Redox-photosensitized reaction of indene using photosensitive surfactant in emulsion: dependence on oil droplet size and surfactant charge

Article abstract of DOI:10.1016/j.tetlet.2006.03.038

A redox-photosensitized reaction of indene 2 using a photosensitive surfactant 1a in an oil-in-water emulsion proceeded efficiently to give alcohol 3 as a major product and is strongly influenced by the oil droplet size and surfactant charge.

Full text of DOI:10.1016/j.tetlet.2006.03.038

Tetrahedron Letters 47 (2006) 3257–3260
Redox-photosensitized reaction of indene using
photosensitive surfactant in emulsion: dependence on
oil droplet size and surfactant charge
*
*
Yasuharu Yoshimi, Tatsuya Itou and Minoru Hatanaka
Department of Applied Chemistry and Biotechnology, Faculty of Engineering, University of Fukui, 3-9-1 Bunkyo,
Fukui 910-8507, Japan
Received 3 February 2006; revised 3 March 2006; accepted 8 March 2006
Available online 23 March 2006
Abstract—A redox-photosensitized reaction of indene 2 using a photosensitive surfactant 1a in an oil-in-water emulsion proceeded
efficiently to give alcohol 3 as a major product and is strongly influenced by the oil droplet size and surfactant charge.
ꢀ
2006 Elsevier Ltd. All rights reserved.
In the recent past, photochemical reactions in micro-
heterogeneous systems, such as micelles, emulsions,
and vesicles, have been actively investigated. For exam-
remarkable electron transfer process since a photosensi-
tizer can act as a redox carrier. Thus far, studies have
been carried out in homogeneous systems such as meth-
1
5
ple, the photoinduced electron transfer between hydro-
philic and hydrophobic solutes has been suggested to
mimic photosynthetic pathways since this partition be-
tween these solutes could retard the recombination of
anol–acetonitrile and water–acetonitrile solutions. In
this letter, we would like to report that the photosensi-
6
tive surfactant 1a in which the naphthyl group is
attached to hydrophobic part as photosensitizer
(Fig. 1) can be utilized successfully in a redox-photosen-
sitized reaction in a microheterogeneous system in
water. Detailed experiments showed that the character-
istic feature of the emulsion system is that the efficiency
of this photoreaction is highly dependent on the oil
droplet size and the charge of the surfactant used.
2
oxidized and reduced species. In particular, efficient
charge separation between reactive species would be
achieved in microheterogeneous media. However, there
have been few previous reports on organic photo-
3
chemical reactions in such systems. We have recently
reported that the photochemical polar addition by using
a photosensitive surfactant proceeded smoothly in a
4
microheterogeneous system. With regard to this, our
An oil-in-water emulsion was formed when an aqueous
solution of the surfactant 1a (5 mM) was mixed with the
indene 2 (10 mM), 1,4-dicyanobenzene (DCB, 3 mM),
attention is confined to a redox-photosensitized reaction
in this system. The redox-photosensitized reaction is a
+
−
N (CH CH ) Br
R
H3C
2
3 3
1
c
+
−
1
a; R = CH N (CH CH ) Br
2
2
3 3
1
b; R = CO H
2
Figure 1. Photosensitive surfactants 1a, b and non-photosensitive surfactant 1c.
Keywords: Redox-photosensitized reaction; Photosensitive surfactant; Emulsion; Oil droplet size; Surfactant charge.
*
Corresponding authors. Tel.: +81 776 27 8633; fax: +81 766 27 8747 (Y.Y.); e-mail: yoshimi@acbio2.acbio.fukui-u.ac.jp
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.038

