94%) of lactones.2 Gallium(III) chloride was found to be
a very active catalyst, especially in aprotic solvents. Lewis acidic
chlorogallate(III) ionic liquids were also extremely active, and in
addition allowed for the elimination of volatile molecular
solvents.
Chem., 2013, 376, 120.
18 C. Bolm, O. Beckmann, T. Kuhn, C. Palazzi, W. Adam,
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´
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It was demonstrated that water was crucial for the efficient
process activation. The active species, postulated based on
Raman and 71Ga NMR spectroscopies, were hydroxy-
chlorogallate(III) anions, such as [GaCl3(OH)]ꢀ or [GaCl(OH)3]ꢀ,
which are the intermediate products of hydrolysis of both GaCl3
and chlorogallate(III) anions.
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Higher Education, National Science Centre (grant no. UMO-
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COST Action (CM1206 EXIL – Exchange on Ionic Liquids),
which allowed for the Short Term Scientic Mission in The
QUILL Research Centre.
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