Direct bromination of hydrocarbons catalyzed by Li2MnO 3 under oxygen and photo-irradiation conditions

Article abstract of DOI:10.1039/c2ra22197g

A method for the direct bromination of hydrocarbons with Br₂ using a ubiquitous and inexpensive catalyst is highly desirable. Herein, we report the selective mono-bromination of hydrocarbons in good yield using Li₂MnO₃ as a catalyst under irradiation with a fluorescent room light. This new catalyst can be recycled. The effect of light was investigated using action spectra, which revealed that the reaction occurred on the surface of the catalyst.

Full text of DOI:10.1039/c2ra22197g

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Direct bromination of hydrocarbons catalyzed by
Li2MnO3 under oxygen and photo-irradiation
Cite this: DOI: 10.1039/c2ra22197g
conditions3{
Yuta Nishina,*a Junya Moritab and Bunsho Ohtanic
Received 18th September 2012,
Accepted 10th December 2012
DOI: 10.1039/c2ra22197g
www.rsc.org/advances
We envisaged that the formation of a bromine radical species
on the surface of a catalyst would enable a more selective
bromination process. Of the previously reported alkane bromina-
tion processes, we were particularly interested in the MnO2-
promoted reaction, because it has been proposed that the Br2 is
activated on the MnO2 surface.11 An issue associated with the use
of MnO2, however, is that the HBr by-product formed during the
course of the reaction poisons the MnO2 to give non-active
MnBrO(OH) species.11 During our initial attempts to use a
catalytic amount of MnO2 in the bromination of cyclohexane,
however, the formation of MnBr2 was observed following the
A method for the direct bromination of hydrocarbons with Br2
using a ubiquitous and inexpensive catalyst is highly desirable.
Herein, we report the selective mono-bromination of hydrocar-
bons in good yield using Li2MnO3 as a catalyst under irradiation
with a fluorescent room light. This new catalyst can be recycled.
The effect of light was investigated using action spectra, which
revealed that the reaction occurred on the surface of the catalyst.
Organic bromides have been widely used as starting materials and
intermediates in organic synthesis,1 as well as enjoying functional
applications in bioactive materials.2 Classically, alkyl bromides
have been prepared from the reaction of the corresponding alkyl
alcohols with Br2, HBr and other brominating reagents.3 They can
also be synthesized from the corresponding alkanes by reaction
reaction (see ESI ). The development of a form of manganese oxide
3
that is stable under acidic conditions could potentially provide a
platform for the effective catalytic bromination of alkanes. It is
known that manganese oxides containing an alkali metal are
stable under acidic conditions.12 With this in mind, we
synthesized a series of alkali metal-modified manganese oxides
5
with brominating reagents, such as NBS,4 CBr4 and Et4NBr.6
Although alkyl bromides can be synthesized from the correspond-
ing alkanes by reacting with Br2 under photolysis or high
temperature conditions without any catalyst, the reactions do
not exhibit any selectivity,7 because the bromine radicals present
in the reaction mixtures indiscriminately attack the C–H bonds of
the starting material and the products (alkyl bromides). There
have only been a limited number of reports in the literature
concerning the selective synthesis of alkyl bromides from alkanes.
For example, the reactions of alkanes with Br2 in the presence of
(see ESI ) and screened them as catalysts for the bromination of
3
cyclohexane (1a). Consequently, the lithium-modified manganese
oxide, Li2MnO3, was identified as a suitable catalyst and promoted
the bromination reaction to afford the mono-brominated product
2a in 70% yield (Scheme 1).13,14 It is worthy of note that trans-1,2-
dibromocyclohexane (3a)15 was only formed in 2% yield.7 In the
absence of the catalyst, the selectivity of the reaction was poor, in
spite of the inclusion of a large excess of 1a (Scheme 1).
