Stereoselective photochemical reaction of cyclohexyl phenyl ketone within lytropic liquid crystals formed by chiral ionic liquids

Article abstract of DOI:10.1002/cjoc.201300275

Irradiation of cyclohexyl phenyl ketone (1) results in either intra- or intermolecular hydrogen abstraction to afford compounds 1-phenylhept-6-en-1-one (2) and α-cyclohexyl benzyl alcohol (3) as photoproducts, in which 3 has a pair of enantiomers. Herein,

Full text of DOI:10.1002/cjoc.201300275

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DOI: 10.1002/cjoc.201300275
Stereoselective Photochemical Reaction of Cyclohexyl Phenyl
Ketone within Lytropic Liquid Crystals Formed by
Chiral Ionic Liquids
Xiujie Yang,§,a Xinwei Li,§,a,c Bin Chen,*,b Lizhu Wu,*,b Liping Zhang,b
Liqiang Zheng,*,a and Chenho Tunga,b
a Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education,
Jinan, Shandong 250100, China
b Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
c Patent Examination Cooperation Center of the Patent Office, SIPO, Beijing 100083, China
Irradiation of cyclohexyl phenyl ketone (1) results in either intra- or intermolecular hydrogen abstraction to af-
ford compounds 1-phenylhept-6-en-1-one (2) and α-cyclohexyl benzyl alcohol (3) as photoproducts, in which 3 has
a pair of enantiomers. Herein, two types of chiral hexagonal liquid crystals were prepared to direct the enantioselec-
tive photochemical reaction of 1. One of the hexagonal liquid crystals is composed of 1-tetradecyl-3-methyl-
imidazolium bromide/p-xylene/H2O with the modification by a chiral inductor. The other one is formed by chiral
(S)-3-hexadecyl-1-(1-hydroxy-propan-2-yl)-imidazolium bromide/p-xylene/H2O. The product analysis shows that
the latter one can be used as a microreactor to achieve stereoselective photochemical reaction, while the former one
produced compound 3 with no enantioselectivity at all.
Keywords enantioselectivity, chiral ionic liquid, liquid crystal, microreactor
Introduction
form anisotropic lamellar liquid crystals (LLCs) and
hexagonal liquid crystals (HLCs) with p-xylene and
water (Figure 1).[20] The assemblies are optically trans-
parent amenable to photochemical study. Owing to the
large interfaces and various microdomains with differ-
ent polarities presented in liquid crystals (LCs), these
ordered aggregates could incorporate various kinds of
substrates in high concentration. Furthermore, LCs have
Enantioselectivity in ground-state reactions is com-
monly achieved by developing appropriate catalytic
systems, and great success has been made in the past
decades.[1,2] However, there are fewer examples of
asymmetric photochemical transformations.[3,4] In an
effort to achieve stereoselective photochemistry, scien-
tists have made use of inherently chiral hosts,[5-7] or
chiral-modified media[8-12] as microreactors. It has been
established that chiral cavities and/or surfaces are able
to bias the course of photochemical reactions, and thus
leading to stereoselective chemical transformation. Cy-
clodextrins,[5-7] chirally modified zeolites[8] and chiral lac-
tams[10-12] are the most successful chiral microreactors.
Ionic liquids (ILs) represent an important class of
molecular assemblies that have been widely used as
environmentally benign solvents for organic chemical
and enzymatic reactions,[13-16] and as media in extraction
process.[17-19] It has been known that ILs enable surfac-
tants self-assembly to form lyotropic liquid crystals ei-
ther in water or in oil solutions. For example, 1-tetra-
decyl-3-methylimidazolium bromide ([C14mim]Br) can
Figure 1 Schematic representation of the structure of HLC and
LLC. The structures of [C16hpim]Br and [C14mim]Br are also
shown in the figure.
*
E-mail: lqzheng@sdu.edu.cn, lzwu@mail.ipc.ac.cn, chenbin@mail.ipc.ac.cn; Tel.: 0086-0531-88366062; Fax: 0086-0531-88564750. § These authors
contribute equally to this work.
Received March 31, 2013; accepted April 27, 2013; published online May 14, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300275 or from the author.
Chin. J. Chem. 2013, 31, 603—606
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Yang et al.
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very high viscosity, and the “stiff” structure would in-
fluence the arrangement and orientation of sub-
strates.[20-22] In particular, one may synthesize chiral ILs
to prepare LCs, or incorporate a chiral inductor into the
LCs formed by achiral ILs. The prepared LCs might
serve as chiral microreactors to conduct stereoselective
