Synthesis of imines from nitrobenzene and TiO2 particles suspended in alcohols via semiconductor photocatalysis type B

Article abstract of DOI:10.1016/j.tetlet.2010.03.054

UV irradiation on a non-aqueous suspension of titanium dioxide with nitrobenzene and different alcohols in deaerated conditions produces imines and aniline as main products. The conversion of nitrobenzene and the corresponding selectivity of imines or aniline depend on the type of alcohol used. A low conversion (3-12%) and selectivity close to 100% to imines were obtained with methyl, ethyl, or propyl alcohol. Otherwise, using i-propanol only aniline was detected with a conversion of 13%. Finally, a mixture of aniline and imines was formed employing n-butyl, n-amyl, and i-amyl alcohols with the higher conversion (～50%).

Full text of DOI:10.1016/j.tetlet.2010.03.054

Tetrahedron Letters 51 (2010) 2730–2733
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of imines from nitrobenzene and TiO particles suspended
2
in alcohols via semiconductor photocatalysis type B
Omar Rios-Bernÿ, Sergio O. Flores, Iván Córdova, Miguel A. Valenzuela *
Lab. Catálisis y Materiales, ESIQIE-Instituto Politécnico Nacional, Zacatenco, 07738 Mexico, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
UV irradiation on a non-aqueous suspension of titanium dioxide with nitrobenzene and different alcohols
in deaerated conditions produces imines and aniline as main products. The conversion of nitrobenzene
and the corresponding selectivity of imines or aniline depend on the type of alcohol used. A low conver-
sion (3–12%) and selectivity close to 100% to imines were obtained with methyl, ethyl, or propyl alcohol.
Otherwise, using i-propanol only aniline was detected with a conversion of 13%. Finally, a mixture of ani-
line and imines was formed employing n-butyl, n-amyl, and i-amyl alcohols with the higher conversion
Received 11 February 2010
Revised 11 March 2010
Accepted 12 March 2010
Available online 17 March 2010
Keywords:
Imines
Nitrobenzene
Photocatalytic reduction
Titanium dioxide
(
ꢀ50%).
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
Imine, also known as azomethine, is formed by the reversible
tion and then, they are among the most studied photochemical
9
,10
systems.
In addition, the photocatalytic reduction of nitroarenes
has been also reported using different semiconductors and opera-
4
,9–18
condensation between amine and carbonyl groups. This reaction
was discovered by the German chemist Hugo Schiff in 1864 and
since then imines are also called as Schiff’s bases. An imine is a
chemical compound containing a carbon–nitrogen double bond
with a general structure R
the synthesis of nitrogen heterocycles. Imine synthesis is cata-
lyzed with acids and since it implies a condensation reaction, it
is necessary to eliminate the water as formed.3 Therefore, the
exploration of new methods that have a minimal environment im-
pact and low cost is necessary.
Organic synthesis through semiconductor photocatalysis has
become an important research area in photochemistry for the last
two decades. Many reactions, such as oxidation, reduction, and
isomerization of organics, C–H bond activation, C–C and C–N bond
tion conditions.
This reaction can be carried out in the pres-
9–11
ence of a large excess of a sacrificial electron donor (SED).
To
1
scavenge valence band holes is the main function of the sacrificial
electron donors in order to limit the charge recombination degree
1
2
1
R
2
C@NR
3
used as key intermediate for
and then liberating more reductive species. Many solvents, such
as alcohols, organic acids, and their mixtures have been reported as
useful SED in order to improve the global photoredox cycle.9
However, it is not still clear what the influence is of the type of
alcohol added on the selectivity during the photocatalytic reduc-
2
–18
1
3
tion of nitrocompounds. In a previous work, we have studied
the photocatalytic reduction of nitrobenzene using ethanol as
SED finding a low selectivity to aniline and the formation of by-
products such as indoles and quinolines. Note that the nature of
the obtained products should be explained in terms of the specific
mechanistic scheme of semiconductor photocatalysis classified
4
,5
4
–8
forming have been studied using different photocatalysts.
The
1
9–22
most studied photocatalytic reaction has been the oxidation of or-
ganic compounds in water because common semiconductors have
band edges that lie positive of the oxidation potentials of most or-
ganic functional groups. By contrast, photocatalytic reductions are
less frequently found, because the reducing power of a conduction
band electron is significantly lower than the oxidizing power of a
into two categories accordingly.
Semiconductor photocataly-
sis type A means that at least one reduced and one oxidized com-
pounds are obtained and reactions such as dealkylation,
7
cyclization, and hydroalkylation can also occur. On the contrary,
photocatalysis type B after forming the photoredox products can
react under dark conditions by using the catalytic properties of
the semiconductor yielding new products.
8
valence band electron. It is well known that the reduction of
nitroarenes can occur easily as a result of their direct light absorp-
In the present work, we report a new approach of photocata-
lytic reduction of nitrobenzene and its derivatives by using a
non-aqueous suspension of titanium dioxide to observe the effect
1 5
of several aliphatic alcohols (C –C ) on the conversion and
*
Corresponding author.
E-mail address: mavalenz@ipn.mx (M.A. Valenzuela).
selectivity.
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.054

