a
Table 2 One-pot imine synthesis from various alcohols and amines
2
The tandem reactions promoted on Pt@TiO are tolerant
for synthesis of various substituted imines (Table 2). UV
irradiation of alkyl or benzyl alcohols containing various
kinds of aromatic amines with Pt(0.3)@TiO2 produces the
1
8
b
corresponding imines with high yields (>74%).
Amine
conv. (%) (%)
Yield
b
In summary, we found that UV irradiation of Pt@TiO2
Run
R
1
R
2
t/h Catalyst
achieves one-pot imine synthesis. This simple system offers
1
2
3
4
5
6
7
8
9
Methyl
n-Propyl
Cyclohexyl
Ph
H
2
TiO
Pt(0.3)@TiO
TiO
Pt(0.3)@TiO
16 TiO
Pt(0.3)@TiO
2
16
90
12
91
16
91
5
>99
12
>99
7
94
14
80
11
86
14
84
4
97
2
91
6
86
14
83
11
74
significant advantages: (i) no harmful byproduct; (ii) the
reaction proceeds at room temperature; and (iii) most of the
hydrogen atoms removed by alcohol oxidation are recovered
2
2
2
2
2
2
2
2
2
2
2
2
as H gas, a clean energy resource. The system therefore has
potential as a sustainable method for imine synthesis.
This work was supported by a Grant-in-Aid for Scientific
Research (No. 20360359) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
2-Me
3-Me
4-Me
16 TiO
2
Pt(0.3)@TiO
12 TiO
2
10
11
12
13
14
15
16
Pt(0.3)@TiO
12 TiO
2
Pt(0.3)@TiO
4-MeO 16 TiO
Pt(0.3)@TiO
TiO
Pt(0.3)@TiO
2
15
Notes and references
>99
12
83
c
c
4-Cl
4
2
1 (a) J. M. Lee, Y. Na, H. Han and S. Chang, Chem. Soc. Rev., 2004,
33, 302; (b) H. C. Kolb, M. S. VanNieuwenhze and K. B. Sharpless,
Chem. Rev., 1994, 94, 2483; (c) J.-C. Wasilke, S. J. Obrey, R. T. Baker
and G. C. Bazan, Chem. Rev., 2005, 105, 1001.
a
Reaction conditions: alcohol, 5 mL; amine, 50 mmol; catalyst, 5 mg;
b
c
temperature, 298 K; N
2
, 1 atm. Determined by GC. Amine, 20 mmol.
2
(a) C. Gunanathan, Y. Ben-David and D. Milstein, Science, 2007,
17, 790; (b) V. Cadierno, J. Francos, J. Gimeno and N. Nebra,
Chem. Commun., 2007, 2536; (c) T. Zweifel, J.-V. Naubron and
H. Grutzmacher, Angew. Chem., Int. Ed., 2009, 48, 559.
3
by UV light in benzyl alcohol show a distinctive desorption
1
profile assigned to the Brønsted acid site. The desorbed
6
¨
3
(a) L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133;
(b) K. Motokura, N. Fujita, K. Mori, T. Mizugaki, K. Ebitani
and K. Kaneda, J. Am. Chem. Soc., 2005, 127, 9674;
(c) F.-X. Felpin and E. Fouquet, ChemSusChem, 2008, 1, 718;
amount of NH increases with the Pt loadings, indicating that
3
+
higher Pt loadings produce larger amounts of Ti–OH . As
2
+
shown in Table 1, the amount of Ti–OH , roughly determined
2
(d) Y. Shiraishi, Y. Sugano, S. Tanaka and T. Hirai, Angew.
Chem., Int. Ed., 2010, 49, 1656.
4 (a) S. F. Martin, Pure Appl. Chem., 2009, 81, 195; (b) W. Tang and
X. Zhang, Chem. Rev., 2003, 103, 3029.
+
from the mass balance of H , increases with the Pt loadings.
+
This supports the formation of larger amounts of Ti–OH
2
on
the catalysts with higher Pt loadings.
+
2
5 (a) S. Murahashi, Y. Okano, H. Sato, T. Nakae and N. Komiya,
Synlett, 2007, 1675; (b) F. Porta, C. Crotti, S. Cenini and
The protonation of amine by Ti–OH
is confirmed by the
adsorption experiments of aniline with catalysts at 298 K. As
shown in Fig. 2b (white), the fresh catalysts show similar
aniline adsorption. In contrast, the catalysts, when irradiated
by UV light (black), show an increased adsorption with the Pt
´
G. Palmisano, J. Mol. Catal., 1989, 50, 333; (c) H. Ell, J. S.
M. Samec, C. Brasse and J.-E. Backvall, Chem. Commun., 2002, 1144.
S. Sithambaram, R. Kumar, Y.-C. Son and S. L. Suib, J. Catal.,
008, 253, 269.