Toluene was then used as a substrate to determine the nature
of the reaction pathway (radical or ionic). Thus, if Br2 was
homolytically dissociated to form bromine radicals, the bromina-
tion would occur at the C–H bond on the benzyl position,16
whereas if cationic bromine was formed, electrophilic attack
an excess of AcOH,8 a stoichiometric amount of t-BuONa,9
a
catalytic amount of AlCl3 at a low temperature10 and an excess of
MnO2, have been reported.11 Unfortunately, however, these
reactions are relatively inefficient and cannot compete with the
corresponding alcohol-mediated bromination processes.
aResearch Core for Interdisciplinary Sciences, Okayama University, Tsushimanaka,
Kita-ku, Okayama 700-8530, Japan. E-mail: nisina-y@cc.okayama-u.ac.jp;
Fax: 81-86-251-8718; Tel: 81-86-251-8718
bGraduate School of Natural Science and Technology, Okayama University,
Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
cCatalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
Electronic supplementary information (ESI) available: Methods and experimental
3
procedure. See DOI: 10.1039/c2ra22197g
{ Products and yields are determined by GC comparing the standard sample and
using dodecane as an internal standard.
Scheme 1 Bromination of cyclohexane (1a).
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Table 1 The effect of light and oxygen in the bromination of cyclohexane (1a)
Yield (%)b
Entrya
Conditions
Atmosphere
2a
3a
1
2
3
4
5
6
hn, 80 uC
dark, 80 uC
hn, 80 uC
hn, 115 uC
hn, 80 uC
hn (5 min), 80 uC
Ar (1 atm)
Ar (1 atm)
O2 (1 atm)
O2 (1 atm)
air (1 atm)
O2 (1 atm)
70
0
85
84
78
62
1
2
3
4
5
6
a
Reaction conditions: Li2MnO3 (0.05 mmol), 1a (2 mL, 40 equiv.),
Br2 (0.5 mmol). The ratio of 2a to 3a was determined by 1H NMR.
b
Scheme 2 A mechanistic study of Li2MnO3-catalyzed bromination of hydro-
carbons. (1) Bromination of toluene (1b), (2) KIE analysis using cyclohexane (1a) and
cyclohexane-d12 (1a9), (3) bromination of benzene (1c).
providing 2a with an increased yield of 85% (Table 1, entry 3).
Increasing the reaction temperature to 115 uC under an atmo-
sphere of oxygen did not lead to an increase in the reaction yield
(Table 1, entry 4). Interestingly, the reaction also proceeded
smoothly even under air, although there was a moderate decrease
in the yield of the product (Table 1, entry 5). Continuous
irradiation of the reaction mixture was necessary because the
yield of 19 decreased to 62% when the irradiation was performed
for only the first 5 min of the reaction process (Table 1, entry 6).
When cyclooctane (1d), which is more reactive than 1a, was used
as a substrate and the reaction was conducted over a 1 h period
using 0.5 mmol of Br2, bromocyclooctane (2d) was formed in 0.61
mmol. This result indicated that Br2 and ultimately the bromine
radical species were being produced from the reaction of HBr and
oxygen (Scheme 3).
With the optimized conditions in hand from the bromination
of cyclohexane, we then investigated the scope of the reaction
conditions with other substrates (Table 2). Cyclooctane (1d) was
converted into bromocyclooctane (2d) at room temperature
(Table 2, entry 1). Although n-hexane (1e) consists of six 1u C–H
bonds and eight 2u C–H bonds (four bonds at 2- and 3-positions,
respectively), only the 2u C–H bonds were brominated.
Furthermore, a 2 : 1 selectivity for the 2-position over the
3-position was observed, which was attributed to steric hindrance
(Table 2, entry 2). Adamantane (1f), which has four 3u C–H bonds
and twelve 2u C–H bonds, was selectively brominated at the 3u C–
H bonds (Table 2, entry 3). As indicated in Scheme 2, toluene (1b)
was brominated exclusively at the benzylic position under the
current bromination conditions to give benzylbromide (2b)
(Table 2, entry 4). When the benzylic positions of toluene were
blocked, the bromination could occur at the 1u or aromatic C–H
would occur on the aromatic ring.16a,17 When the reaction was
performed without the catalyst under photolysis, only benzylbro-
mide (2b) was formed in 89% yield (Scheme 2, eqn. 1). The same
reaction profile was observed when the Li2MnO3 catalyst was used
with and in the absence of light (Scheme 2, eqn. 1).18 Furthermore,
the kinetic isotope effect (KIE) of the bromination of cyclohexane
(1a and 1a9) with and without Li2MnO3 was investigated. The KIE
values in both cases were similar (3.3 and 3.4),19 which suggested
that the reaction pathways were similar. These results indicated
that Li2MnO3 promoted the formation of bromine radicals. In
contrast, when benzene (1c) was used as the substrate, bromo-
benzene (2c) was obtained in 68% yield (Scheme 2, eqn. 3), which
suggested that cationic bromine species were being formed in the
Li2MnO3 system. Unfortunately, however, the catalytic activity of
Li2MnO3 in the bromination of benzene was lower than that
previously reported for other catalysts, including a zeolite and its
analogues.17 With this in mind, we focused our own research on
the bromination of aliphatic hydrocarbons.