O. Rios-Bernÿ et al. / Tetrahedron Letters 51 (2010) 2730–2733
2731
2
. Results and discussion
nitrobenzene. However, imines, not expected, are also obtained.
For all cases, selectivities for imines are higher than those for ani-
line. Although the conversion of nitrobenzene was higher than that
for C –C alcohols, the condensation reaction between aniline and
1 3
the corresponding aldehyde was very slow which again must be
explained by a steric effect using long chain alcohols.
Irradiation with UV-light (k ꢁ 365 nm) of an alcoholic suspen-
sion of titanium dioxide containing nitrobenzene (outgassed to
discard the presence of oxygen) yields imines or amines. In a blank
experiment, irradiation of an alcoholic solution of nitrobenzene
alone led to no reaction product, indicating that TiO
for the reaction. On the other hand, without irradiation under UV-
light, no reaction product was detected either.
Irradiation for 24 h in the presence of alcoholic solvents gave
the corresponding imines, aniline, or a mixture of imine/aniline
without further reduction of imines (Table 1). The structure of each
product was confirmed by GC/MS (see Supplementary data). As can
be seen in Table 1, the conversion and selectivity were strongly
2
was essential
These results indicated that the photocatalytic reduction of
nitrobenzene was faster compared with the condensation reaction
regardless of the type of alcohol used. However, for these experi-
mental conditions, the condensation reaction is highly sensitive
to the presence of oxygen in the reaction media. After irradiation,
if the reactor is maintained open to the environment, the dissolved
oxygen inside the reaction mixture favors the formation of indoles
and quinolines. On the contrary, if the reactor is maintained in
nitrogen atmosphere a high selectivity to imines is obtained.
A reaction path for the production of imines or amines is pro-
posed in two steps, as depicted in Figure 1. In the first step (I),
1
6
1 3
solvent dependent. For short chain primary alcohols, C –C , nitro-
benzene conversion exhibited values below 12% and the corre-
sponding selectivities were close to 100% toward imines.
Isopropyl alcohol is a special case for its photocatalytic oxida-
tion. Acetone is the only possible product to obtain and no subse-
quent reaction can take place. Nitrobenzene conversion just
reaches 13%, similar to those of short chain alcohols. Besides, it
has to be highlighted that selectivity toward aniline achieved
the photoexcitation of TiO
to its band gap value, promoting electrons (e ) in the conduction
band and leaving positive holes (h ). The photoreduction of nitro-
2
occurs at wavelength corresponding
À
+
benzene takes place only in the presence of a SED participating
in the photoinduced process and sustaining oxidation and produc-
ing the electrons and protons required for the reduction of nitro-
1
00%. An explanation of this behavior can be done in terms of
9
,10,17,18
the steric effect of acetone and acetaldehyde, the former is much
less suitable to undergo condensation with aniline to form an
benzene.
In the second step (II), the process is carried out
at dark conditions, the condensation between aldehyde and aniline
forms an imine. In fact, the condensation reaction to form imines is
catalyzed in acidic conditions that are provided by the TiO surface.
2
2
3
imine.
A different product distribution is observed for C
–C
4 5
alcohols
used as SEDs. An average nitrobenzene conversion value of 50%
is achieved, that is, almost fivefold increase when compared to
C –C alcohols. Two products were found: aniline and imines. Ani-
1 3
The acid sites could be titanol groups, TiÁ Á ÁOH, generated during
24
the oxidation of alcohols. It has been reported as a potential reac-
tion, which includes the further reduction of imine into hydroxyl-
amine depending on the thermodynamic equilibrium.23 According
line is the expected product for the photocatalytic reduction of
Table 1
Conversion of nitrobenzene and selectivity after 24 h of irradiation time as a function of the type of alcohol
Alcohol
Nitrobenzene conversion (%)
Reaction products
Selectivity (%)
100
N
Methanol
3
N
Ethanol
12
9
100
100
100
N
n-Propanol
i-Propanol
NH2
13
N
NH2
n-Butanol
n-Pentanol
i-Pentanol
54
49
52
15, 85
25, 75
42, 58
N
NH2
N
NH2