photochemical reactions.[21] However, the use of lyo-
tropic liquid crystals (LCs) as chiral-confined media is
rarely reported.[23-25]
In order to examine whether liquid crystal chiral mi-
croreactor could be used as a confined medium for en-
antioselective photochemical reactions, we synthesized
(S)-3-hexadecyl-1-(1-hydroxy-propan-2-yl)imidazolium
bromide ([C16hpim]Br),[26] a chiral IL, and made use of
the surfactant to prepare LCs with p-xylene and H2O
(chiral microreactor mode 1). At the same time, we also
prepared LCs by [C14mim]Br, which was modified by a
chiral inductor (chiral microreactor mode 2). The photo-
reaction of cyclohexyl phenyl ketone (1, Scheme 1) was
studied, which undergoes intramolecular and intermo-
lecular hydrogen abstraction to yield product 1-phenyl-
hept-6-en-1-one (2) and α-cyclohexyl benzyl alcohol (3),
respectively. The ratio of 2/3 mainly depends on
whether a hydrogen donor is presented in the reaction.
Irradiation of 1 gives 2 exclusively in the absence of
hydrogen donor. However, in the presence of a large
amount of hydrogen donors, 3 is yielded as the only
product. Prolonged irradiation leads to secondary reac-
tions of 2. In our experiments, we focused on the enan-
tioselectivity of product 3, and the conversion of the
reaction was kept in the range of 20%－30%.
diagram was constructed by the titration of a series of
[C16hpim]Br/p-xylene mixtures with water at room
temperature. Appropriate amounts of [C16hpim]Br and
p-xylene were weighed into bottles. Samples were
shaken for sufficient time to attain equilibrium and then
progressively enriched with water (added drop by drop).
The amount of added water at which transition occurred
was derived from weight measurements. By repeating
this experimental procedure for other combinations of
[C16hpim]Br to p-xylene weight ratio, the phase
boundaries were determined. The presence of liquid
crystalline mesophase structures was determined by
using cross polarizers. Temperature was thermostatted
at (25±0.1) ℃. The textures of samples were obtained
by polarized optical microscopy (POM, Olympus
BX51p) with a cooled CCD (Evolution MP5.1RTV,
Q-imaging, Canada) for image capture, and temperature
was controlled at 25 ℃ with a Linkam THSME600
liquid crystal freezing and heating stage system with a
TP94 temperature controller (Linkam Scientific Instru-
ment Ltd., UK). Small-angle X-ray scattering (SAXS)
measurements were performed using a Kratky Compact
Camera (HMBG, Austria) with Ni and W filtered Cu
Kα radiation (wavelength λ＝0.15418 nm) generated by
a PW3830 X-ray generator (5 kV×40 mA). Scattering
intensities were plotted versus reciprocal spacing (q＝
4π•sin θ/λ), where θ was the scattering angle. The ex-
1
posure time was 600 s for all samples. H NMR meas-
urements were carried out with a Brucker 400 NMR
spectrometer at 298 K.
Photoirradiation of cyclohexyl phenyl ketone
Scheme 1 Photochemical reaction of cyclohexyl phenyl ketone
Irradiation of cyclohexyl phenyl ketone (1) in the
hexagonal liquid crystals was carried out in Pyrex reac-
tors. The samples used for the photochemical reaction
of 1 were simply prepared by sonicating the mixture of
p-xylene, the IL ([C14mim]Br or [C16hpim]Br), water,
chiral inductor (if necessary), cyclohexyl phenyl ketone
(1) and hydrogen donor (Scheme 2) at room temperature
over 24 h till the mixture became uniformly transparent.
The concentration of the ketone (1) was 2 mg/g of LC
sample. Prior to irradiation, the samples were degassed
by freeze-thaw cycles. A 500 W high-pressure mercury
lamp was used as the light source. After the conversion
reached ca. 30%, the products were obtained by extrac-
O
1
hν
O
H
OH
HO
H
+
+
3
ent-3
2
Experimental
Materials
Scheme 2 Chemical structures of chiral inductors
The amphiphilic ionic liquids, [C14mim]Br and
[C16hpim]Br were synthesized according to the proce-
dure reported in the previous work.[26] Cyclohexyl
phenyl ketone, triethylamine, (−)-N-methylephedrine,
(−)-norephedrine, (−)-prolinol and (−)-menthol were
purchased from Acros and were used without further
purification. p-Xylene was provided by Beijing Chemi-
cal Reagent Company. Double-distilled water was used
throughout the experiment.
*
H
N
HO
H
H
*
OH
H
CH3
(−)-norephedrine
(−)-menthol
CH3
HO
H
H
N
N
*
CH3
OH
H
CH3
(−)-N-methylephedrine
Apparatus and procedures
(−)-prolinol
To investigate the microemulsion regions, a phase
604
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© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2013, 31, 603—606