J. W. Kim, J. He, K. Yamaguchi and N. Mizuno, Chem. Lett.,
2009, 920.
¨
6
7
2
+
loadings, indicating that aniline is protonated by Ti–OH2 . In
addition, as shown in Fig. 2a (black), the UV-irradiated
catalysts, when used for condensation of aniline with benz-
aldehyde, show a decreased yield of 1 with the Pt loadings.
8 M. S. Kwon, S. Kim, S. Park, W. Bosco, R. K. Chidrala and
J. Park, J. Org. Chem., 2009, 74, 2877.
9
H. Sun, F.-Z. Su, J. Ni, Y. Cao, H.-Y. He and K.-N. Fan, Angew.
Chem., Int. Ed., 2009, 48, 4390.
These indicate that the catalysts with higher Pt loadings
+
10 (a) A. Hakki, R. Dillert and D. Bahnemann, Catal. Today, 2009,
44, 154; (b) K. Selvam, B. Krishnakumar, R. Velmurugan and
M. Swaminathan, Catal. Commun., 2009, 11, 280; (c) K. Selvam
and M. Swaminathan, Tetrahedron Lett., 2010, 51, 4911.
11 F.-C. Wang, C.-H. Liu, C.-W. Liu, J.-H. Chao and C.-H. Lin,
J. Phys. Chem. C, 2009, 113, 13832.
1
produce larger amounts of Ti–OH
2
during UV irradiation,
which promotes the protonation of amines and suppresses the
1
7
condensation. Efficient imine synthesis, therefore, requires a
catalyst with an appropriate Pt loading (0.1–0.3%) that promotes
1
1
2 W. B. Jennings and C. J. Lovely, Tetrahedron, 1991, 47, 5561.
3 (a) Y. Z. Yang, C.-H. Chang and H. Idriss, Appl. Catal., B, 2006,
67, 217; (b) J. M. Herrmann, Top. Catal., 2006, 39, 3.
efficient photooxidation of alcohols while producing a smaller
+
amount of Ti–OH2
.
+
It is noted that the Ti–OH2 species are deprotonated by
14 P. Qu and G. J. Meyer, Langmuir, 2001, 17, 6720.
1
4
1
1
5 Organic Chemistry, ed. S. H. Pine, McGrow-Hill, New York, 1987.
6 (a) S. Brandenberger, O. Krocher, A. Wokaun, A. Tissler and
R. Althoff, J. Catal., 2009, 268, 297; (b) R. Barthos, F. Lnyi,
G. Onyestyak and J. Valyon, Solid State Ionics, 2001, 141–142, 253.
17 The amount of Lewis acid site on Pt@TiO scarcely decreases. As
shown in Fig. S4, ESIw, the NH desorption peak at ca. 500 K,
simple water washing of catalysts. Upon water washing
Fig. 2, gray), the UV-irradiated Pt(0.3)@TiO2 shows high
¨
(
activity for condensation of aldehyde with aniline (Fig. 2a)
and a low adsorption amount of aniline (Fig. 2b) similar to
´
2
3
those of the fresh catalyst. This indicates that accumulation of
+
assigned to the Lewis acid site (ref. 16a), scarcely changes even
Ti–OH
2
is suppressed by water washing. As shown in
, when washed with
after UV irradiation in benzyl alcohol. This again suggests that the
Ti–OH2 species suppress the imine formation.
18 The imine synthesis from anilines with an electron-withdrawing
+
Table 1 (runs 8 and 9), Pt(0.3)@TiO
2
water, is reusable for imine synthesis at least two times without
loss of activity and selectivity. In addition, TEM analysis of
the catalyst recovered after the reaction reveals that the Pt
particle size scarcely changes during reaction (Fig. 1b).
group (–NO
related systems: S. Bhagat and A. K. Chakraborti, J. Org. Chem.,
007, 72, 1263. This is because the decreased nucleophilicity of the
amine moiety suppresses the condensation.
2
and –CN) was unsuccessful (o10% yield), as the
2
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4811–4813 4813