2
732
O. Rios-Bernÿ et al. / Tetrahedron Letters 51 (2010) 2730–2733
O
O
N
+
6H+
e-
6e-
NH2
2H2O +
(
I)
hν
+
2
H +
O
h+
R
2
h+
OH
R
H
H
H
H
OH
HO
H
Ti
O
Ti
O + H+
H
N
R
N
R
H
O
R
R
NH2
H
H
(II)
OH
O
H+
H
HN
R
HN
R
N
R
HN
R
-H2O
Figure 1. Proposed reaction mechanism for the photocatalytic formation of imines on TiO
2
suspension.
to our operation conditions, that reaction is favored to the forma-
tion of imines even though the presence of water shifts the equilib-
rium to imine reduction.
butyl, n-amyl, and i-amyl alcohols were analytical grade (J.T. Ba-
ker). Nitrobenzene was high purity grade (>99%, Fluka). All re-
agents were used without further purification.
Further work is required in order to prove in more detail the
role of the type of SED on yield to imines to have a one-pot synthe-
sis at room temperature and atmospheric pressure.
4.2. Methods
The alcoholic suspensions (5 mL) containing 126 mmol/L of
nitrobenzene were prepared in a vial that was capped with butyl
rubber septum, crimped with an aluminum seal, and then out-
gassed for 5 min in an ultrasonic bath. The suspensions were thor-
oughly purged with argon before irradiation. The irradiation was
carried out with a black light lamp (15 W) for 24 h, under constant
stirring and the vial was thermostated at 25 °C. After irradiation,
the mixture was maintained closed and analyzed to identify the
reaction products by means of a GC/MS system (Perkin–Elmer
Autosystem XL-TurboMass) using a SPB-225 (Supelco) capillary
column, 0.25 mm  30 m. Gas chromatograph was operated with
the following conditions: inlet temperature 200 °C, oven program:
3
. Conclusion
Different kinds of imines have been prepared through a semi-
conductor photocatalysis type B in which the reduction product
coming from nitrobenzene and the oxidation product of an alcohol
are linked. It seemed that the rate-limiting step of the reaction net-
work was the condensation between the aniline and aldehyde or
1 3
ketone. When a C –C alcohol is used as a solvent, a low conversion
of nitrobenzene but almost 100% selectivity to the corresponding
imine is obtained. If i-propanol is used only aniline is obtained.
The long chain alcohols such as n-butyl, n-amyl, and i-amyl alco-
hols produce a mixture of aniline and imines. These results were
explained in terms of a steric effect presented by each alcohol
1
30 °C (2 min), 230 °C (5 min), 17 °C/min. Blank experiments were
carried out in order to discriminate the effect of TiO
2
or photo-
chemical excitation.
2
and with the presence of acid sites on the surface of TiO which
could be responsible for the conversion of aniline and aldehydes
to imines.
Acknowledgments
This research has been supported by Conacyt, SIP, and COFAA of
IPN.
4
4
. Experimental
.1. Materials
Supplementary data
2
Titanium dioxide (TiO , Aldrich, 325 mesh, 98% anatase, 2% ru-
Supplementary data (mass spectrometry patterns for the reac-
tion products) associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2010.03.054.
tile) was used in all photocatalytic experiments in a concentration
of 1 g/L. Methyl alcohol and ethyl alcohol were absolute grade (J.T.
Baker); n-propyl alcohol was analytical grade (Merck); i-propyl, n-