Stereoselective Photochemical Reaction of Cyclohexyl Phenyl Ketone within Lytropic Liquid Crystals
tion with ether for gas chromatograph (GC) detection
crystals (LCs) were constructed by two modes: 1) HLCs
microreactors were prepared by chiral ionic liquids
(ILs), [C16hpim]Br (mode 1 microreactor); 2) HLCs
microreactors were prepared by achiral ionic liquids
(ILs), [C14mim]Br with a chiral inductor (mode 2 micro-
reactor). The samples for the photochemical reaction of
cyclohexyl phenyl ketone (1) were simply prepared by
sonicating the mixture of p-xylene, ILs ([C14mim]Br or
[C16hpim]Br), water, chiral inductor (if necessary),
cyclohexyl phenyl ketone (1) and hydrogen donor
(Scheme 2) at room temperature over 24 h till the mix-
tures became uniformly transparent. Generally, the
weight ratio of p-xylene∶ILs∶water was 20∶40∶40
or 25∶50∶25. In mode 1 microreactor, the mole ratio
of hydrogen donor to substrate was fixed at 7∶1, while
in mode 2 microreactor, the concentration of the chiral
inductor was 7 times that of the ketone (1).
using a chiral column Supelco β-dex 325. The structure
1
of products was identified mainly by GC-MS and H
NMR spectra.
Results and Discussion
The ternary phase diagram of [C16hpim]Br, p-xylene
and water system is constructed and presented in Figure
2a. There is a region of liquid crystalline phase in the
phase diagram. Figure S1 shows the POM picture of
sample (the ratio of p-xylene/[C16hpim]Br/water is
0.25∶0.50∶0.25), indicating the formation of a hex-
agonal liquid crystalline phase. SAXS experiment was
also carried out to understand the internal structure of
the liquid crystalline phases. From the scattering spectra
in Figure S2, the values of the scattering vector q corre-
sponding to the scattering peaks of the sample (the ratio
of p-xylene/[C16hpim]Br/water is 0.25∶0.50∶0.25)
appear in the ratio q1∶q2∶q3＝1∶ 3 ∶2, a charac-
teristic of hexagonal periodicity, indicating that the
sample is hexagonal liquid crystalline (HLCs). For
comparison, the ternary phase diagram of [C14mim]Br,
p-xylene and water was also examined (Figure 2b),[20]
where LLCs and HLCs are clearly shown. Herein, we
use HLCs as microreactors to carry out photochemical
reaction of cyclohexyl phenyl ketone (1, Scheme 1). As
mentioned above, the chiral microreactors of liquid
Table 1 Product distributions of 1 in hexagonal liquid crystals
Molar
ratio of ee in 3
2∶3
Hydrogen
donor
Reaction
media
Entry
1
2
3
4
5
6
7
8
No
HLC-1a
95∶5
－
triethylamine
No
triethylamine
HLC-1a 0∶100 13% A
HLC-2b 100∶0
HLC-2b
3∶97
－
0%
(−)-N-methylephedrine HLC-2b 40∶60 1% B
(−)-prolinol
(−)-norephedrine
(−)-menthol
HLC-2b 42∶58 1% B
HLC-2b 100∶0
HLC-2b 100∶0
－
－
a The ratio of p-xylene∶[C16hpim]Br∶H2O is 0.25∶0.50∶0.25.
b The ratio of p-xylene∶[C14mim]Br∶H2O is 0.20∶0.40∶0.40,
the molar ratio of chiral inductor to substrate is fixed at 7∶1.
The product distributions both in mode 1 and mode 2
microreactors are given in Table 1. Evidently, in the
absence of a hydrogen donor, irradiation of 1 in HLCs
prepared by [C14mim]Br led to intramolecular hydrogen
abstraction to produce compound 2 (Table 1, entry 3,
Figure S3), while in HLCs formed by [C16hpim]Br,
small amount of 3 (5%, Table 1, entry 1) is also de-
tected. In the later case, 3 is probably produced by hy-
drogen abstraction from the hydrocarbon chain of the IL
and/or the methyl group of p-xylene. In the presence of
a hydrogen donor, the intermolecular hydrogen abstrac-
tion product becomes dominant. Irradiation of 1 in
mode 1 microreactor exclusively gave 3 in the presence
of triethylamine (Table 1, entry 2). On the other hand,
the photochemical reaction of 1 in HLCs made of
[C14mim]Br in the presence of triethylamine only
yielded trace of 2 (3%) (Table 1, entry 4, Figure S4).
The chiral inductor, (−)-prolinol, (−)-N-methylephedrine,
(−)-norephedrine and (−)-menthol (Scheme 2) can also
act as hydrogen donor. However, in the model 2 micro-
reactors shown in Table 1 (entries 5－8), compound 3
was detected as a part of products or even absent. This
observation suggests that ketone 1 and the hydrogen
Figure 2 Ternary phase diagram of [C16hpim]Br/p-xylene/H2O
(a) and [C14mim]Br/p-xylene/H2O (b) at room temperature.
Chin. J. Chem. 2013, 31, 603—606
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Acknowledgement
The authors thank the National Key Basic Research
Program of China (Nos. 2010CB833304 and
2013CB834505), the National Natural Science Founda-
tion of China (Nos. 91027041 and 20920102033) for
financial support, and Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, TIPC, CAS
for financial support.
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Figure 3 GC trace of product 3 obtained in HLC-1 (up) and
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Chin. J. Chem. 2013, 31, 603—606