Br2 can be readily dissociated under irradiation of light with a
wavelength of 350–550 nm to give bromine radicals.20 It was
envisaged that the rate of the reaction could be accelerated in the
presence of a catalytic amount of Li2MnO3 and under irradiation
with weak light. Fluorescent room light was selected as a light
source for the investigation and cyclohexane (1a) was consequently
converted into bromocyclohexane (2a) in 70% yield using a lower
reaction temperature and a shorter reaction time (Table 1, entry 1).
When a strong light source was used, such as a high pressure
mercury-vapor lamp (Ushio, UM-452, 450 W) or a xenon lamp
(Asahi Spectra, MAX-302, 300 W), complex polybrominated
products derived from the photoinduced formation of free
bromine radicals were obtained. In the absence of light, 2a was
not formed at 80 uC following a 10 min reaction period (Table 1,
entry 2). It is known that HBr is converted into Br2 under
photolysis with oxygen.21 The effect of oxygen was therefore
investigated and the bromination reaction was conducted with a
weak light source at 80 uC under an atmosphere of oxygen,
Scheme 3 Bromination of cyclooctane (1d).
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Table 2 The substrate scope in the Li2MnO3-catalyzed bromination of hydro-
carbons under oxygen and photo-irradiation
Entrya
1
R–H
T/uC
Time/h
0.2
Product yield
25
Fig. 1 XRD analysis of Li2MnO3 before (top) and after (bottom) the reaction.
2b
3c
40
0.5
24
encouraged that NBS could be used for bromination of a wide
range of substrates because Br2 cannot undergo mono-bromina-
tion in the presence of an alkene and this discovery expanded the
scope and utility of the current procedure.23,24 This increase in
scope was exemplified by the conversion of allylbenzene (1h) to
cinnamyl bromide (2h) when NBS was used as the brominating
reagent (Scheme 4, eqn. 2).
The reusability of the catalyst was examined. X-ray diffraction
(XRD) measurements were used to confirm that the structure of
the catalyst did not change following the bromination of
cyclohexane 1a (Fig. 1). The catalyst showed resistance to the
formed HBr,12 and was therefore recyclable and could be reused
without any adverse impact on the product yields for at least five
100
4
5
25
80
0.2
3
a
Reaction conditions: Li2MnO3 (0.05 mmol), 1 (40 equiv.), Br2 (0.5
mmol). Yield was determined by 1H NMR using CH2ClCH2Cl as an
b
consecutive cycles (see ESI ).
c
3
internal standard. Adamantane (1f) was used in 2 equiv. and
Next, we investigated the wavelength dependency in the
bromination of cyclohexane (1a) with and without Li2MnO3
(Fig. 2). An action spectrum plot of apparent quantum efficiency
against wavelength is recognized as a valid and feasible method
for determining whether a reaction is or is not photocatalysed. The
apparent quantum efficiency is defined as the ratio of the
consumed photogenerated electrons to the incident photons.25
A mixture of Br2 in cyclohexane absorbed light at wavelengths
of 380–500 nm (Fig. 2, —). A similar tendency was observed in the
action spectrum for the bromination of cyclohexane (1a) without
the catalyst (Fig. 2, m), which indicated that Br2 itself was activated
CH2Cl2 (1 mL) was used as a solvent.