3258
Y. Yoshimi et al. / Tetrahedron Letters 47 (2006) 3257–3260
hν
OH
OH +
+
1
, DCB, CB
/
H O
2
2
3
4
5
CN
Scheme 1.
and cyanobenzene (CB, 30 mM) under an argon atmo-
sphere for 12 h. The emulsion was irradiated with a
Next, we studied the effect of oil droplet size on the reac-
tion course. In the absence of stirring, the yield of 3 de-
creased, implying that stirring is essential to retain the
particle size (entry 7). The particle size formed in entry
1 was proven to be 600 nm by dynamic light scattering
5
00 W high-pressure mercury lamp through a Pyrex fil-
ter (>300 nm) under vigorous stirring for 24 h at room
temperature to afford mainly 2-indanol 3 (31%) accom-
8
panied by 4 (4%) and the dimer 5 (1%) in isolated yields
measurement. Further addition of 1a (7.5 mM) caused
7
(
Scheme 1, entry 1 in Table 1). In the absence of CB,
a decrease in the oil droplet size (370 nm) and an in-
crease in the yield of 3 (50%, entry 8). However, when
the concentration of 1a was greater than 10 mM, a clear
solution was formed due to a decrease in the oil droplet
DCB dissolved little in the mixture, and the yield of
the photoproduct 3 was very low (4%). The use of benz-
ene in the place of CB produced a low yield of 3 (4%).
Thus, CB serves as a polar medium in this system and
this photoreaction takes place in the oil droplet of CB
in water.
9
size, and the yield of 3 was lower (entries 9 and 10).
The best result was achieved by the addition of cetyl-
+
trimethylammonium bromide (CTAB, CH (CH ) N -
3
2 15
À
(
CH ) Br , 1 mM) in the presence of 1a (5 mM). The
3 3
The results of our study, in which we investigated the
effects of additives and solvents, are shown in Table 1.
When the mixture was irradiated in the absence of 1a,
alcohol 3 was not obtained (entry 2). Instead of 1a, the
addition of 1-methylnaphthalene as photosensitizer re-
sulted in a low yield of 3 (entry 3). The use of the sur-
factant 1c that had no photosensitizer activity was not
effective both in the absence and presence of 1-methyl-
naphthalene as compared with 1a (entries 4 and 5).
In a homogeneous system (aqueous acetonitrile solu-
tion; water/acetonitrile = 1:9), a low yield of alcohol
addition of CTAB decreased the droplet size (500 nm)
and yielded 63% of alcohol 3 (entry 11). Higher concen-
trations of CTAB caused a decrease in the yield similar
to that observed in the case of 1a (entries 12 and 13). It
was found that a small particle size resulted in a higher
yield of 3; however, a decrease in this size below a cer-
tain limit reduced the yield of 3.
The anionic surfactant 1b was also examined. When the
emulsion containing the anionic surfactant 1b (5 mM),
NaOH (25 mM), 2 (10 mM), DCB (3 mM), and CB
(30 mM) was subjected to the photoreaction, a 23%
yield of the decarboxylated product 6 was obtained
along with the recovery of 1b (38%) and a poor yield
3
was observed (entry 6). These results indicate that
the formation of an emulsion provides high local con-
centrations of the reactant and the alkylnaphthalene
part of 1a as photosensitizer; in this emulsion, the sur-
factant 1a serves efficiently as a mediator for photo-
chemical electron transfer leading to a higher yield
of 3.
1
0
of 3 (1%), as shown in Scheme 2. The yield of the
decarboxylated product 6 was observed to be low (6%)
in the absence of 2, and 6 was not obtained when
DCB was not added. From these results, we noted that
Table 1. Product yields in a redox-photosensitized reaction of 2 under various conditions
a
b
Entry
Surfactant (mM)
Additive
Product yields/%
3
4
5
1
2
3
4
5
1a (5)
—
—
1c (5)
1c (5)
1a (5)
1a (5)
1a (7.5)
1a (10)
DCB, CB
DCB, CB
DCB, CB, 1-methylnaphthalene
DCB, CB
DCB, CB, 1-methylnaphthalene
DCB, CB
DCB, CB
DCB, CB
DCB, CB
DCB, CB
31
0
9
0
13
4
17
50
29
9
63
19
6
4
0
6
6
4
3
4
18
4
4
1
9
Trace
15
8
0
c
6
7
d
Trace
8
9
0
1
2
3
2
2
3
0
0
5
1
1
1
1
1a (15)
1a (5), CTAB (1)
1a (5), CTAB (5)
1a (5), CTAB (10)
DCB, CB
DCB, CB
DCB, CB
8
11
4
a
b
c
[
DCB] = 3 mM, [CB] = 30 mM, [1-methylnaphthalene] = 5 mM.
Isolated yields.
In H O/CH CN = 1:9.
Under no stirring.
2
3
d