O. Rios-Bernÿ et al. / Tetrahedron Letters 51 (2010) 2730–2733
2733
1
2. Ahn, W. Y.; Sheeley, S. A.; Rajh, T.; Kropek, D. M. Appl. Catal. B: Environ. 2007,
References and notes
7
4, 103–110.
1
1
1
1
3. Ferry, J. L.; Glaze, W. H. Langmuir 1998, 14, 3551–3555.
1
2
3
4
.
.
.
.
Meyer, C. D.; Joiner, C. S.; Stoddart, J. F. Chem. Soc. Rev. 2007, 36, 1705–1723.
Martin, S. F. Pure Appl. Chem. 2009, 81, 195–204.
Layer, R. W. Chem. Rev. 1963, 63, 489–510.
4. Ferry, J. L.; Glaze, W. H. J. Phys. Chem. B 1998, 102, 2239–2244.
5. Zhang, T.; You, L.; Zhang, Y. Dyes Pigments 2006, 68, 95–100.
6. Flores, S. O.; Rios-Bernÿ, O.; Valenzuela, M. A.; Cordova, I.; Gomez, R.;
Gutierrez, R. Top. Catal. 2007, 44, 507–511.
Palmisano, G.; Augugliaro, V.; Plagiaro, M.; Palmisano, L. Chem. Commun. 2007,
33, 3425–3437.
1
7. Brezova, V.; Blazkova, A.; Surina, I.; Havlinova, B. J. Photochem. Photobiol. A
5
6
7
8
9
.
.
.
.
.
Ohtani, B.; Ohno, T. In Photocatalysis Science and Technology; Kaneko, M., Okura,
I., Eds.; Kodansha-Springer, 2002. Chapter 4.
Fox, M. A.. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH, 2001;
Vol. 1,. Chapter 8.
Kisch, H.; Hopfner, M.. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH, 2001; Vol. 4,. Chapter 6.
Maurel, V.; Mouesca, J. M.; Desfonds, G.; Gambarelli, S. J. Phys. Chem. A 2005,
1997, 107, 233–237.
1
1
8. Mahdvai, F.; Bruton, T.; Li, Y. J. Org. Chem. 1993, 58, 744–746.
9. Keck, H.; Schindler, W.; Knoch, F.; Kisch, H. Chem Eur. 1997, 3, 1638–
1
645.
0. Reinheimer, A.; van Eldik, R.; Horst Kisch, H. J. Phys. Chem. B 2000, 104, 1014–
024.
1. Hopfner, M.; Weiß, H.; Meissner, D.; Heinemann, F. W.; Horst Kisch, H.
Photochem. Photobiol. Sci. 2002, 1, 696–703.
2
2
2
1
109, 148–156.
Brezova, V.; Tarabek, P.; Dvoranova, D.; Stasko, A.; Biskupic, S. J. Photochem.
Photobiol. A 2003, 155, 179–198.
2. Pehlivanugullari, H. C.; Sumer, E.; Kisch, H. Res. Chem. Intermed. 2007, 33, 297–
309.
1
1
0. Maldotti, A.; Andreotti, L.; Molinari, A.; Tollari, S.; Penoni, A.; Cenini, S. J.
Photochem. Photobiol. A 2000, 133, 129–133.
1. Kulik, S. V.; Korzhack, A. V.; Kuchmii, S. Y.; Kryukov, A. I. Theor. Exp. Chem. 1994,
2
2
3. Wang, H.; Partch, R. E.; Li, Y. J. Org. Chem. 1997, 62, 5222–5225.
4. Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99–104.
30, 171–175.