Communication
that electron transfer from the photo-reduced I to oxygen pro-
ceeds. When visible light (l>400 nm) was irradiated to a 1,2-
dichloroethane solution containing 1a (0.1m) and a catalytic
amount of I (1 mol%) in 1 atm of oxygen, the catalytic oxida-
tion of 1a to p-anisaldehyde proceeded, and the turnover
number reached up to 72 after 6 h. Similarly, upon addition of
nitrobenzene to the solution of the photo-reduced I (in 1 atm
of Ar), the color of the solution changed from dark blue to the
original pale yellow within 2 s, and the UV/Vis spectrum
became almost identical to that of the original I (Figure 2b,d,
step 2). These results show that the electrons in the photo-re-
duced I can efficiently transfer to the suitable electron accept-
ors and that I can act as the photoredox catalyst.
Table 1. Photocatalytic one-pot synthesis of N-(4-methoxybenzylidene)a-
niline from nitrobenzene and 1a by irradiation with visible light.[a]
Entry
Catalyst ([mol%])
t [h]
Yield [%]
1
I (1.5)
I (1.0)
I (1.5), dark
I (1.5), without 1a
TBA4[a-SiW12O40] (1.5)
TBA4H4[a-SiW11O39] (1.5)
TBA4H6[a-SiW9O34] (1.5)
22
4
70
96
<0.1
<0.1
<0.1
31
2[b]
3[c]
4[d]
5
22
22
22
22
22
6
7
48
CÀN bond formation is one of the important reactions be-
cause nitrogen-containing compounds (in particular, amines
and imines) are versatile synthons for pharmaceuticals and bio-
active compounds, and several synthetic methods for nitro-
gen-containing compounds have been established.[14] In partic-
ular, N-arylimines are important intermediates and have fre-
quently been synthesized by dehydrative condensation of ani-
lines and aldehydes or ketones. Aniline derivatives have gener-
ally been synthesized by the reduction of nitroarenes using
hydrogen, and this procedure intrinsically leads to problems in
the case of substrates containing other reducible functional
groups.[15] Therefore, if the chemoselective synthesis of N-aryli-
mines is achieved directly from inexpensive and readily avail-
able nitroarenes and alcohols using visible light, it would be
one of the most efficient synthetic routes. The present photo-
redox system would efficiently promote the one-pot synthesis
of N-arylimines from nitroarenes and alcohols because 1) I acts
as the visible-light-responsive catalyst for efficient charge
transfer, 2) both the oxidation of alcohols and the reduction of
nitroarenes simultaneously proceed to form the corresponding
aldehydes and anilines by the multielectron redox catalysis,
and 3) dehydrative condensation of anilines and aldehydes is
efficiently promoted by the presence of I.[16]
As expected, N-(4-methoxybenzylidene)aniline was formed
from 1a and nitrobenzene in the presence of a catalytic
amount of I by irradiation with visible light (l>400 nm)
(Table 1, entry 1). The reaction did not proceed in the dark
(Table 1, entry 3). In the absence of 1a, no reduction of nitro-
benzene proceeded (Table 1, entry 4). Alcohols are the indis-
pensable components in the present photoredox system and
serve the following important functions: 1) Formation of the
visible-light responsive catalyst in situ by their coordination to
the lacuna of I, 2) hydrogen and electron sources for reduction
of nitroarenes, and 3) alkylating reagents.
[a] Reaction conditions: catalyst, nitrobenzene (0.2 mmol), 1a (4 mmol),
acetonitrile (2 mL), photo-irradiation (l>400 nm) for indicated times at
308C, under Ar (1 atm). Yields were determined by GC using naphthalene
as an internal standard. [b] The reaction was carried out in a mixed sol-
vent of acetonitrile and toluene (2 mL, 1:9 v/v). Photo-irradiation for 4 h
at 608C followed by heating at 1008C for further 2 h. [c] Without photo-
irradiation. [d] The reduction of nitrobenzene did not proceed at all.
nation of 1a to the lacunary sites of I in the less polar solvents.
At the initial stage of the reaction (photo-irradiation for
30 min), nitrosobenzene (two-electron reduction product) and
N-phenylhydroxylamine (four-electron reduction product) were
also observed, albeit in only small amounts (below 1 mmol), in
addition to aniline (six-electron reduction product) (Figure S8,
Supporting Information). These results suggest that the reduc-
tion of nitrobenzene to aniline proceeds by three successive
two-electron reductions by the photo-reduced I.
In the presence of I, various kinds of substituted nitroarenes
could selectively be converted into the corresponding N-aryli-
mines by irradiation with visible light (l>400 nm)
(Table 2).[18,19] Nitrobenzene and its derivatives with electron-
donating (Table 2, entries 7–11) as well as electron-withdrawing
(Table 2, entries 12–14) substituents were converted into the
corresponding N-arylimines in high yields (80–99%). In addi-
tion, 4-methylbenzyl alcohol (1b), 4-chlorobenzyl alcohol (1c),
and 4-bromobenzyl alcohol (1c) could also be utilized in the
present system (Table 2, entries 4–6). Remarkably, the present
photocatalytic system was applicable to nitroarenes possessing
additional reducible groups such as the C=C bond (Table 2,
entry 10), CꢀC bond (Table 2, entry 11), Cl (Table 2, entry 12),
and C=O (Table 2, entries 13 and 14); nitro groups were selec-
tively reduced to the amino groups and the corresponding N-
arylimines were successfully obtained.[20]
After the reaction, the catalyst could easily be retrieved as
precipitate by addition of diethyl ether (ca. 15 mL). The CSI-MS
and IR spectra show that the structure of the retrieved catalyst
is preserved (Figures S9 and S10, Supporting Information). The
retrieved I could be reused without an appreciable loss of its
catalytic performance (Table 2, entries 1–3).
N-(4-Methoxybenzylidene)aniline was not formed at all by
using fully occupied TBA4[a-SiW12O40] because of the absence
of the visible-light-responsive charge transfer (Table 1, entry 5;
Figure S1, Supporting Information). The catalytic activities of
TBA4H4[a-SiW11O39] (Table 1, entry 6) and TBA4H4[a-SiW9O34][17]
(Table 1, entry 7) were lower than that of I. The catalytic per-
formance of I was improved when the reaction was carried out
in the mixed solvent of acetonitrile and toluene (1:9 v/v) at
608C, affording the corresponding N-arylimine in 96% yield
(Table 1, entry 2). This is likely because of the efficient coordi-
In conclusion, we have successfully developed an efficient
visible-light-responsive photoredox system with lacunary POM
TBA4H4[g-SiW10O36] (I). The coordination of alcohols to the
lacuna of I generated a new highest occupied molecular orbi-
tal and enabled visible-light-responsive multielectron transfer.
Chem. Asian J. 2014, 9, 1 – 6
3
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