bonds. Further to an investigation with tert-butylbenzene (1g),
p-bromination of the aromatic ring was achieved, but required a
higher reaction temperature and an extended reaction time to
provide a good yield of the product (Table 2, entry 5). Based on
these experiments, the order of C–H bond reactivity was proposed
to be 3u > 2u > aromatic > 1u.22
N-bromosuccinimide was also investigated in the current
reaction system and performed effectively as a brominating
reagent. Thus, cyclooctane (1d) was converted to bromocyclooc-
tane (2d) following irradiation with a fluorescent light at 40 uC for
30 min when NBS was used instead of Br2 (Scheme 4, eqn. 1). It is
clear that more extreme conditions were required when NBS was
used in this particular case relative to Br2 (Table 2, entry 1). In
spite of the requirement for more aggressive conditions, we were
Fig. 2 Action spectra of photoinduced bromination of cyclohexane in the presence
(&) and absence (m) of Li2MnO3, and the photo absorption spectrum of Br2 (—) in
cyclohexane.
Scheme 4 Bromination of (1) cyclooctane (1d) and (2) allylbenzene (1h) with NBS.
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Grignard Reagents, ed. G. S. Silverman, P. E. Rakita, Marcel
by light to promote the reaction. The action spectrum of the
bromination reaction in the presence of Li2MnO3 contained a
maximum peak at 350 nm (Fig. 2, &) that had shifted to a shorter
wavelength than those in the previous two spectra, indicating that
the reaction with Li2MnO3 proceeded efficiently in the ultraviolet
region. Stoimenov et al. reported that when Br2 was adsorbed onto
the surface of a metal oxide, its photo absorption spectrum shifted
to a wavelength shorter than that of free Br2.26 Unfortunately, the
photo reflection and absorption spectra of Li2MnO3 and its
composite with Br2 could not be measured because it was a black
and solid-state material. Taken together, the shorter wavelength
shift of the action spectrum with Li2MnO3 could indicate that Br2
was adsorbed onto the Li2MnO3 surface and subsequently
functioned as a photoabsorber. Furthermore, the fact that the
apparent quantum efficiency with Li2MnO3 exceeded unity (1.0)
suggests that the photoinduced bromination reaction involves a
radical chain reaction. The formation of a bromine radical species
on the surface could contribute to selective bromination and
halogens could interact with metal oxides both with and without
polarization.26 This could well be true, because Li2MnO3 effectively
promoted both the radical- (Scheme 2, eqn. 1–2 and Table 2,
entries 1–4) and cationic- (Scheme 2, eqn. 3 and Table 2, entry 5)
derived bromination reactions. A simple metal oxide, such as
Fe2O3 or MnO2, could not catalyze the bromination of an aromatic
ring and it is therefore clear that Li2MnO3 exhibits both metal
oxide and metalate chemical character towards Br2.17
´
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Conclusions
We have successfully developed a novel bromination catalyst
system for use under photo-irradiation conditions. Furthermore,
the catalyst is stable in the presence of HBr and is recyclable.
Selective mono-bromination reactions were achieved by surface
reaction. Our future work will focus on modifications to the
surface conditions and particle size, with the aim of developing
further reactive and selective catalysts that are more effective than
the existing systems for the halogenation of alkanes.
Acknowledgements
13 Li MnO was synthesized from Mn O and Li CO (see ESI ).
3
2
3
2
3
2
3
Financial support for this study was provided by the
Development of Human Resources in Science and
Technology, The Circle for the Promotion of Science and
Engineering and the Hatakeyama Culture Fundation. We also
received generous support from Mr. Noriyasu Kimura for XRD
analysis and Prof. Kazuhiko Takai for his generous support
and comments on this research. This study was also supported
by the Cooperative Research Program of Catalysis Research
Center, Hokkaido University (Grant #11B2001).
Other alkali metal-modified manganese oxides were prepared
in the same procedure, however, they were complex mixtures
and XRD analysis could not determine their structures.
14 Na and K-modified manganese oxide gave 2a in 53 and 64%
yields, respectively. After the reaction, the structure of the oxide
changed to give MBr (M = Na and K, by XRD analysis),
therefore, they are not suitable for catalyst.
15 Trans-1,2-dibromobenzene is formed selectively. Although the
reason is unclear, one possible route would be dehydrobromi-
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