Y. Yoshimi et al. / Tetrahedron Letters 47 (2006) 3257–3260
CH3
3259
photosensitive surfactant 1a in a microheterogeneous
system to yield alcohol 3 as a major product and is
strongly influenced by the droplet size and the surfactant
charge. The selectivity and efficiency of a photoinduced
electron transfer reaction in an emulsion can be con-
trolled through the droplet size and ionic surfactant;
thus, photoreactions using photosensitive surfactants
in emulsions can be used as a new synthetic method.
h
3
+
1b , DCB, CB,
6
NaOH / H O
1%
2
23%
2
Scheme 2.
the photogenerated cation radicals (the cation radicals
of 2 and the naphthyl moiety of 1b) react with the car-
1
1
boxylate anion to give the decarboxylated product.
Acknowledgements
Although a similar photochemical decarboxylation of
the carboxylate anion via electron transfer has already
1
2
This work was supported by a Grant-in-Aid for Scien-
tific Research (No. 15750081) from the Japan Society
of Promotion of Science. We thank Professor Isa and
Dr. Sugihara, University of Fukui, for helpful
discussions.
been reported, the reaction between a cation radical
and a carboxylate anion has not been investigated. A
detailed study of this mechanism and the development
of this type of photoreaction are now in progress in this
laboratory and will be reported in separate publications.
These results may be rationalized as follows. The cation
radical of the naphthyl moiety of 1a and the anion rad-
ical of DCB are formed via the electron transfer between
the excited 1a and DCB in the oil droplet. The cation
radical of 1a is reduced by the indene 2 to produce the
cation radical of 2; on the addition of water, this radical
gives alcohol 3 at the interface (Fig. 2, A). The decrease
in the oil droplet size on the addition of CTAB im-
proved the yield of 3 due to an increase in the oil–water
surface area (Fig. 2, B). However, in the presence of a
higher concentration of the cationic surfactant, the
influence of the cationic charge increases by further
decreasing the droplet size. In this case, the formation
of the cation radicals might be prevented by the surfac-
tant charge and results in a lower yield of 3 (Fig. 2, C).
On the other hand, the use of the anionic surfactant 1b
quenches the photogenerated cation radicals at the
interface, and the formation of alcohol 3 is also re-
tarded. The formation of the decarboxylated product 6
indicates that the electron transfer from the carboxylate
anion of 1b to the cation radicals takes place at the inter-
face. It is evident from these results that the highest yield
of 3 is achieved when an appropriate oil droplet size is
attained in the presence of the cationic surfactant.
References and notes
1
2
. For reviews, see: (a) Kalyanasundaram, K. Photochemis-
try in Microheterogeneous System; Academic Press: New
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3
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(
9
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4
. Yoshimi, Y.; Higuchi, M.; Itou, T.; Hatanaka, M. Chem.
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In conclusion, we found that the redox-photosensitized
reaction of indene proceeds efficiently by using the
5. (a) Pac, C.; Nakasone, A.; Sakurai, H. J. Am. Chem.
Soc. 1977, 99, 5806; (b) Majima, T.; Pac, C.; Nakasone,
CN
N+(CH CH ) Br−
2 3 3
+
CN
DCB
1a
hν
+
−
N (CH2CH3)3Br
DCB + 1a
hν
DCB
+
1a
Addition of CTAB (< 1 mM)
Addition of CTAB (> 5 mM)
+
2
1a
DCB
+
oil droplet
2
OH
2
3
+H2O
2
OH
A
B
3
+
H O
2
With decreasing the oil droplet size,
the product yield was improved.
C
aqueous phase
With further decreasing the oil droplet
size, the photoreaction was suppressed
by the surfactant charge.
Figure 2. A plausible reaction course of a redox photosensitization of 2 using the cationic surfactants 1a and CTAB in an emulsion.

3260
Y. Yoshimi et al. / Tetrahedron Letters 47 (2006) 3257–3260
A.; Sakurai, H. J. Chem. Soc., Chem. Commun. 1978,
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4
dynamic light scattering spectrophotometer (DLS-7000,
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9. These droplet sizes could not be measured by dynamic
light scattering since very little light was scattered.
10. When the basic aqueous solution (NaOH, 25 mM)
including CB, DCB, and 1-methylnaphthalene (30, 3,
and 5 mM, respectively) was irradiated, the product yields
were the same as those mentioned in entry 3 in Table 1.
This result indicates that the efficiency of this photoreac-
tion does not differ if the pH range is changed.
2
004, 69, 4997.
. Spectra data for 1a; mp 108 ꢁC; H NMR (500 MHz,
CDCl 8.04 (d, 1H, J = 8.5 Hz), 7.84 (d, 1H,
J = 8.1 Hz), 7.69 (d, 1H, J = 7.9 Hz), 7.51–7.37 (m, 3H),
1
6
3
)
d
11. The intramolecular electron transfer between the excited
naphthyl moiety and the carboxylate anion of 1b did not
À4
7
2
4
1
1
2
.31 (d, 1H, J = 6.1 Hz), 3.50 (q, 6H, J = 7.3 Hz), 3.25 (t,
occur; the fluorescence spectrum of 1b (6 · 10 M) in the
H, J = 8.6 Hz), 3.06 (t, 2H, J = 7.9 Hz), 1.77–1.67 (m,
aqueous NaOH (25 mM) solution was very close to that in
the acetonitrile solution. Thus, the photogenerated cation
radicals might be quenched by the anionic surfactant at
the interface.
1
3
2
H), 1.46–1.26 (m, 23H); C NMR (125 MHz, D O) d
39.3, 134.7, 132.6, 129.6, 127.3, 126.6, 126.5, 126.4, 126.3,
24.5, 57.2, 53.5, 33.7, 31.5, 30.5, 30.5, 30.4, 30.3, 30.2,
+
9.7, 26.8, 22.0, 7.8; MS(FAB) 383 (M ÀBr); Anal. Calcd
12. (a) Kurauchi, Y.; Nobuhara, H.; Ohga, K. Bull. Chem. Soc.
Jpn. 1986, 59, 897; (b) Griesbeck, A.; Henz, A.; Peters, K.;
Peters, E.; Schnering, H. Angew. Chem., Int. Ed. Engl. 1995,
34, 474; (c) Yokoi, H.; Nakano, T.; Fujita, W.; Ishiguro,
K.; Sawaki, Y. J. Am. Chem. Soc. 1998, 120, 12453; (d)
Sandanayaka, A.; Zhang, H.; Takaguchi, Y.; Sako, Y.;
Tamura, M.; Araki, Y.; Ito, O. Chem. Commun. 2005, 5160.
for C27H37NBr: C, 70.11; H, 9.59; N, 3.03. Found: C,
70.40; H, 9.60; N, 3.26.
7
. After the photoreaction, the emulsion was added to a 10%
aqueous NaOH solution. The mixture was extracted with
ether and ethyl acetate to give the products, CB and DCB,
and bring about the recovery of 1a (93%).